Allegro Common Lisp Function prints fine in debug window, but when using the aserve html macro, only nil is printed. (html ((:pre ) (:princ (show-frame symbol)))) The show-frame function prints to *standard-output*, and not to *html-stream*. Function prints to *standard-output* or another output stream which you know. Wrap printing code with (with-output-to-string (*output-stream*) ...) where *output-stream* is the stream that the function normally prints to. (html ((:pre ) (:princ (with-output-to-string (*standard-output*) (show-frame symbol))))) Java Code compiles fine, but when trying to run, get an error that says: "Exception in thread "main" java.lang.NoClassDefFoundError: ..." java MyClass The java interpreter cannot find your class. This is a problem with the classpath, a variable which tells java where to find your classes. Must be able to set system properties. Set the system property "classpath" to point to where your java classes are. set classpath="." java MyClass> None. Pass the classpath into the java interpreter. java -classpath "." MyClass Python Import a module in the program, but still can't use its functions. import math x = sin(1.0) Importing a module does not import the functions into the current symbol table. None. Call the function with the module name in front of the function. import math x = math.sin(1.0) There are no other functions with the same name from a different module or your code. Import the functions from the module into the current symbol table. from math import * # This imports all functions, not just sin() x = sin(1.0) or from math import sin x = sin(1.0) Common Lisp The body of the do loop is executed for the last item of a list (do ((lst lst (cdr lst))) ((endp lst)) (setf x (car lst))) The end condition i.e. (endp lst) is checked before the body is executed The update varibale must be a list Change the do to a dolist (dolist (obj lst) (setf x obj)) C Sometimes causes an error when fgetc() is called again but the file has already ended. int count_line_size( FILE * fp ) { char ch; int cnt = 0; while( (ch = fgetc(fp)) != EOF && ch != '\n') cnt++; return cnt; } The return of fgetc() is cast to a char ch. Now since EOF isn't a character but an integer code when ch is cast back to an int for the EOF test the cast may change the value and the loop wont terminate in time. The programmer wants the result of fgetc() from a file to be stored as a char. Keep ch as int instead of a char as char is subtype of int int count_line_size( FILE * fp ) { int ch; int cnt = 0; // NOTE: while( (ch = fgetc(fp)) != EOF && ((int) ch) != '\n') // is a hidden cast while( (ch = fgetc(fp)) != EOF && ch != '\n') cnt++; return cnt; } C == returns false when comparing 2 equal strings char st1[] = "abc"; char st2[] = "abc"; if ( st1 == st2 ) printf("Yes"); else printf("No"); This prints No == compares the value of st1 and st2 as pointers instead of strings Two strings are being compared Use strcmp(st1,st2) instead of ==, it returns 0 if they equal and 1 if they are not if ( strcmp(st1,st2) == 0 ) printf("Yes"); else printf("No"); Common Lisp, C++ You notice that some variable that you did not mean to change has changed value on its own (setf x "hello world") (setf y x) (setf (aref x 0) #\H) When using a destructive modifier (setf (aref )) x and y are assigned to the smae object so when x is changed, y is changed as well two variables are assigned to a single object and one of them is destructively modified use a copy instead of an assingment to prevent one variable from changing another (setf x "hello world") (setf y "hello world") C++ Your program crashes unexpectedly when the value of a variable is read or you notice that a variable has an unexpected value x= x+1 You are referring to a pointer and you have not allocated enough space to hold the value for the pointer's contents you have a pointer which you are referring to and not enough memory has been allocated allocate enough space before using the pointer x= (char*)malloc(sizeof(char*255)) Bug 3 C++ Your program crashes while in a large loop for (i=0;i<1000;i++) {x=new int;} you are allocating memory and not freeing it which cause a memory leak and a crash once memory runs out you allocate memory without freeing it inside of a loop free the memory after you no longer need it {x=new int; free x;} Common Lisp Returned