Decision Structures

In this lesson, we will learn what are and how to use the relational operators and the if decision structure, addressing the following topics:

• Relational operators
• True or False
• IF decision structure

Relational operators

Relational operators are used to determine whether a relationship between two values is true or false.

Examples:

• ---> true
• ---> false

The main operators that will be used in C are represented in the table below:

Operator	Relationship
<	Less than
>	Greater than
<=	Less than or equal to
>=	Greater than or equal to
==	Equal to
!=	Different

Observations:

• The relation is always applied to compare the value on the left of the operator with the value on the right, in that order.
• Note that the relation of equality is expressed by an operator with two equal signs. This occurs because a single equal sign is used to assign values, as seen in past lessons.

True or False

In programming in general, the values of false and true are assigned to numbers. That is, if a certain comparison is true, then it will be worth . Otherwise, it will be worth .

Knowing this, we can print the result of a comparison using the printf command:

printf("%d\n", 1 < 2);
printf("%d\n", 3 > 4);
/* Output:
1
0

*/

Notice that the output of the first command was , because the comparison is true.

The output of the second command was , because the comparison is false.

Decision structure IF

We can apply the relational operators above to create a decision structure based on the veracity of a statement. That is, if such a statement is true, execute a certain code.

This structure is the if, and does exactly what was said above. Its syntax is as follows:

if( condition ){
    Code
}

Now let's explain what this statement means.

When the program reaches this structure, it will check if the statement/condition contained within the parentheses is true. If it is, then the code contained within the braces will be executed. Otherwise, this code will be ignored and the program will follow its execution after the braces.

Example:

if( 1 < 2 ){
    printf("1 less than 2\n");
}
/* Output:
1 less than 2

*/

Note that since the comparison is true, then the printf command was executed.

Observation: Notice that the comparison used resulted in , because it is true. In C, any value other than will be considered as true. For a better understanding, see the following code:

if(3){
    printf("Value different from 0\n");
}
/* Output:
Value different from 0

*/

As is different from , then it is considered as true, allowing the execution of the prinft command.

Taking into account all the knowledge obtained so far, we can create a program that reads a certain value and tells us if it is even or odd.

Before you continue reading, we recommend that you try to perform this task by yourself. In case you don't succeed, see below a way to do it:

#include <stdio.h>

int main(){
    int x;

    scanf("%d", &x);

    if(x % 2 == 0){
        printf("%d is even!\n", x);
    }

    if(x % 2 != 0){
        printf("%d is odd!\n", x);
    }
}

Explanation:

For a value to be even, the rest of its division by must be equal to , and that is exactly what the code above does.

It starts by declaring the variable , reads it with scanf and checks if , i.e. checks if the remainder of the division of by is .

If this statement is true, the first printf will be executed.

Regardless of whether the condition in the first if is true or false, the second if will also be checked.

If has a remainder other than in the split for , then the second printf will be executed.

See you in the next class!