Conditional Statements: `if...else`

Conditional statements are a cruicial component of Python, enabling programs to execute actions based on specified conditions.

One of the fundamental conditional statements is the if...else statement, along with its extended form, the if...elif...else statement. These constructs operate in a manner similar to the use of "if" in human language but utilize Python code to interact with programs. Let's explore some examples below to enhance your understanding of their usage.

Other conditional constructs, such as try, except, and finally statements, are used for error handling. This more advanced topic will not be discussed in this tutorial.

if...else statement

The example below illustrates the fundamental logic of an if...else statment in the simplest format.

if True:    print('The "if" block ran')else:    print('The "else" block ran')

Output:

The "if" block ran

Explanation:

The if keyword is followed by the boolean value True, which constitutes the condition to be evaluated.
In this case, since the condition is always true, the code within the if block will be executed.
Consequently, the program will print The "if" block ran. The code in the else block will never be executed.

Let's modify the code to execute the else block.

if False:    print('The "if" block ran')else:    print('The "else" block ran')

Output:

"else" block ran

Explanation:

In this scenario, the if keyword is followed by the boolean value False, which forms the condition to be evaluated.
The program first proceeds to the if statement and evaluate the condition. Since the condition is always false, so the code within the if block will not be executed.
Instead, the program will proceed to the else block and execute the code within it.
Consequently, the program will print The "else" block ran.

Now that we've grasped the basic logic of the if...else statement, let's explore the integration of comparison operations and conditional statements to enahnce their utility.

Example

# Create a variable xx = 20
# Compare the equality of the x variable and 20if x == 20:    # If the equality comparison evaluates to True, then execute this code    print(x)else:    # If the quality comparison evaluates to False, then execute this code    print("x doesn't equal to 20")

Output:

20

Explanation:

This code is an example of a if...else statement that checks the value of a variable x.

The if keyword is followed by a equality comparison: x == 20. This condition compares the equality beween the variable x and the number 20.
If the condition is true, meaning that x is equal to 20, then the code within the if block is executed. In this case, it prints the value of x.
If the condition is false, indicating that x is not equal to 20, then the code within the else block is executed. This block prints the message "x doesn't equal to 20".
In this example, the if block gets executed and print the value of x.

If we set x to 10, the else block gets executed instead:

# Create a variable xx = 10
# Compare the equality of the x variable and 20if x == 20:    # If the equality comparison evaluates to True, then execute this code    print(x)else:    # If the quality comparison evaluates to False, then execute this code    print("x doesn't equal to 20")

Output:

x doesn't equal to 20

After setting x to 10, the expression x == 20 evaluates to False, promoting the program to execute the else block.

Let's see another example.

# Create variables x and yx = 100y = 10
# Compare whether the value of x is less than or equal to the value of yif x <= y:    # If the value of x is less than or equal to the value of y, then execute this code    print('x <= y')else:    # If the value of x is larger than the value of y, then execute this code    print('x > y')

Output:
x > y

Explanation:

This code is to compare whether the value of x is less than or equal to the value of y.

The if keyword is followed by a condition: x <= y. This condition checks if the value of x is less than or equal to the value of y.
If the condition is true, meaning that x is indeed less than or equal to y, then the code within the if block is executed. In this case, it prints the message "x <= y".
If the condition is false, indicating that x is greater than y, then the code within the else block is executed. This block prints the message "x > y".

For demonstration purposes, the examples above solely utilize the print() function. It's worth noting that within the if...else blocks, a variety of actions beyond printing can be executed.

Example

Use if...else statement to conditionally change the value of a variable.

# Create variables x, y and cx = 100y = 10c = 0
# Compare whether the value of x is less than or equal to the value of yif x <= y:    # If the value of x is less than or equal to the value of y, then execute this code    c = c + 1else:    # If the value of x is larger than the value of y, then execute this code    c = c - 1
print("c changes from 0 to", c)

Output:

c changes from 0 to -1

Explanation:

This code sets three variables, x, y, and c, to specific values and then uses an if...else statement to conditionally change the variable c based on the comparison of x and y.

The if condition checks if the value of x is less than or equal to the value of y.
If the condition is true, meaning that y is indeed less than or equal to y, then the code within the if block is executed. In this case, it adds 1 to the value of c, effectively incrementing it by 1.
If the condition is false, indicating that x is greater than y, then the code within the else block is executed. This block subtracts 1 from the value of c, effectively decrementing it by 1.

So, based on the values assigned to x and y in this code, it substracts c by 1 because x (100) is greater than y (10). Therefore, after this code snippet, the value of c becomes -1.

`if...elif...else` statement

The if...elif...else statement expands upon the basic if...else by adding more conditional layers. You can include multiple elif statements for additional conditions. Let's explore how this structure operates.

Example

# Create variables x and yx = 100y = 100
# Compare whether x is less than yif x < y:    # If x is less than y, then execute this code    print('x < y')# Compare whether x is equal to yelif x == y:    # If x is euqal to y, then execute this code    print('x = y')else:    # If x is larger than y, then execute this code    print('x > y')

Output:
x = y

In the example, we divide x <= y to two distinct condtions x < y and x == y, enabling the program to respond to different scenarios.

The example below uses both comparison and logical operations in the conditional statements, creating a more complex setup. Can you deduce the conditions covered by the else statement based on preceeding if and elif statements?

Example

# Create variables x and yx = 10y = 10
# Check if both x and y are less than 10.if x < 10 and y < 10:    c = x + y# Check if x is less than 10 and y is greater than 10.elif x < 10 and y > 10:    c = x * y# Check if x is greater than 10 and y is less than 10.elif x > 10 and y < 10:    c = y / x# what condition is included in the else statement?else:    c = 0
# Print variable cprint(c)

Output:

0

Explanation:

The code starts with an if statement with the condition x < 10 and y < 10. This condition checks if both x and y are less than 10.
- If this condition is true, it adds x and y together and assigns the result to c. In this case, c becomes 20.
- If the condition is false, it moves on to the next elif (else if) statement.
The next elif statement has the condition x < 10 and y > 10, which checks if x is less than 10 and y is greater than 10.
- If this condition is true, it multiplies x and y together and assigns the result to c. In this case, c becomes 100.
- If the condition is false, it moves on to the next elif statement.
The final elif statement has the condition x > 10 and y < 10, which checks if x is greater than 10 and y is less than 10.
- If this condition is true, it divides y by x and assigns the result to c. In this case, c becomes 1.
- If the condition is false, it falls into the else block.
The else block runs when none of the preceding conditions are met, occurring when both x and y are greater than or equal to 10. In this scenario, it sets c to 0.

Recap

Conditional statements are a powerful tool that enable programs to operate differently under various conditions. We'll explore more use cases as we delve into additional topics in Python.

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Conditional Statements: if...else

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Recap

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Conditional Statements: `if...else`

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