a function from another function, but upon attempting to call it, error occurs (defun foo (n) (lambda (o) (+ o n))) ;; produces an error ((foo 12) 8) ;; works as intended (funcall (foo 12) 8) When you return a lambda expression, funcall must be used to call the function. Use funcall on the returned function. Lisp-based languages Attempted to copy a list and then alter an element in the copy, but both the copy and original are changed. A statement like (setf foo bar) does not set the "contents" of foo to those of bar, but merely sets foo to point to the same memory that bar points to. There are a number of copy functions built-in to remedy this situation. Consider the following, depending on the data structure in use: copy-list copy-seq copy-structure copy-tree (setf lst1 '(a b c d e)) (setf lst2 lst1) ;should be (setf lst2 (copy-list lst1)) (setf (car lst2) 'm) ;(car lst1) and (car lst2) are now 'm C/C++ Appears as if pointer already points to something when you check it, and then you use a bad pointer. Many times, you can't actually set a pointer to something until some time after you've declared or initialized it. One usually checks to make sure the variable doesn't point to anything before assigning it but checking if it's NULL. However, if a pointer is not explicitly initialized to NULL it will contain a random address which is non-null, thus confusing -- and indeed "breaking" -- the code which assigns the pointer to a valid location in memory. Always initialize pointers to NULL. MyClass::MyClass() { memberPointerVar = NULL; //the repair! } void MyClass::CalledAtSomeLaterTime() { if (memberPointerVar == NULL) memberPointerVar = CreateNew...; } Common Lisp A DO variable that is regularly updated with a conditional returns as nil or 0 when the loop is complete (do ((output nil (unless (null input) (cons output (car input))))) ((null input) output)) The update in place for DO variables must return a value, so if a conditional such as (unless...) or (if...) with no "else" clause is used, and the the current conditions do not satisfy the conditional, the variable will be updated using nil or 0, depending on the type of the variable. If (unless...) is used to update the DO variable. Change the (unless...) to an (if...) with an "else" clause that sets the value of the variable to itself. (do ((output nil (if (not (null input)) (cons output (car input)) output))) ((null input) output)) If (if...then...) is used to update the DO variable, but no "else" is provided. Add an else clause the the (if...) to set the value of the variable to itself. (do ((output nil (if (not (null input)) (cons output (car input)) output))) ((null input) output)) Common Lisp A program returns an "Expected Type" error (defun add (x y) (+ x y)) (add 1 '(2)) [not a terribly complicated example] It is not always good to assume the type of an input. Because Common Lisp does not check for matching types until their values are called, "expected type" errors are particularly harmful in Common Lisp, as the program will not blow up until sometime in the middle of the program. A function has an input that is later used in a type-specific function. Check the type of the input before using it in the function. (defun add (x y) (if (not (or (numberp x) (numberp y))) (error "Number Expected")) (+ x y)) C++ (but not limmited to C++) Unexpected output or an error is received after a loop has been called. int sample_array[10]; for(int counter = 0; counter <= 10; counter++) {}; cout << sample_array[counter]; It is easy to be "one off" in the number or iterations performed in c++ loops. "Is counter a link or is it the post?" A 'for' loop is set to iterate based on a counter indexed at 0. change the loop-continuation condition so that the iteration continues only while the counter is less than the desired number, not less than or equal to. int sample_array[10]; for(int counter = 0; counter < 10; counter++) {}; cout << sample_array[counter]; C Dynamically appending a string using realloc results in no update to the string str = (const char*)realloc(str,len+1)); str[len+1] = c; if char is being set to string[length+1] and realloc resizes the string anything greater than strlen realloc to strlen+2, replace the last character (old null ptr) with the character, add a null at the end. Replace the const char* with a char* str = (char*)realloc(str,len+2)); str[len++] = c; str[len] = 0; if strcat is preferred realloc with 2, but simply concatenate the pointer to the character to the string (since strcat takes a char*) <include "string.h"> str = (char*)realloc(str,len+2)); str = strcat(str, &c); Common Lisp A for loop operates on one or more lists and it either returns an unexpected nil or a list that is not the same size as one of the input lists. (loop for e1 in l1 do (list e1 'value)) (loop for e1 in '(1 2 3) and e2 in '(a b c d) collect (list e1 e2)) there is more than one variable in the loop and just one collect assure the input lists are the same size before using them in the loop. (loop for e1 in '(1 2 3 -1) and e2 in '(a b c d) collect (list e1 e2)) there is a DO and an expected return value afterwards replace the do with collect or finally (loop for e1 in l1 collect (list e1 'value)) there are any number of variables in the loop and there is a 'finally' if possible, get rid of the 'finally'; else make sure 'finally' returns a collection the same size of the input sequence (got rid of any unnecessary finally) (loop for e1 in l1 collect (list e1 'value)) Perl regular expression takes a long time to find last element of a string. if ($string =~ /(.*)$tag/) {return $1;} there is a '.*' before the desired expression or a '(?!.*) after the desired expression reverse the list and search for the first match. return the reverse of the outcome if (reverse($string) =~ /$reverse($tag)(.*)/) {return reverse($1);} Perl You cannot substitute a function by directly refering to it within the standard perl substitute syntax. s/include_$page/&page_content($page)/; the substitute content string will never be used again simply place an '/e' after the substute syntax so it is evaluated as an expression s/include_$page/&page_content($page)/e; the substitute content string may be used again grab the return value for the function in a variable before substituting reverse the list and search for the first match. return the reverse of the outcome $pagecontent = &page_content($page); s/include_$page/&pagecontent/; Common Lisp I want to compare whether or not two strings are equivalent, but using equal returns nil if the strings are capitalized in different ways. (equal "Test" "test") Use string-equal instead of equal if you don't want the test to be case sensitive. (string-equal "Test" "test") C++ I want to prompt the user to input a character, then a string, but receive an empty string. cout << "Type a character: "; cin >> ch; cout << "Type a string: "; cin.getline(str,20); ignore(INT_MAX, '\n'); Input string is read correctly by the program. C++ I want to declare a null string, but receive an error message. String s(); As entered, you are actually declaring a function which returns a string. Instead use String s; A null valued string has been declared. Common Lisp The computer says that two values aren't equal when I know they are. (equal 1.5 (/ 3 2)) equal tests that two items are the same, in both value and type. It must therefore be used with care on some items such as numbers, which can come in many different types and still have the same value. The comparison function is testing numbers, and can be changed. Change equal to =. (= 1.5 (/ 3 2)) The types of the objects can be changed to match. Change the type of one object to that of the other. (equal 1.5 (coerce (/ 3 2) (type-of 1.5))) Common Lisp I assign several values using a let, but the values are not being assigned correctly. (setf x 1) (let ((x 2) (y x)) (print (list (x y)))) When multiple variables are being assigned at once, the let statement maintains the old variable scoping until it's done with all assignments; therefore, an assignment that depends on a prior assignment in the same let will instead use the prior variable's previous value. One assignment depends on a prior assignment in the same let. Change let to let*. (setf x 1) (let* ((x 2) (y x)) (print (list (x y)))) C/C++ I know that valid strings are being sent to stdout to be printed, but nothing shows up to the screen. printf("Hello!"); The computer will only write items from a stream when that stream gets flushed (usually done when it sees a '\n' or the keyword endl). We're in C++, printing to stdout, and it's ok to print a new-line after this output. Send an endl to the stream after the desired output. printf("Hello!"); cout<<endl; We're in C, using a compiler that flushes upon seeing a '\n', and it's ok to print a new-line after this output. Append a '\n' to the output going to the stream. printf("Hello!