CS 211:
Introduction to Computer Programming II

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

Pass by value

	void swap(int x, int y) {
	int old_x = x;
	x = y;
	y = old_x;
	}
	int main() {
	int x = 10, y = 20;
	swap(x, y);
	cout << "x: " << x;
	cout << " y: " << y;
	cout << "\n";
	return 1;
	}

Pass by reference

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

Pass by value reference

	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;
	}

Slide 5

Memory

	Array of bytes
	Byte == 8 bits
		Often written in Hex, base 16
	Every variable
		Some storage in memory
		A type
		A size
			ints = 4 bytes
			floats = 4 bytes
			doubles = 8 bytes
			char = 1 byte
			pointer (of any type) = 4 bytes

Increment shorthands

	int main() {
	int x = 100;
	int z = 3;
	x = x + 1;
	// The following only work on lvalues
	x += 1; // Add 1 to x
	x += z, x += z * z; // Add 3, then add 9
	x++; // Increment x, ret oldx
	++x; // Increment x, ret newx
	}

for statement

	int fib_pure_iter(int n) {
	int a = 1;
	int b = 0;
	int old_a = 1;
	for (int i = 0; i < n; i += 1)
	{
	old_a = a;
	a = a + b;
	b = old_a;
	}
	return b;
	}

for statement

	int fib_pure_iter(int n) {
	int a = 1;
	int b = 0;
	int old_a = 1;
	for (int i = 0; i < n; i++)
	{
	old_a = a;
	a = a + b;
	b = old_a;
	}
	return b;
	}

Decrement shorthands

	int main() {
	int x = 100;
	int z = 4;
	x = x - 1;
	// The following only work on lvalues
	x -= 1; // Subtract 1 from x
	x -= z, x -= z * z; // Sub 4, then sub 16
	x--; // Decrement x, ret oldx
	--x; // Decrement x, ret newx
	}

while statement

	int fib_pure_iter(int n) {
	int a = 1;
	int b = 0;
	int old_a = 1;
	while (0 < n--) {
	old_a = a;
	a = a + b;
	b = old_a;
	}
	return b;
	}

Arrays

	// Arrays: homogeneous, sequential,
	// indexed, contiguous storage, zero based
	int main() {
	int int_array[10]; // Array of 10 integers
	// 40 bytes of storage
	double double_array[5]; // Array of 5 doubles
	// 20 bytes of storage
	char char_array[10]; // Array of 10 chars
	// 10 bytes of storage
	}

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";
	}
	}

Arrays: access

	#include <iostream>
	using namespace std;
	int main() {
	char hw = "Hello, World";
	for (int j = 0; hw[j]; j++) {
	cout << hw[j];
	}
	cout << "\n";
	for (int k = 12; 0 <= k; k--) {
	cout << hw[k];
	}
	cut << "\n";
	}

Help us help you

	Bring the source code
	Trim to smallest broken example
	What are you trying to get done?
	How is your code supposed to be doing it?
	How did you run your code?
	What did you expect?
	What did you get?

That’s a Wrap

	Takeaway
		Pointer basics
		Array basics
	Reading
		4.1 – 4.4
		5.1 – 5.4