If Statements

Section 1.2 If Statements

One of the most basic things in programming is the if statement. This is a block of code that runs "if" something is true. This could be a mathematical statement like "if x is bigger than 5" or it would be something like "if the list contains apples." If the condition is false, the code simply doesn’t run. We will learn how to use if statements in this section.

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Subsection 1.2.1 Checking Equality

So far, we have been using the equals sign (=) to set a variable to a value. But, to check if two things are equal in python, we use a double equals sign (==). This is common in many programming languages.

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Below, we set the value of x to be 5. Then we check if x is equal to 5. If it is, we add 1 to x. Finally, we print the value of x. Python uses indentation to show what code is inside the if statement. The line that is indented after the if statement is the only line that will run if the condition is true.

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What if instead we checked if x was equal to 7? Let’s adjust the code and see what happens.

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This time, the statement we were checking was false, so the code indented after the if statement did not run. The values of x is still 5. We did not add 1 to it.

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Subsection 1.2.2 Checking Inequality

It is also possible to check if two things are not equal. This is done with the "not equal" operator (!=). Let’s check if x is not equal to 7, and then add 1 if that is the case.

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Since x is not 7, the code ran and added 1 to x. The value of x is now 6.

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When you read inequality, you probably thought about "greater than" or "less than or equal." Those are also possible in if statements. The symbols you would use are:

Greater than: >

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Less than: <

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Greater than or equal to: >=

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Less than or equal to: <=

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In the code below, we check if x is less than or equal to 10. If it is, we multiply x by 2.

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If x were 15, the code above would not have multiplied x by 2, and the value of x would have stayed 15.

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Subsection 1.2.3 Else Statements

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Since x is not greater than 5, the if condition is false, and we do not add 10 to x. But, since if was false, the else statement runs, and x is multiplied by 4. The value of x is now 8.

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Exercises 1.2.4 Exercises

1.

Set a variable n equal to 20. Check if n is less than 100, and if it is, add 50 to n. Finally, print n. What is the value of n?

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Solution.

The value of n is 70 because 20 is less than 100, so we added 50 to it.

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n = 20
if n < 100:
    n = n + 50
print(n)

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2.

Set a variable k equal to 95. Check if k is greater than or equal to 100. If it is, subtract 20 from k. Otherwise, subtract 5 from k. Finally, print k. What is the value of k?

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Hint.

You will need to use an else statement.

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Solution.

k = 95 if k >= 100: k = k - 20 else: k = k - 5 print(k)

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The value of k is 90 because 95 is not greater than or equal to 100, so the if statement does not run. Instead, the else statement is applied. So, 5 is subtracted from k.

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3.

Set a variable equal to 6, check if it is less than 12, and if it is, take it to the power of 3. What is the value of your variable?

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Hint 1.

You will need to name your variable first.

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Hint 2.

Remember that to take something to a power in python, you use **.

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Solution.

We will call our variable a. You could name it something else if you want. a = 6 if a < 12: a = a ** 3 print(a)

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