\n"); The above cases don't apply Call fflush() to flush the output stream. printf("Hello!"); fflush(stdout); C/C++ A parameter is passed into a function that changes its value, but it remains unchanged void assign( int i ) { i = 10; } assign( j ); pass modified parameter by reference void assign( int * i ) { *i = 10; } assign( &j ); C++ return value is corrupt SomeClass& generate_return() { SomeClass to_return; return to_return; } local object is stack allocated return local objects by value SomeClass generate_return() { SomeClass to_return; return( to_return ); } Common Lisp function recursing over a list never ends (defun infinite (list) (cons (process (car list)) (infinite (cdr list)))) list does not end with a non-nil cdr return nil on null list argument (defun infinite (list) (and (not (null list)) (cons (process (car list)) (infinite (cdr list))))) C / C++ I check to see if a variable equals a number, but it always returns TRUE int num = 5; if (num = 10) cout << "This is shouldn't be true!" << endl; none use the == operator in cases of conditionality num = 5; if (num == 10) cout << "This is won't get printed because it's false!" << endl; C A case is hit in a switch statement, but it doesn't stop there int num = 0; switch (num) { case 0: printf ("ZERO\n"); case 1: printf ("ONE\n"); case 2: printf ("TWO\n"); } actually prints out: ZERO ONE TWO not the intended: ZERO none use the BREAK statement after each case to break out of the switch statement int num = 0; switch (num) { case 0: printf ("ZERO\n"); break; case 1: printf ("ONE\n"); break; case 2: printf ("TWO\n"); break; } Common Lisp I use a function to try to find an element in a list, but it doesn't find an element that it should. >> (remove '(a) '((a) (b) (c) (d))) ((A) (B) (C) (D)) Equality condition TEST #'EQUAL This returns true if the elements would print the same way. The default for these functions is to use #'EQL >> (remove '(a) '((a) (b) (c) (d)) :test #'equal) ((B) (C) (D)) Common Lisp I get this error message for a variable I defined in "let". (let (x 0) x) (let ((x 0)) x) Common Lisp I get this error message for no apparent reason even though I think I defined all the variables I am using. (do ((j 1 (1+ j))) (> j 10) (princ j)) You are probably using the lisp syntax incorrectly. In the example given, you should have specified a return value for the end condition (do ((j 1 (1+ j))) ((> j 10) 10) (princ j)) Common Lisp I get error messages when I pass a list of variable names to a function that expects a list of values as its input. (setf *x* 1) (setf *y* 2) (defun foo (lst) (car lst)) (foo '(*x* *y*)) returns *x* instead of 1 Using quote creates a list; however it also cancels the interpretation of the variables as such. Instead, use list. (foo (list *x* *y*)) Common Lisp I INCF a variable that has been set to the CDR of an ASSOC, but only the variable is updated and not the actual number in the original list. (let ((a (list (cons 'x 1)))) (setf b (cdr (assoc 'x a))) (incf b) (print a)) b becomes simply the number one, and does not point to the number one in the list If the original list must have the incrementation performed set b to the ASSOC and incf the CDR of B (let ((a (list (cons 'x 1)))) (setf b (assoc 'x a)) (incf (cdr b)) (print a)) Objective-C Objects are not deallocated when a method completes that allocs an object and returns a result. (NSString *) someMethod() { NSString *result = [[NSString alloc] initWithFormat:@"$i", 1] return result; } after being initted the NSString object has a retain count of 1 and therefore will not be deallocated after the method finishes. you want to use the same init method, want to return the result, but do not want to retain the object Add the object to the autorelease pool. It will be deallocated when you move outside of the scope in which it was created (NSString *)someMethod { NSString *result = [[NSString alloc] initWithFormat:@"$i", 1] [result autorelease] return result; } You don't mind using a slightly different method, want to return the result, but do not want to retain the object Use a method that returns an autoreleased object like