CS 211:
Introduction to Computer Programming II

	Instructor: Brian M. Dennis
	Teaching Assistants:
	Tom Lechner, Bin Lin
	http://www.cs.northwestern.edu/~bmd/cs211/

Pointers

	int* swap(int* x_ptr, int *y_ptr) {
	int* z_ptr, *z;
	int old_x = *x_ptr;
	*x_ptr = *y_ptr;
	*y_ptr = old_x;
	return y_ptr;
	}
	int main() {
	int x = 10, y = 20;
	swap(&x, &y);
	cout << "x: " << x;
	cout << " y: " << y;
	cout << "\n";
	return 1;
	}

Arrays: access

	#include <iostream>
	using namespace std;
	int fib(int n);
	int main() {
	int fib_vals[10];
	for(int i = 0; i < 10; i++) {
	fib_vals[i] = fib(i);
	}
	int j = 10;
	while(j--) {
	cout << "Fib: " << j << " = "
	<< fib_vals[j] << "\n";
	}
	}

Pointer Initialization

	#include <iostream>
	using namespace std;
	int main() {
	int* int_ptr = 0;
	int_ptr = NULL;
	char *c_ptr = NULL;
	c_ptr = 0;
	char c = 'A';
	char *c_ptr2 = &c;
	c_ptr = c_ptr2;
	// Do some work
	return 0;
	}

Static Cast

	#include <iostream>
	using namespace std;
	int main() {
	int* int_ptr = 0;
	int_ptr = NULL;
	char *c_ptr = NULL;
	c_ptr = 0;
	char c = 'A', *c_ptr2 = &c;
	int_ptr = c_ptr; // Shouldn't compile
	return 0;
	}

C style Cast

	#include <iostream>
	using namespace std;
	int main() {
	int* int_ptr = 0;
	int_ptr = NULL;
	char *c_ptr = NULL;
	c_ptr = 0;
	char c = 'A', *c_ptr2 = &c;
	int_ptr = (int*)c_ptr; // Should compile
	return 0;
	}

Static Cast

	#include <iostream>
	using namespace std;
	int main() {
	int* int_ptr = 0;
	int_ptr = NULL;
	char *c_ptr = NULL;
	c_ptr = 0;
	char c = 'A', *c_ptr2 = &c;
	int_ptr = static_cast<int*>(c_ptr);
	return 0;
	}

Void*

	#include <iostream>
	using namespace std;
	int main() {
	int* int_ptr = 0;
	int_ptr = NULL;
	char *c_ptr = NULL;
	c_ptr = 0;
	char c = 'A', *c_ptr2 = &c;
	void* vptr = c_ptr2;
	int_ptr = (int*)static_cast<void*>(c_ptr);
	return 0;
	}

Array Sizing

	// Array sizes have to be constant expressions
	#include <iostream>
	using namespace std;
	int main() {
	int cnt = 100;
	int a[100], b[cnt];
	cin >> cnt;
	int b[cnt];
	// Do some other stuff
	}

Array Initialization

	// Arrays can be initialized
	#include <iostream>
	using namespace std;
	int main() {
	char *strings[] = {
	"one", "two", "three"};
	for (int i = 0; i < 3; i++) {
	cout << strings[i] << endl;
	}
	int ints[] = { 1, 2, 3, 5, 8 };
	return 0;
	}

Arrays of pointers
& Pointers to pointers

	// Arrays can be initialized
	#include <iostream>
	using namespace std;
	int main() {
	char *strings[] = {
	"one", "two", "three"};
	strings[1] = "Hello, world";
	for (int i = 0; i < 3; i++) {
	cout << strings[i] << endl;
	}
	char **s_ptr = strings;
	char **s_ptr = &strings[0];
	}

The True Main

	// main actually gets command line arguments
	#include <iostream>
	using namespace std;
	int main(int argc, char* argv[]) {
	for (int i = 0; i < argc; i++) {
	cout << argv[i] << endl;
	}
	return 0;
	}

