Lecture 4, Tue 10/08

Repetition with loops

Code from Lecture

Control Structures: Loops

while loop

A while loop is used to repeat code while some condition is true.

	while(BOOLEAN_EXPRESSION) {
		// Code to run when the BOOLEAN_EXPRESSION is true.
	}

Check if the BOOLEAN_EXPRESSION is true.

• If true, the statements inside the loop’s block will execute.
• at the end of the loop block, go back to while.
• If false, the statements in the loop will not execute.
• the program execution after the loop continues.

Example usage: say we want users to enter a non-negative number. Use a while loop, so that users must keep entering values until the boolean statement is satisfied

    while(num < 0) {
    	//keep asking user, taking in input
    }

Example

int i = 0;
while (i < 10)
	cout << "i = " << i << endl;
	// add i++ afterwards to eliminate an infinite loop.
	// i++ → i = i + 1;
	// Remember to enclose this statement with { }

• Note that a single statement after a while loop (similar to an if statement) is considered part of the loop without the { }.
  • If more than one statement is part of the loop, this must be contained within the { }

The boolean statement is checked first, and the body is skipped if the requirements are met. Once the bool statement (num < 0) is no longer true, i.e., the user provided a non-negative number, the loop will stop.

The while loop has two flavors: a usual while that we looked at above, and do-while loop, which is used when the body of the loop needs to be executed at least once.

do-while loops

A while loop is used to repeat code until some condition is no longer true

	do {
		// Code
		// This code is executed once
		// and is executed again if the BOOLEAN_EXPRESSION below is true
	} while(BOOLEAN_EXPRESSION);

1. Execute the code in the loop
2. Check if BOOLEAN_EXPRESSION is true.
   • If true, then go back to 1.
   • If false, then exit the loop and resume program execution.

Note: You must have a semicolon (;) after the while statement, if you are using a do-while loop (no semicolon if it is just a while).

In all loops, remember to update the variable controlling the loop each time (examined in bool test), otherwise an infinite loop will occur. After any while loop, we know that the variable being examined meets a particular condition (e.g., num >= 0), otherwise, we would not have exited the loop.

Example

int i = 0;
do {
	cout << "i = " << i << endl;
	i++;
} while (i < 0);

• Outputs “i = 0” once regardless of what the BOOLEAN_EXPRESSION evaluates to.
• Change to while (i < 10) to print “i = [0 - 9]”.

for loops

The while and for loops can be interchanged, but some formats are more convenient than others in different situations.

The for loop is used to repeat code (usually a fixed number of times).

	for (INITIALIZATION; BOOLEAN_EXPRESSION; UPDATE) {
		// Code to run when the BOOLEAN_EXPRESSION is true.
	}

1. Execute the INITIALIZATION statement.
2. Check if the BOOLEAN_EXPRESSION is true.
   • if true, execute code in the loop.
   • THEN execute UPDATE statement.
   • Go back to the BOOLEAN_EXPRESSION.
   • if false, do not execute code in the loop.
   • exit the loop and resume program execution.

Example

for (int i = 0; i < 10; i++) {
	cout << “i = “ << i << endl;
}

Nested Loops

Other loops within a loop can be defined.

Example

for (int i = 0; i < 5; i++) {
	cout << “—“ << endl;
	cout << “i = “ << i << endl;
	for (int j = 0; j < 5; j++) {
		cout << “j = “ << j << endl;
	}
}

Skipping and Terminating Loop Execution

continue

• Once a continue line is reached, go back to the top of the loop immediately

break

• Once a break line is reached, the program will exit (break out of) the loop immediately

In both cases, the program ignores anything else in the block that comes after continue/break, i.e., the rest of that block below the continue/break line is not executed

Example

for (int i = 0; i < 10; i++) {
	if (i == 4)
		continue;
	if (i == 7)
		break;
	cout << “i = “ << i << endl;
}

Formatting output to the terminal

• Several ways to do this.
• We can customize the cout function to display floating point numbers as follows:

cout.setf(ios::fixed);
cout.setf(ios::showpoint);
cout.precision(2); // prints two decimal spaces for floating point values.

Example: A number guessing game

#include <iostream>

using namespace std;

const int ANSWER = 42; // const values cannot be modified

int main(int argc, char *argv[]) {

	int input = 0;

	do {
		cout << "Guess a number between [0 - 100]: ";
		cin >> input;
		if (input == -1)
			break;
		if (input < ANSWER) {
			cout << "Too small" << endl;
			continue;
		}
		if (input > ANSWER) {
			cout << "Too big" << endl;
			continue;
		}
		if (input == ANSWER) {
			cout << "WINNER! ANSWER = " << ANSWER << endl;
			break;
		}
	} while (true);

	cout << "Thanks for playing!" << endl;
}

Math Puzzle One of the powers of computing is being able to do a brute-force search for a solution to a problem. Trial and error works just fine for some problems. In fact, computers can be especially good at such problems. Consider this:

Horses cost $10, pigs cost $3, and rabbits are only $0.50. A farmer buys 100 animals for $100, How many of each animal did he buy?

Write a program to do this.

Acknowledgements: Thanks to Richert Wang for the original version of these notes