if... else

Definition

The if statement is used to execute a block of code when a condition is true. It is possible to add an else to the if statement to execute a different block of code when this condition is not true.

Syntax

if

A simple if statement has the following syntax:

if ( condition ) {
    block1;
}

if ( condition ) {
    block1;
}

if condition:
    block1

Note: Python uses indentation instead of braces { } to structure blocks of code.

if ... else

We can add an else and get a full if ... else statement:

if ( condition ) {
    block1;
} else {
    block2;
}

if ( condition ) {
    block1;
} else {
    block2;
}

if condition:
    block1
else:
    block2

The condition is an expression that evaluates to either true or false.
The block1 is a block of code that gets executed when the condition evaluates to true.
The block2 is a block of code that gets executed when the condition evaluates to false.

Note: With this structure, the code executed is always either block1 or block2. Both blocks cannot be executed at the same time.

if ... else if ... else

When the original condition is false and we reach the else, we can check for another condition using the else if:

if ( condition ) {
    block1;
} else if ( condition2 ) {
    block2;
} else {
    block3;
}

if ( condition ) {
    block1;
} else if ( condition2 ) {
    block2;
} else {
    block3;
}

if condition:
    block1
elif condition2:
    block2
else:
    block3

Note: The python keyword 'elif ' is short for 'else if'.

The condition is an expression that evaluates to either true or false.
The block1 is a block of code that gets executed when the condition evaluates to true.
The block2 is a block of code that gets executed when the condition evaluates to false and condition2 evaluates to true.
The block3 is a block of code that gets executed when both condition and condition2 evaluate to false.

Here's a video explaining this example

The if ... else if ... else is a useful construct when there are more than 2 cases to the outcome. For example, we can use a simple if ... else to check the result of flipping a coin, because the result will either be heads or tails so 2 possibilities. This would result in the following code:

if ( coin == heads ) {
    message = "You got heads";
} else {
    message = "You got tails";
}

if ( coin == heads ) {
    message = "You got heads";
} else {
    message = "You got tails";
}

if coin == heads:
    message = 'You got heads'
else:
    message = 'You got tails'

However, if you wanted to check the results of rolling a dice, which has 6 possibilities, a simple if .. else statement will not be sufficient. In this case, the if ... else if ... else construct will be handy:

if ( side == one ) {
    message = "You scored 1 point";
} else if ( side == two ) {
    message = "You scored 2 points";
} else if ( side == three ) {
    message = "You scored 3 points";
} else if ( side == four ) {
    message = "You scored 4 points";
} else if ( side == five ) {
    message = "You scored 5 points";
} else {
    message = "You scored 6 points";
}

if ( side == one ) {
    message = "You scored 1 point";
} else if ( side == two ) {
    message = "You scored 2 points";
} else if ( side == three ) {
    message = "You scored 3 points";
} else if ( side == four ) {
    message = "You scored 4 points";
} else if ( side == five ) {
    message = "You scored 5 points";
} else {
    message = "You scored 6 points";
}

if side == one:
    message = 'You scored 1 point'
elif side == two:
    message = 'You scored 2 points'
elif side == three:
    message = 'You scored 3 points'
elif side == four:
    message = 'You scored 4 points'
elif side == five:
    message = 'You scored 5 points'
else:
    message = 'You scored 6 points'

Note: There is no need to have an if for the sixth option side == six since this is the only possibility if none of the previous conditions are true

Examples

Example 1: Ticket Price

You want to check the ticket price the user should pay. The rule to determine the price is as follows:

Free for anyone up to the age of 12
£10 for above 12, up to the age of 16
£20 for above 16

Let's create the code for this, one step at a time.

Note: there are many solutions to this problem and it is important that you write code that you feel comfortable with and that you tackle the problem using a logic that feels natural to you.

Note: We will declare the variables as we start using them in the code.

Entry is Free for anyone up to the age of 12 age will be the variable representing the user's age, and will contain a number. entry will be the variable representing the entry price, and will contain a string.

var age, entry;
if ( age <= 12 ) {
    entry = "Free";
}

int age;
String entry;
if ( age <= 12 ) {
    entry = "Free";
}

age = 2
if  age <= 12:
    entry = 'Free'

If the user is above 12, then the entry will be £10 if they are up to the age of 16.

var age, entry;

if ( age <= 12 ) {
    entry = "Free";
} else if ( age <= 16 ) {
    entry = "£10";
}

int age;
String entry;

if ( age <= 12 ) {
    entry = "Free";
} else if ( age <= 16 ) {
    entry = "£10";
}

age = 2
if  age <= 12:
    entry = 'Free'
elif age <= 16:
    entry = '£10'