stringWithFomat instead of initWithFormat (NSString *) someMethod() { NSString *result = [[NSString alloc] stringWithFormat:@"$i", 1] return result; } perl unexpected results or behavior when making a system call in perl, e.g., instead of getting a single column list of simply the files in the directory one gets a recursive listing of all subdirectories. $myVar = system("ls -1"); print $myVar; ls is already aliased in environmental variables to something like alias ls="ls -R", You do not wish to alter the environmental variables use the full path of the command you wish to call $myVar = system("/bin/ls -1"); print $myVar; you can change the environmental variables (this is not highly recommended as it allows for the pitfall to occur again) remove the alias from shell init scripts $myVar = system("ls -1"); print $myVar; C++ (Open GL) While having open GL render a scene pixel by pixel, a large clump of pixels did not show up on the screen, even though their color values were being calculated and their (x, y) coordinates were being sent to the glVertex2f draw command. (Note: For brevity's sake, I'm skipping some sections of the code because this was a good-sized chunk of a program I wrote for my CS 351 class last quarter. Any skipped lines will be denoted with ...) glBegin(GL_POINTS); if (! textured) { glColor3ub(color,(color-50)%256,(color+100)%256); for (int x_cur = x_start; x_cur< x_end; x_cur++) { glVertex2f(x_cur,y_cur); } ... } else { ... glColor3ub(red_final, green_final, blue_final); glVertex2f(x_cur, y_cur); } glEnd(); Only certain functions can be called between glBegin and glEnd, and weird results (like missing pixels) can occur if other functions are used. only some of the pixels are missing Move the glBegin call so it occurs directly before the glVertex2f call and put the glEnd call directly after it. if (! textured) { glColor3ub(color,(color-50)%256,(color+100)%256); for (int x_cur = x_start; x_cur< x_end; x_cur++) { glBegin(GL_POINTS); glVertex2f(x_cur,y_cur); glEnd(); } ... } else { ... glBegin(GL_POINTS); glColor3ub(red_final, green_final, blue_final); glVertex2f(x_cur, y_cur); glEnd(); } if none of the pixels are being drawn call glFlush and glSwapBuffers after glEnd. (Note: calling glSwapBuffers within a loop can lead to flickering.) glBegin(GL_POINTS); if (! textured) { glColor3ub(color,(color-50)%256,(color+100)%256); for (int x_cur = x_start; x_cur< x_end; x_cur++) { glVertex2f(x_cur,y_cur); } ... } else { ... glColor3ub(red_final, green_final, blue_final); glVertex2f(x_cur, y_cur); } glEnd(); glFlush(); glSwapBuffers(); Common Lisp Every time I try to use read-from-string to parse a list with a comma in it, I get an error saying "comma not inside a backquote". (read-from-string "Hello, how are you today?" 5) (Note: Usually this error comes up when reading in a file, but I thought I would use a simple string instead to make it easier to understand.) Commas are a type of read-macro that cannot be parsed by the basic Lisp read functions. if the commas are a necessary part of the string use read-char to process the string letter by letter (let ((strm (make-string-input-stream "Hello, how are you today?" 5))) (do ((c (read-char strm) (read-char strm)) (s "" (concatenate 'string s (string c)))) ((eql c #\space)) s))) if the commas are not necessary and can be discarded use remove to take all the commas out of the string (read-from-string (remove #\, "Hello, how are you today?") 5) C++ I wrote a function to read in an array of values, but when I call the function and try to access the array it returns, all the values are wrong. They seem to be random gibberish. #include "stdafx.h" #include <iostream> using namespace std; int* get_values(){ int vals[3]; for (int ii = 0; ii< 3; ii++) cin >> vals[ii]; return vals; } int _tmain(int argc, _TCHAR* argv[]){ int* vals = get_values(); for (int ii = 0; ii< 3; ii++) cout << vals[ii] << endl; return 0; } Returning a local variable, unless it was created with new, is always a bad idea because when the function exits, the destructor frees all of the memory that was being used. Since a pointer was returned, the value at the memory location the pointer refers to is saved by the new pointer, but the rest of the values in something like an array can easily be lost. none pass an array into get_values and edit it directly #include "stdafx.h" #include <iostream> using namespace std; void get_values(int* vals) { for (int ii = 0; ii< 3; ii++) cin >> vals[ii]; } int _tmain(int argc, _TCHAR* argv[]){ int vals[3]; get_values(vals); for (int ii = 0; ii< 3; ii++) cout << vals[ii] << endl; return 0; } OpenGL After manipulating a model/the world with commands such as glScalef() it no longer displays or lights correctly. glPushMatrix(); glTranslatef(0, -275, -3); glScalef(50, 50, 15); glDraw3DText(score); glPopMatrix(); Scaling the models cause all of the obect's normal vectors to be incorrect with the new coordinates. glEnable(GL_NORMALIZE) normalizes the vectors prior to manipulation, thus preserving their integrity. Before manipulating the objects, enable normalizing. This will correct the effects of the transformation. glEnable(GL_NORMALIZE); glPushMatrix(); glTranslatef(0, -275, -3); glScalef(50, 50, 15); glDraw3DText(score); glPopMatrix(); N/A Upon running a program with an iterative or recursive loop, there is a memory overflow or the program never ends. (defun iter (lst) (... (iter lst)...) In recursive/iterative functions, there needs to be an exit case that is properly reached in order for the program to stop. Otherwise it will keep looping until broken manually. Using recursion Make sure you are correctly updating the recursing variable (defun iter (lst) (... (iter (cdr lst))...) Using recursion Make sure you have a valid exit case, one that will always be reached (defun iter (lst) (if (null lst) ...) (iter (cdr lst))...) Using iteration In an iterative loop, such as DO or FOR, track the counter variable. Make sure it is getting updated correctly, and not modified inadvertantly. some functions may have side effects you are unaware of. Using recursion It is possible to create a recursive program that simply requires too much memory. If the program is recursing a very large number of times, you might be overflowing the stack. Try another method of computation. C++ While running, the program breaks with either a memory exception or with no explanation at all vector <int> vec; vec[58] = 27; return 0; for (int i = 0; i < vec.length(); i++) { cout<< vec[i]; You are trying to access a variable that is nonexistant or pointing to something other than what you intend. Using std library variables Make sure that the vector/queue/etc... is correctly initialized vec.clear(); vec.resize(100); Using std library variables Make sure that you are neve accessing an illegal index. This can often happen in loops. Remember that the length of a variable is out of its indexing range, arrays are zero-indexed and thus you need to use (length - 1) to avoid stepping out of bounds. Counter/iterating variables often can cause this error. for (int i = 0; i < (vec.length() - 1); i++) { cout<< vec[i]; Lisp Program breaks with message: "unbound variable <var>", yet it appears to be defined. (let ((mid (floor (+ end start) 2)) (current (aref v mid))) ... UNBOUND VARIABLE "mid" DO and LET do not allow you to do this, as what appears proper order in your writing code may not be how the compiler runs Variable in a DO or LET statement cannot be used in the definition or update clause of other variables in the same declaration block. You must use either another DO/LET statement or use Lisp funtions DO* or LET* (let* ((mid (floor (+ end start) 2)) (current (aref v mid))) or (let ((mid (floor (+ end start) 2))) (let ((current (aref v mid))) ... Common Lisp The recursion function runs into a infinite loop (defun flat (lst) (cond ((listp (car lst)) (flat (append (car lst) (cdr lst)))) (t (cons (car lst) (flat (cdr lst))))))) This code runs into an infinite loop because it doesn't test to see if the list lst is empty. No preconditions check to see if the list is empty before continuing. (defun flat (lst) (cond ((null lst) nil) ((listp (car lst)) (flat (append (car lst) (cdr lst)))) (t (cons (car lst) (flat (cdr lst))))))) C++ The FOR loop makes one extra iteration or one less iteration. This usually happens with large number of iterations. //to print 1 through 1000 int some_value = 1000; for (int i = 1 ; i < some_value ; i++) { cout << i; } //OR to print 1 to 999 int some_value = 1000; for (int i = 1 ; i <= some_value ; i++) { cout << i; } The first code prints once less than desired and the second code prints once more than desired. No