Conditional Statement#

There comes situations in real life when we need to make some decisions and based on these decisions, we decide what should we do next. Similar situations arise in programming also where we need to make some decisions and based on these decisions we will execute the next block of code, or not.

Decision-making statements in programming languages decide the direction of the flow of program execution. These decision-making statements include:

  • functions

  • conditional statements, and

  • loops

Control flow refers to the order in which individual statements, instructions, or function calls are executed or evaluated. We start to learn control flow by diving into the conditional statements, i.e., if-else conditions.

The conditional statement checks to see if a statement is True or False. So, executing a conditional statement outputs a Boolean.

Here is a simple example:

a = 62
b = 71
print(a == b)

False

Note

  • “=” is the assignment operator.

  • “==” is a comparison operator.

Comparison operators are used to form conditions. Python supports the usual logical conditions from mathematics:

Symbol

Meaning

Example

Result

==

Equal to

a == b

False

!=

Not equal to

a != b

True

>

Greater than

a > b

False

<

Less than

a < b

True

>=

Greater than or equal to

a >= b

False

<=

Less than or equal to

a <= b

True

1. if-else statement#

if statement is one of the most common conditional statements. It is used to decide whether a line of code or a block of codes will be executed if the condition is true, i.e., True. Otherwise, the codes immediately following it will not be executed.

if a == 71:
    print('a equals to 71')

if a == 62:
    print('a equals to 62')

a equals to 62

Use in or not in with a list to test if a list contains a specific element.

states = ['Florida', 'New York', 'California']

if 'Florida' in states:
    print('Florida is in the list.')
    
if 'Texas' not in states:
    print('Texas is not in the list.')

Florida is in the list.
Texas is not in the list.

Code Block Definition

Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Missing indentation will raise an exception (IndentationError).

  • By convention, indentation is four white spaces.

  • The colon after the if statement is required.

  • Most code blocks, such as loops and functions start by a colon.

  • Most IDEs will automatically add indentation after colon is typed.

# the following code will raise an Indentation Error
if a == 62:
print('a equals to 62')

  File "C:\Users\chjch\AppData\Local\Temp\ipykernel_3388\4165433191.py", line 3
    print('a equals to 62')
    ^
IndentationError: expected an indented block

We now understand that if statement can determine whether to run certain statements. But what if we want to do something else if the condition is false (False). Here comes the else statement.

x = '+'

if x == '+':
    print('x is a plus sign')
else:
    print('x is not a plus sign')

x is a plus sign

The Else-If statement (elif) is used when the else condition might not suffice. This is used mostly in those cases when you have to justify more than two results.

x = '-'

if x == '+':
    print('x is a plus sign')
elif x == "-":
    print('x is a minus sign')
else:
    print('x is neither a plus nor a minus sign')

x is a minus sign

2. Comparing two numbers#

num_1 = 784533
num_2 = 763092

if num_1 > num_2:
    print(str(num_1) + " is greater than " + str(num_2)) 
elif num_1 < num_2:
    print(str(num_1) + " is smaller than " + str(num_2))
else:  
    print(str(num_1) + " is equal to " + str(num_2))

784533 is greater than 763092

Tip

The input statement allows user to dynamically insert inputs. Then, use the inserted values to carry out the rest of statements. The type of values inserted by input is string (str).

num_1 = int(input("Enter first number: "))
num_2 = int(input("Enter second number: "))

if num_1 > num_2:
    print(str(num_1) + " is greater than " + str(num_2))
elif num_1 < num_2:
    print(str(num_1) + " is smaller than " + str(num_2))
else:
    print(str(num_1) + " is equal to " + str(num_2))

Warning

String comparison always compare the first character according to the ASCII table. This can results in conterintuitive results. See example below.

'45' > '245'

True

3. Compound condition#

In many situations, a decision is not made only by a single condition. For example, a number is an even number if it is an integer and can be completely divided by 2. To write a more complicated condition, we can use compound Boolean expressions which are combinations of multiple conditions, i.e., variables and values that produce a Boolean value.

To write compound conditions we need to use logical operators (or boolean operators).

Assume a = True and b = False, then

Symbol

Meaning

Example

Result

and

Both expressions are True

a and b

False

or

At least one of the expressions is True

a or b

True

not

The expression is not True

not a

False

 
True or False  # Returns True if one of the statements is true

True

True and False  # Returns True if both statements are true

False

Tip

You can directly use True or False in your code or assign them to a variable. You can then update the variable’s value later.

not True  # Reverse the result

False

Now let’s see how we can extend the example of comparing two numbers by adding compound conditions.

num_1 = int(input("Enter first number: "))

if num_1 > 10:
    print(str(num_1) + " is greater than 10.")
elif num_1 > 5 and num_1 <= 10:
    print(str(num_1) + " is smaller or equal to 10 but greater than 5.")
elif num_1 > 0 and num_1 <= 5:
    print(str(num_1) + " is smaller or equal to 5 but greater than 0.")
else:
    print(str(num_1) + " is negative.")