Then if they are above the age of 16 (i.e. neither 12 or under nor 16 or under), we know that they are above 16 and must pay £20.

var age, entry;

if ( age <= 12 ) {
    entry = "Free";
} else if ( age <= 16 ) {
    entry = "£10";
} else {
    entry = "£20";
}

int age;
String entry;

if ( age <= 12 ) {
    entry = "Free";
} else if ( age <= 16 ) {
    entry = "£10";
} else {
    entry = "£20";
}

age = 2
if  age <= 12:
    entry = 'Free'
elif age <= 16:
    entry = '£10'
else:
    entry = '£20'

View this example in the Code Visualiser. You may want to animate this code also using the control panel on the right handside.

Example 2: Module Outcome

To pass the module, a student needs a minimum module average mark of 40% and also meet the minimum qualifying mark for the various module components that have one.
Anyone that gets below 40% but above 30% or above 30% and doesn't meet any of the qualifying mark will get a referral in the module components for which they have not achieved a mark of at least 40.
Anyone with an average mark below 40% has failed the module.

Note: there are many solutions to this problem and it is important that you write code that you feel comfortable with and that you tackle the problem using a logic that feels natural to you.

Note: We will declare the variables as we start using them in the code.

All students with an average mark above 40% may have passed the module. average will be the variable representing the module average, and will contain a number. outcome will be the variable representing the module outcome, and will contain a string.

var average = 0,
    outcome = "";

if ( average >= 40 ) {
    outcome = "Pass";
}

double average = 0;
String outcome = "";

if ( average >= 40 ) {
	outcome = "Pass";
}

average = 0
outcome = ''

if average >= 40:
    outcome = 'Pass'

Let's consider that the module has a coursework and an exam, and that only the exam has a qualifying mark of 30%. The exam variable will represent the exam mark, and will contain a number. To pass the module, the student needs an average mark of at least 40% (already in place) but also an exam mark above 30. If not, they get a referral. We can do this inside another if...else block:

var average = 0,
    exam = 0,
    outcome = "";

if ( average >= 40 ) {
	if ( exam >= 30 ) {
		outcome = "Pass";
    } else {
		outcome = "Referral";
    }
}

double average = 0;
int exam = 0;
String outcome = "";

if ( average >= 40 ) {
	if ( exam >= 30 ) {
		outcome = "Pass";
	} else {
		outcome = "Referral";
    }
}

average = 0
exam = 0
outcome = ''

if average >= 40:
    if exam >= 30:
	    outcome = 'Pass'
    else:
        outcome = 'Referral'

We'll now add an else to the original if so that anyone with an average above 30% will get a referral:

var average = 0,
    exam = 0,
    outcome = "";

if ( average >= 40 ) {
	if ( exam >= 30 ) {
		outcome = "Pass";
    } else {
    	outcome = "Referral";
    }
} else if ( average >= 30 ) {
	outcome = "Referral";
}

double average = 0;
int exam = 0;
String outcome = "";

if ( average >= 40 ) {
	if ( exam >= 30 ) {
		outcome = "Pass";
    } else {
    	outcome = "Referral";
    }
} else if ( average >= 30 ) {
	outcome = "Referral";
}

average = 0
exam = 0
outcome = ''

if average >= 40:
    if exam >= 30:
        outcome = 'Pass'
    else:
        outcome = 'Referral'
elif average >= 30:
    outcome = 'Referral'

Finally, anyone students that don't get an average above 40% or an average above 30% get a fail in the module:

var average = 0,
    exam = 0,
    outcome = "";

if ( average >= 40 ) {
	if ( exam >= 30 ) {
		outcome = "Pass";
    } else {
    	outcome = "Referral";
    }
} else if ( average >= 30 ) {
	outcome = "Referral";
} else {
	outcome = "Fail";
}

double average = 0;
int exam = 0;
String outcome = "";

if ( average >= 40 ) {
	if ( exam >= 30 ) {
		outcome = "Pass";
    } else {
    	outcome = "Referral";
    }
} else if ( average >= 30 ) {
	outcome = "Referral";
} else {
	outcome = "Fail";
}

average = 0
exam = 0
outcome = ''

if average >= 40:
    if exam >= 30:
	    outcome = 'Pass'
    else:
	    outcome = 'Referral'
elif average >= 30:
    outcome = 'Referral'
else:
    outcome = 'Fail'

if else

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