preconditions Check to make sure when < or <= is more appropriate. //to print 1 through 1000 int some_value = 1000; for (int i = 1 ; i <= some_value ; i++) { cout << i; } //OR to print 1 to 999 int some_value = 1000; for (int i = 1 ; i <= some_value ; i++) { cout << i; } Common Lisp The second variable is an unbounded variable (let ((x 2) (y (+ x 1))) (+ x y)) The second variable has no knowledge of the first variable. Eventhough the first variable was written before the second variable, they are both declared the same time. The above expression is equivalent to ((lambda (x y) (+ x y)) 2 (+ x 1)) No preconditions use let* (let* ((x 1) (y (+ x 1))) (+ x y)) C When I divide two integers and print the result to the screen, I get 0. printf("one-third of 11 is %f\n", 11/3); The division function decides what data type to return by its inputs. When its inputs are both integers, it return arguably the most reasonable integer, floor(x/y). none replace the integer arguments of / with their float counterparts. printf("one-third of 11 is %f\n", 11.0/3.0); Lisp After making a copy of list lst, using DELETE on lst deletes elements from the copy. (setf copy lst) (delete 'a lst) (print copy) setting copy to lst with SETF points the variable copy to the address in memory where lst is stored. Thus, copy is not a copy but another pointer to the same lst. So deleting an element from lst is the same as deleting an element from copy. none use COPY-LIST (setf copy (copy-list lst)) (delete 'a lst (print copy) Lisp After building a circular list, attempting to print the list results in an infinite printout. (setf x '(1)) (setf (cdr x) x) (print x) When print is given a cons cell whose car is an atom, it prints the car and then, if the cdr is not atom, recursively prints the cdr of the cons. Since we have created a circular list, the cdr is never an atom. none Set the global variable *print-circle* to t. (setf *print-circle* t) (print x) Lisp Value of a variable in do loop not updated (defun occurrences (occ-list) ;;Bad Line here (do ((search-list occ-list (if (eq? (cdr search-list) "test") 32)) ;;Rest of do loop Doesn't match the correct do syntax, therefore it is not updated when if evaluates to false. Make sure a default argument is always provided (do ((search-list occ-list (if (eq? (cdr search-list) "test") 32 search-list)) C Prints garbage/crashes char *teststring="test" for(int i=0, i<5, i++){ teststring[i]='f'; } printf("%s", teststring); Program doesn't know when to stop printing-keeps printing garbage until it attempts to access protected memory. Make sure not to overwrite the last 0 in a c-string for(int i=0, i<4, i++) C GPF, illegal memory access when large inputs are used buffer=char[10000]; //buffer is filled with data from input stream... void setmemory(buffer){ allocatedmemory=malloc(sizeof(buffer)); usememory(allocatedmemory); } void usememory(char *inbuffer){ //do something with the memory //fill the buffer with data again, until the data stream is used up.. setmemory(buffer); Program uses up the heap, and attempts to write over protected memory Call free after the memory is used. void usememory(char *inbuffer){ //do something with the memory free(inbuffer); //fill the buffer with data again, until the data stream is used up.. setmemory(buffer); Stack Machine / Assembly in WinScheme leftover results remaining on the stack following a procedure call crash the virtual machine when returned. In this example, if the value is not null, both 1 and 0 will be pushed on the stack and returned, leading to too many results returned. null (link 1) (load-local 0) (jump-if-zero is-null) (push 1) is-null (push 0) (return) If the value is not null return after (push 1) ... (jump-if-zero is-null) (push 1) (return) is-null .... Common Lisp only a single value is modified in each iteration (do ((num (1- (length v)) (1- num)) (lst nil (if (eql k (char v num)) (adjoin (char v (1- num)))))) Multiple values changed in the do iteration If and cond require base cases, otherwise the if just returns a single value (if (eql k (char v num)) (adjoin (char v (1- num))) lst))) Common Lisp Two strings that are visually equivalent are interpreted as incorrect due to case issues. (equal (subseq line 0 5) "<pre>") If subseq will return the same result, only uppercase. replace the equivalent string. (equal (subseq line 0 5) "<PRE>") If the case returned by subseq is unimportant Lowercase the result from subseq, resulting in equal values (equal (string-downcase (subseq line 0 5)) "<pre>") C++ I try to print out a computation with a macro and get the wrong output on the screen. #define A 6 #define B 5 #define C A+B ... cout << "We want 22. We get " << 2*C <<"."