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Python 3 If Statement - With complete source code

Python 3 If Statement - With complete source code

Python If Statement

Applications rarely follow one flow during execution. Applications usually have to make decisions based on which path to take based on some conditions. for example, let's say a user wants to login to the application. The application will have to determine if the authentication is valid then grant access or deny access if the authentication fails.

This is where control structures come in. In this lesson, we will look at how the if statement works in python

Topics to be covered

We will cover the following topics in this lesson.

  • If statement pseudocode
  • If statement syntax
  • Logical operators
  • Simple console application that allows user to select an option then take appropriate action.

If statement pseudocode

The pseudocode for the simplest if statement is as follows

if condition is true then
	execute code
end if

HERE,

  • The above statement only executes the code within the if statement if the condition is true.

Sometimes, we want to execute some code regardless of the condition. This is where the else part comes in.

The following is the pseudocode for the if... else statement.

if condition is true then
		execute code for the true path
	else
		execute code for the false path
end if

HERE,

  • condition is an expression that evaluates to either true or false. The code that comes immediately after the condition is executed if the expression for the condition evaluates to true else the code for the false path is executed.

If statements can also evaluate multiple conditions to decide on the code to execute.

The following is the pseudocode for multiple conditions that need to be evaluated.

if condition_1 is true then
	execute code for condition_1
else if condition_2 is true then
	execute code for condition_2
else
	execute default code if everything else fails
end if

If statement syntax

In this section, we will look at how to implement the if statement in python.

The syntax is as follows

condition_1 = True

if condition_1:
	print('if condition is true')

HERE,

  • condition_1 = True we define a variable called condition_1 and set its value to True. Remember to always capitalize the letter T when writing True because python is case sensitive.
  • if condition_1: is a standard python syntax for defining if conditions. We start with the if keyword followed by a condition that must evaluate to either True or False then we end with a full colon :.
  • print('if condition is true') represents the code that is within the if statement. The code must always be indented under the if condition: block. To end the if statement block, you simply remove the indenting.

Let's now look at the syntax for if... else

condition_1 = False

if condition_1:
    print('condition_1 is true')
else:
    print('condition_1 is false')

HERE,

  • condition_1 = False we define a variable condition_1 and we assign a value of False to it.
  • if condition_1: evaluates our condition.
  • else: we use the else keyword followed by the full colon : to define the else block. The code under else is also indented just like we did if the condition is true. The else keyword is aligned along the same vertical as the if statement.

Let's now look at the syntax for the if statement with multiple conditions.

condition_1 = True
condition_2 = True

if condition_1:
    print('condition_1 is true')
elif condition_2:
    print('condition_2 is true')
else:
    print('all conditions are false')

HERE,

  • if condition_1: evaluates the first condition to see if it is true.
  • elif condition_2: defines the else if condition. We use the keyword elif for else if statement then we end with the full colon ':'.
  • else: defines the block of code that must be executed if all of the conditions evaluate to false.

Executing the above code gives the following results.

condition_1 is true

Notice only one line has been printed even if both conditions are true. This is because once the first condition evaluates to true, python stops evaluating the other condition(s) that we define below the condition that evaluates to true.

Logical and comparison operators

Logical operators allow us to work with multiple conditions in the same expression. Let's suppose that we are working on a loan approval system. We can check the customer's earning potential and credit score to determine whether they qualify or not.

The following are the logical operators that we will deal with

  • and - this logical operator evaluates to true if both conditions are true. if any of the conditions is false then it will evaluate to false. Using our above example, good credit score with no earning potential is bad, good earning potential but bad credit score is also bad. we only need to approval for customers who have both good credit score and earning potential.
  • or - this logical operator evaluates to true if one of the conditions or both are true. for example, let's say we want to reset the password for the user. we can ask them for either the username or email address to determine that the user is legit.

Let's now look at a practical example.

condition_1 = True
condition_2 = True

if condition_1 and condition_2:
    print('duff beer')

HERE,

  • condition_1 = True; condition_2 = True defines two conditions whose values have been set to true.
  • if condition_1 and condition_2: uses the and logical operator to evaluate both conditions at the same time.

Running the above code produces the following results.

duff beer

Let's now modify the code to the following

condition_1 = True
condition_2 = False

if condition_1 and condition_2:
    print('duff beer')

The above code does not print duff beer because one of our conditions is false.

Let's now look at comparison operators, just like the name suggests, they compare things. We will explore the following operators

  • == this operator is used to compared two items like if condition == True. Note: the single equal sign = is used to assign values while the double == is used to compare them.
  • != this is the not equal to operator. We can use it to compare two items and if they are not equal then True is return else False is returned.

The following examples show you how to

Console menu application using if statement

Our application will print out a simple heading and a couple of options with index numbers. The application will then ask the user to enter a number that corrects to the option index then use an if statement to print out the option that the user has selected.

The code is as follows

title = 'Database CRUD Application'
print('*' * len(title))
print(title)
print('*' * len(title))
print('')
print('Select an option by entering the option number\n')

options = """1. List all records
2. Search for a record
3. Create new record
4. Update record
5. Delete record
6. Quit
"""

print(options)

selection = input('Enter your selection: ')

if selection == '1':
    print('You selected "List all records"')

elif selection == '2':
    print('You selected "Search for record"')

elif selection == '3':
    print('You selected "Create new record"')

elif selection == '4':
    print('You selected "Update record"')

elif selection == '5':
    print('You selected "Delete record"')

elif selection == '6':
    print('You selected "Quit Application"')

Running the above code produces the following results

Python If Statement

Let's look at another example that uses the if statement.

We will create a list of numbers from 0 to 10 then we will only print out even numbers from the list.

numbers = list(range(11))

for number in numbers:
    if number % 2 == 0:
        print(number)

HERE,

  • numbers = list(range(11)) generates a list of numbers from 0 to 10 using the range function and then uses the list function to convert the results to a list.
  • for number in numbers: loops through each number in the list and assigns the value to a local for loop variable number
  • if number % 2 == 0: the if statement divides the number by 2 and sees if the remainder is 0 indicating that it is an even number. Notice how we are using the comparison operator == in our expression which is the condition. As you can see, conditions can be simple or complex expressions

Lesson challenge

Modify the above program to print only odd numbers and share your answer in the comments section. You can share even if other students have already shared.

Summary

If statements allow us to control the execution flow of the program. The simplest form of the if statement has only a single block of code which is executed if the statement is true. The next simplest form has an else part that is executed if the condition is false. The elif statement is used to set a condition if the first condition is false.

What next?

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