Python if then else end

Lesson 3
Conditions: if, then, else

All the programs in the first lesson were executed sequentially, line after line. No line could be skipped.

Let’s consider the following problem: for the given integer X determine its absolute value. If X>0 then the program should print the value X, otherwise it should print -X. This behavior can’t be reached using the sequential program. The program should conditionally select the next step. That’s where the conditions help:

x = int(input()) if x > 0: print(x) else: print(-x)

This program uses a conditional statement if . After the if we put a condition (x > 0) following by a colon. After that we put a block of instructions which will be executed only if the condition is true (i.e. evaluates to True ). This block may be followed by the word else , colon and another block of instructions which will be executed only if the condition is false (i.e. evaluates to False ). Is the case above, the condition is false, so the ‘else’ block is executed. Each block should be indented using spaces.

To sum up, the conditional statement in Python has the following syntax:

if condition: true-block several instructions that are executed if the condition evaluates to True else: false-block several instructions that are executed if the condition evaluates to False 

The else keyword with the ‘false’ block may be omitted in case nothing should be done if the condition is false. For example, we can replace the variable x with its absolute value like this:

Indentation is a general way in Python to separate blocks of code. All instructions within the same block should be indented in the same way, i.e. they should have the same number of spaces at the beginning of the line. It’s recommended to use 4 spaces for indentation.

The indentation is what makes Python different from the most of other language, in which the curly braces < and >are used to form the blocks.

By the way, there’s a builtin-function for absolute value in Python:

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Python If . Else

Python supports the usual logical conditions from mathematics:

• Equals: a == b
• Not Equals: a != b
• Less than: a < b
• Less than or equal to: a
• Greater than: a > b
• Greater than or equal to: a >= b

These conditions can be used in several ways, most commonly in «if statements» and loops.

An «if statement» is written by using the if keyword.

Example

In this example we use two variables, a and b , which are used as part of the if statement to test whether b is greater than a . As a is 33 , and b is 200 , we know that 200 is greater than 33, and so we print to screen that «b is greater than a».

Indentation

Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.

Example

If statement, without indentation (will raise an error):

Elif

The elif keyword is Python’s way of saying «if the previous conditions were not true, then try this condition».

Example

In this example a is equal to b , so the first condition is not true, but the elif condition is true, so we print to screen that «a and b are equal».

Else

The else keyword catches anything which isn’t caught by the preceding conditions.

Example

a = 200
b = 33
if b > a:
print(«b is greater than a»)
elif a == b:
print(«a and b are equal»)
else:
print(«a is greater than b»)

In this example a is greater than b , so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that «a is greater than b».

You can also have an else without the elif :

Example

Short Hand If

If you have only one statement to execute, you can put it on the same line as the if statement.

Example

Short Hand If . Else

If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:

Example

One line if else statement:

This technique is known as Ternary Operators, or Conditional Expressions.

You can also have multiple else statements on the same line:

Example

One line if else statement, with 3 conditions:

And

The and keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b , AND if c is greater than a :

Or

The or keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b , OR if a is greater than c :

Not

The not keyword is a logical operator, and is used to reverse the result of the conditional statement:

Example

Test if a is NOT greater than b :

Nested If

You can have if statements inside if statements, this is called nested if statements.

Example

if x > 10:
print(«Above ten,»)
if x > 20:
print(«and also above 20!»)
else:
print(«but not above 20.»)

The pass Statement

if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.

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