The Iteration Structure Cont.

Let's look at some more examples of the iteration structure. Let's write a program that asks the user for two integers, then prints a list of all the integers between those two:

#include <stdio.h;>

int main () {
	int	i, start, end;

	/* prompt for and read lower and upper limits */

	printf ("Enter two integers: ");
	scanf ("%i %i", &start, &end);	

	/* complain if end < start */

	if (end < start) {
		printf ("No integers between %i and %i!\n", start, end);
		exit (1);
	}

	/* i starts out at 'start' */

	i = start;

	/* continue printing and incrementing i until it reaches 'end' */

	while (i <= end) {
		printf ("%i\n", i);
		i++;
	}
	exit (0);
}

The `for` Control Structure

while is the only iteration structure you'll ever need in C. However, a certain kind of loop that has an initialization, simple comparison in the while, and end-of-loop operation like an increment occurs frequently in programming, so C provides a shorthand in the form of the for control structure. It looks like this:

	for ( expr1; expr2; expr3 )
		stmt;

expr1 is done once, right before the loop begins. expr2 is evaluated at the beginning of the loop; the loop terminates when expr2 becomes false, just like in a while loop. expr3 is evaluated at the end of each iteration of the loop. It is equivalent to the following:

	expr1;
	while ( expr2 ) {
		stmt;
		expr3;
	}

but shorter to write and often easier for other programmers to read.

In the previous program, for example, we can replace the code beginning with i = start with this:

	for (i=start; i<=end; i++)
		printf ("%i\n", i);

This is much shorter and easier to read. Note that you can also use a compound statement with a for, just like with while and if.

Now let's look at a program that asks the user for an integer and then determines whether the integer is a prime number. Recall that a prime number is a positive integer that has exactly two factors: itself and 1. So, for example, 13 is prime because no two numbers, other than 1 and 13, can be multiplied to yield 13. For another example, 15 is not prime since it is the product of 3 and 5 (a number that isn't prime and isn't 1 is said to be composite). One way to determine whether a number n is prime is to see whether it is evenly divided by any of the integers from 2 through n-1; if not, then it is prime. Here's the program:

#include <stdio.h>

int main () {
        int     i, 
		n, 
		prime; /* this will be a truth value */

	/* prompt for and read integer */

	printf ("Enter an integer: ");
	scanf ("%d", &n);

	/* start out assuming the number is prime (1 stands for "true") */

	prime = 1;

	/* run through all the numbers from 2..n-1 */

	for (i=2; i<n; i++) {

		/* if the remainder when n is divided by i is 0,
		 * the i is a factor of n so n can't be prime
		 */

		if (n % i == 0) prime = 0;
	}

	if (prime == 1) 
		printf ("%d is prime\n", n);
	else
		printf ("%d is not prime\n", n);
	exit (0);
}

Nested Loops

Let's extend this work and get a program that prints out the prime numbers between 2 and 100. We'll have to have a loop inside another loop, a concept known as as nested loops. We'll run through all the integers from 2 through 100, testing each for primality. Each time we find a prime number, we'll print it:

#include <stdio.h>

int main () {
        int     i, 
		n, 
		prime; /* this will be a truth value */

	/* let n run through all the numbers from 2..100 */

	for (n=2; n<=100; n++) {

		/* start out assuming n is prime */

		prime = 1;

		/* run through all the numbers from 2..n-1 */

		for (i=2; i<n; i++) {

			/* if the remainder when n is divided by i is 0,
			 * the i is a factor of n so n can't be prime
			 */

			if (n % i == 0) prime = 0;
		}

		/* if we still think that n is prime, print it out */

		if (prime == 1) 
			printf ("%i\n", n);
	}
	exit (0);
}

The Iteration Structure Cont.

The for Control Structure

Nested Loops

The `for` Control Structure