<<endl; O/P: We want 22. We get 17. This is a case of the processor/pre-compiler doing what you said and not what you mean. Remember order of operations matter in C++, and the pre-processor does direct text substitution. So, when the substitution is made, order of operations is preserved and the compiler does exactly what you asked for. As a general rule of thumb, you should probably explicitly put parentheses around macro expressions for your desired order of operations. None Explicitly tell the compiler how you would like the macro to be computed with parentheses: Change #define C A+B to #define C (A+B) We want 22. We get 22. lisp I initialize a variable inside a do/let, initialize another variable to it and get error unbound-variable from the debugger. (do ((lst alist (cdr lst)) (key (caar lst)) (val (cadr lst))) ((null lst) ...) ... The scope of the variables does not include the initial values, because DO and LET variables are created in parallel, initialized, then the action begins. None use a LET inside the DO for these variables or just refer to them explicitly (i.e. (car lst)): (do ... (let ((key (car lst)) (val (cdr lst))) ... Perl, C++? I get the wrong output from a ?: expression in Perl $hour=16; ( $hour >= 12 ? $declination = "PM" : $declination = "AM" ); print $declination; O/P: AM This is a case of the compiler doing exactly what you said and not what you wanted. ?: binds tighter than =. So, the ?: is performed and basically you get the equivalent to: (test ? $d="..." : $d="..."). Notice the order of operations. None Explicitly tell the compiler how you would like the macro to be computed with parentheses (in Perl there are too many ways to do things to list all): ($hour >= 12) ? ($declination = "PM") : ("$declination = "AM"); PM None Perhaps a cleaner solution would look like: $d = ($h >= 12) ? "PM" : "AM"; PM C++ The numbers generated by "RAND" are not the same every time I run the program. rand(); The pseudo-random number generator needs to be seeded. Include 'time' header and seed(time(null)) in code. #include <time.h> ... seed(time(null)); rand(); ... C++ I read through a text file, reach EOF, then try to read through it again, but can't seem to read anything at all the second time. while(!in_file.eof()) { ... } in_file.seekg(ios:beg); while(!in_file.eof()) { ... } The status flags need to be reset while(!in_file.eof()) { ... } in_file.seekg(ios:beg); in_file.clear(); while(!in_file.eof()) { ... } C++ When reading binary file, I hit EOF() before the end of the file. in_file.open("filename.txt", ios:in); while(!in_file.eof()) { ... } add binary to the open line in_file.open("filename.txt", ios:in ios:binary); while(!in_file.eof()) { ... } Common Lisp I define multiple variables using let and I get an error when trying to access them. (let ((x 1) (y (+ x 1)) (+ x y))) none use let* in place of let (let* ((x 1) (y (+ x 1)) (+ x y))) none cascade another let within the first let (let ((x 1)) (let ((y (+ x 1))) (+ x y))) C I am using a array and it outputs the wrong numbers when I call it. int numbers[] = { 001, 010, 014 }; cout << numbers[1]; It outputs 8!? The numbers you are defining start with 0's take out the starting zeros (C evaluates these numbers in base 8) int numbers[] = { 1, 10, 14 }; cout << numbers[1]; It outputs 10! C/C++ I have an if statement that I know should execute, but it doesn't. if( 0 < a < 5) cout << a; none seperate the 2 conditions of the if statement using an & if( ( 0 < a ) && ( a < 5) ) cout << a; Common Lisp Passed the LISP bug finder but will loop endlessly if certain input is used (defun printdotsrec (int) (cond ((not (= int 0)) (printdotsrec (1- int)) (format t ".")))) In this code I assumed that the input is a positive number, if a negative number is passed in then the function will loop endlessly. It passed the LISP bug finder but its clearly a bug. I made the assumption that the user will provide valid input do not make assumptions about input and check for negative numbers ((> int 0)