3.1. If-statements#

Let’s develop a decision-making program! We want to develop a program that prompts the user to enter their age. If it was below the legal age to work in Ontario, Canada, the program prints “You are not yet eligible to work in Ontario.”, else it prints “You are eligible to work in Ontario.”

In C, we use the if statement to make decisions. The if statement is a conditional statement that executes a block of code if a condition is true. The syntax of the if statement is as follows:

if (condition) {
  // code to execute if condition is true
}

If we want to execute another block of code if the condition is false. We can have an else to the if-statement. The syntax of the if-else statement is as follows:

if (condition) {
  // code to execute if condition is true
} else {
  // code to execute if condition is false
}

3.1.1. What can this condition be?#

  1. The condition can be a bool variable. Recall bool variable takes either a true or false value. Note: if the bool variable is false, the true block of code will never be executed. If the bool variable is true, the false block of code will never be executed.

    Code 

     
 #include <stdbool.h>
#include <stdio.h>
 
    
int main(void) {
  bool flag = true;
  if (flag) {
    printf("The flag is true.");
  } else {
    printf("The flag is false.");
  }
  return 0;
}

  2. The condition can be a numerical value. Recall that true is stored as 1, and false is stored as 0 as we discussed in Boolean section. To be more accurate, C is only strict in the representing false as 0. While true can be any non-zero number. In other words, any non-zero value in the condition makes the condition true. While a zero value in the condition makes the condition false.

    Code

     
 #include <stdio.h>
 
    
int main(void) {
  if (3) {
    printf("The condition is true.");
  } else {
    printf("The condition is false.");
  }
  return 0;
}

  3. The condition can be a “relational expression” that evaluates to true or false. Relational expressions have relational operators summarized in the table below.

    Relational Operator

    Meaning

    ==

    Equal to

    !=

    Not equal to

    <

    Less than

    >

    Greater than

    <=

    Less than or equal to

    >=

    Greater than or equal to

    For example, the code below is a program that prompts the user if they are eligible to work in Ontario, based on their age. Download eligible-age.c to get the following code.

    Code

     
 #include <stdio.h>
int main(void) {
  int age = 0;
  printf("Enter your age: ");
  scanf("%d", &age);
  
    
  if (age < 14) {  // Condition checking if age is less than 14
    printf("You are not yet eligible to work in Ontario.");
  } else {
    printf("You are eligible to work in Ontario.");
  }
  return 0;
}

    Another example, let’s write a program in C that identifies if a shape is rectangle or square based on the two sides given by the user. Download square-rectangle.c to get the following code.

    Code

     
 #include <stdio.h>
int main(void) {
  int height = 0, width = 0;
  printf("Please enter the height and width of your shape: ");
  scanf("%d %d", &height, &width);
 
    
  if (height == width) {
    printf("The shape is a square.");
  } else {
    printf("The shape is a rectangle.");
  }
  return 0;
}

    Equal to \(==\) Vs. Assignment \(=\)

    One of the most common mistakes is that people confuse the relational operator == with the assignment operator =. The relational operator == compares the right hand side with the left hand side and returns true if they are equal and false otherwise. The assignment operator = assigns the value on the right hand side to the variable on the left hand side.

    What would happen if you got confused and wrote if (x = 5) instead of if(x == 5), for example?

    In if (x = 5), 5 is assigned to x, which returns 5 (recall Assignment operators). The condition here will always be true, since the numerical value in place of the condition is 5. This is not your intention indeed. Your intention is to check if x is equal to 5. To do this, you should write if (x == 5).

3.1.2. What can we do with relational operators?#

Using relational operators, we can:

  1. Compare the values of int and double variables, e.g. (3 >= 2) or (7.2 > 5.1) or (-3.2 <= 1),

  2. Mix arithmetic and relational operators, where arithmetic operations have higher precedence, e.g., in (x + 2 == 5), x + 2 is evaluated first and compared with 5 to see if they are equal, and

  3. Compare the values of two char variables, where the ASCII codes of the characters are compared, e.g., 'a' < 'b' \(\rightarrow\) true since the ASCII code of 'a' is lower than 'b'.

  4. Compare the values of char and int values, e.g., ('0' == 0) \(\rightarrow\) false since '0' has an ASCII code of 48, which is not equal to 0.

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