if and if-else Statements

Learn about the different conditional statements in C++.

We'll cover the following

This lesson about conditional statements which are used to execute a certain statement or a block of code based on some condition(s).

Selection or conditional statements help create a flow in which we decide which set of statements will execute and which won’t, based on whether the condition is true or false.

The types of selection statements include:

  • The if statement
  • The if-else statement
  • The switch statement

In this lesson, we’ll look at the if and if-else statements.

The if statement (syntax)

The if statement is the same as it is in the English language. For example, if it’s raining, I will use an umbrella, otherwise I will not.

In C++, the if statement is the same. If the condition is true, then the statement inside the if is executed. Otherwise it is not. Let’s see how to use the if statement below:

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if(condition)
   statement; // a statement ;
/* 
In case of associating multiple statements inside an `if`
*/  
if(condition)
{   // Adding a block statement
   s1; // statement#1 or a block of statements
   s2; // statement#2 or a block of statements
   .
   .
}

Working of the if statment.

See the animation below to understand how the if statements works.

Now let’s write code using the if statement.

#include<iostream>
using namespace std;

//example of if statement
int main()
{
    int a;
    cout << "Number: ";
    cin >> a;

    if(a < 30)
    {
        cout << "a is smaller than 30." << endl;
    }
    return 0;
}
Example of if statement

In line 9, we take a number as input from the user in variable a.

In line 11, we check if the given input a is less than 30, and then execute all that is inside the curly brackets.

So if a is indeed less than 30, it prints a is smaller than 30 in line 13. Otherwise, if the condition is false (i.e., a is greater than 30), then it simply runs what is after the if statement, which is return 0 in this case.

Now, what if we had several if conditions that we wanted to apply? In that case, the syntax would be:

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if(condition 1)
   // statement or a block of statements
if(condition 2)
   // statement or a block of statements
if(condition 3)
   // statement or a block of statements

Let’s look at a code example now.

#include<iostream>
using namespace std;

//example of if statement(variant 2)
int main()
{
    int a;
    cout << "Number: ";
    cin >> a;
    
    if(a < 30)
        cout << "a is smaller than 30." << endl;

    if(a > 30)
        cout << "a is greater than 30." << endl;

    if(a == 30)
        cout << "a is equal to 30." << endl;
    
    return 0;
}
Example of several if statements

The code above is an extended version of the previous example. In this example, we are checking whether a is smaller, greater, or equal to 30 using three different if conditions and statements.

If the input a is less than 30, then a is smaller than 30 is printed. If input a is greater than 30, then a is greater than 30 is printed, and if input a is equal to 30, then a is equal to 30 is printed. Depending on the value of a, the respective if statement is printed.

Also, note how we did not write any curly brakcets {} after if conditions, and the code still executed fine.

If we write if statements without the curly brackets {}, only the immediate statement/expression after the if condition is executed. Here, since we only need one statement after if to execute, the curly brakcets {} are not needed. But to execute a block of statements, we would need to use the curly brackets {}.

We do not put a semicolon ; after the if condition. If we put a ;, after the end of if condition, the if condition would do nothing, and no statement would be associated with the if statement. The block statement (the statements inside {...} will be considered as separate independent statements not associated with the if statement.

The if-else statement (syntax)

We can write statements/expressions in the else block that we would like to execute when the if condition is not true.

Let’s see how to use the if-else statement:

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if(condition 1)
{ 
     // statement or a block of statements
}
else
{
    // statement or a block of statements
}

Suppose we had to write a program that would take five two-digit numbers from the user and display them on the console. However, the user could mistakenly enter a one-digit number or a three-digit number.

So we would need a condition to ensure that the user entered only a two-digit number. Only then will the program take the number and display it on the console. So for this situation, we can use an if-else statement. We could add a condition that if the number is a two-digit number, display the number; else, the number entered is not a two-digit number as shown below.

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#include <iostream>
using namespace std;
int main()
{
	int num;
	cin >> num;
	if (num <= 99 && num >= 10)
		cout << num;
	else
		cout << "The number entered is not a 2-digit number." << endl;
	return 0;
}

Enter the input below

How if-else works?

Let’s examine an example from the animation below to understand how it works.

In the animation above, we have a simple case where we print Pass only if the score is greater than or equal to 50 and is less than or equal to max (100). Otherwise, we print Fail in case the score is less than 50.

Now let’s see a code example that uses an if-else statement.

//example of if else statement(variant 1)
#include<iostream>
using namespace std;

int main()
{
    int num;
    cout << "Number: ";
    cin >> num;

    if(num > 0)
        cout << "number is positive." << endl;
    else
        cout << "number is negative" << endl;
    
    return 0;
}
Example of if-else statement

In this example, the if statement is on line 11 with condition (num > 0). If the number entered by the user is greater than 0 then the body of if will be executed otherwise the body of else will be executed.

Let’s see another example of the if-else statement.

//example of if else statement(variant 2)
#include<iostream>
using namespace std;

int main()
{
    int num;
    cout << "Number: ";
    cin >> num;

    if(num > 10)
        cout << "Number is greater than 10." << endl;
    else if(num < 10)
        cout << "Number is less than 10." << endl;
    else if(num == 10)
        cout << "Number is equal to 10." << endl;
    else
        cout << "Invalid input." << endl;

    return 0;
}
Example of multiple if-else statements

In this example, we have three conditions. The user will enter any integer in num. If the input num integer is greater than 10, it will print Number is greater than 10.. If the input num integer is less than 10, it will print Number is less than 10.. Or if the input integer is equal to 10, it will execute the third if part and print Number is equal to 10.. Otherwise, the last else will be executed, which will print Invalid input..