The True Main

	// main actually gets command line arguments
	#include <iostream>
	using namespace std;
	int main(int argc, char* argv[]) {
	char **t_ptr = argv;
	while (t_ptr < argv + argc) {
	for (int i = 0; (*t_ptr)[i]; i++) {
	cout << (*t_ptr)[i];
	}
	cout << endl;
	t_ptr++;
	}
	return 0;
	}

The True Main

	// main actually gets command line arguments
	#include <iostream>
	using namespace std;
	int main(int argc, char* argv[]) {
	char **t_ptr = argv;
	while (t_ptr < argv + argc) {
	char c, *t_string = *t_ptr;
	while (c = *t_string) {
	cout << c;
	t_string++;
	}
	cout << endl;
	t_ptr++;
	}
	return 0;
	}

const declaration

	// nonconstant ptr to nonconstant data
	#include <cctype>
	void convertToUppercase(char *sPtr) {
	while (*sPtr != '\0') {
	if (islower( *sPtr )) {
	*sPtr = toupper( *sPtr );
	}
	++sPtr;
	}
	}

const declaration

	// nonconstant ptr to constant data
	void printCharacters(const char* sPtr) {
	for ( ; *sPtr != '\0'; sPtr++) {
	cout << *sPtr;
	*sPtr = 'A';
	}
	}

const declaration

	// constant ptr to nonconstant data
	int main() {
	int x, y;
	int * const ptr = &x;
	*ptr = 7;
	ptr = &x;
	return 0;
	}

const declaration

	// constant ptr to constant data
	int main() {
	int x = 5, y;
	const int *const ptr = &x;
	*ptr = 7;
	ptr = &y;
	return 0;
	}

Linear Search

	#include <iostream>
	using namespace std;
	int linearSearch(const int a[], int k, int size);
	int main() {
	const int arraySize = 100;
	int a[arraySize];
	int key;
	for (int i = 0; I < arraySize; i++) {
	a[i] = 2 * I;
	}
	cout << "Enter integer search key: ";
	int element = linearSearch(a, key, arraySize);
	if (element != -1) {
	cout << Found value in element: " << element << endl;
	} else {
	cout << "Value not found" << endl;
	}
	return 0;
	}

Linear Search

	int linearSearch(const int array[], int key, int sizeOfArray) {
	for (int j = 0; j < sizeOfArray; j++) {
	if (array[j] == key) {
	return j;
	}
	}
	return -1;
	}
	int linearSearch(const int array[], int key, int sizeOfArray) {
	const int *t_array = array;
	while (t_array < (array + sizeOfArray)) {
	if (*t_array++ == key) {
	return (t_array – array – 1);
	}
	}
	return -1;
	}

Binary Search

	#include <iostream>
	using namespace std;
	int binarySearch(const int a[], int k, int low, int high, int size)
	int main() {
	const int arraySize = 100;
	int a[arraySize];
	int key;
	for (int i = 0; i < arraySize; i++) {
	a[i] = 2 * i;
	}
	cout << "Enter integer search key: ";
	int element = binarySearch(a, key, 0, arraySize – 1, arraySize);
	if (element != -1) {
	cout << Found value in element: " << element << endl;
	} else {
	cout << "Value not found" << endl;
	}
	return 0;
	}

Binary Search

	int binarySearch(const int a[], int searchKey, int low, int high,
	int size)
	{
	int middle;
	while (low <= high) {
	middle = (low + high) / 2;
	if (searchKey == a[middle]) {
	return middle;
	} else if (searchKey < a[middle]) {
	high = middle – 1;
	} else {
	low = middle + 1;
	}
	}
	return -1;
	}

That’s a Wrap

	Takeaways
		Array initialization
		Pointer initialization
		Const
		Pointer arithmetic
		Array / pointer duality
	Reading
		4.5 – 4.9
		5.5 – 5.10
		2.11 – 2.12
		3.10, 3.17