Tutorials - Introduction to C - Control Structures

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OldSite

Member since
October 11, 2006

There are a few control structures in C that you can make use of, very similar in syntax to those in Perl and PHP.

If/Else

These allow you to execute code IF a boolean expression is true:

if(1 == 1) {
    printf("1 is equal to 1");
}
else {
    printf("1 is not equal to 1");
}

This speaks for itself really. If 1 is equal to 1, print out so, if not, print out that it isn’t. Obviously 1 will always be equal to 1, so the printf() statement between the curly braces after the if() function will be executed. If 1 wasn’t equal to 1, then the else would be executed.

#include <stdio.h>

int main() {
    int number;
    printf("Enter a number:");
    scanf("%d", &number);
    if(number == 1) {
        printf("The number you entered is 1.");
    }
    else if(number == 2) {
        printf("The number you entered is 2.");
    }
    else if(number == 3) {
        printf("The number you entered is 3.");
    }
    else if(number == 4) {
        printf("The number you entered is 4.");
    }
    else {
        printf("The number you entered is not 1, 2, 3 or 4.");
    }
}

It’s pretty easy to see what’s going on here, these functions allow is to test if a boolen expression, or series of boolean expressions are true, and execute code depending on those tests.

Boolen Expressions

a == b     /* is a equal to b */
a != b     /* is a not equal to b */
a     /* is a true */
!b     /* is a not true */
a > b     /* is a greater than b */
a < b     /* is a less than b */
a => b     /* is a greater than or equal to b */
a =< b     /* is a less than or equal to b */
a && b     /* a AND b */
a || b     /* a OR b */

Any expression that returns TRUE will cause a conditional statement to execute the code contained within its brackets. The computer sees a boolean expression as either true or false. 0 is false, and anything else is true.

While

The while function is a loop, while the expression is true it will continue to execute the code in its brackets.

int a = 0;

while(a < 10) {
    print("%dn", a);
    a++; /* add one to a */
}

This simply means, while the expression is true, execute the code between the curly brackets. The while function checks the expression, if it’s true it executes the code then jumps back and tests the expression again, looping until it checks and finds the expression is false, then it continues executing any code after the while() function. In the example above it keeps checking and adding one to a each time, when a reaches 10, it is no longer less than 10, as stated in the expression, so it will discontinue its loop. So the example will print out the numbers 0 to 9.

We can also do:

int a = 0;

do (
    print("%dn", a);
    a++; /* add one to a */
}
while(a < 10);

Which is the same thing, do the code between the curly braces while the expression in the while() statement is true. If the statement is never true, the code in the braces will not be executed at all. Remember when checking for equality in a boolean expression, don’t confuse a = b with a == b. = is the operator we use to assign a value to a variable, so if we use a single = it will assign b to a and then check whether a is true or not (effectively checking if b is true).

For

We could do the same as we did in the while() examples with for(). For() allows us to include everything in one line.

for(a = 0; a < 10; a++) {
    print("%dn", a);
}

This is neater, it allows us to initialise the variable with a value, then test the expression, then perform an action on the variable. The code will execute while the expression is true, and if it is true, it performs the action and then executes the code between the braces. So we initialise a at zero, test if it’s les than 10, if it is increment it, print out the value in a, and then go back and check the expression again, until it’s no longer true. For() allows more:

for(a = 0, b = 0; a < 10 && b < 20; a++, b = b + 2) {
    print("a is %d and b is %dn", a, b);
}

Seperating them with commas, we can include multiple initialisation variables and perform multiple actions, we cannot use a comma in the boolen expression, although we can test multiple conditions by using the AND (&&) sign. This will initialise both variables at zero, check to see whether a is less than 10 AND b is less than 20, while this expression is true execute. When the expression is false discontinue the loop, effectively counting from 0 – 9 with a, and counting up in two’s with b.

Remember, although you initialise the variables with a value in for(), you still have to declare them first. Have a play around with these functions, get used to them, and when you’re ready read on…

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