What are conditionals in python

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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Python Conditional Statements: IF…Else, ELIF & Switch Case

Conditional Statement in Python perform different computations or actions depending on whether a specific Boolean constraint evaluates to true or false. Conditional statements are handled by IF statements in Python.

What is Python If Statement?

Python if Statement is used for decision-making operations. It contains a body of code which runs only when the condition given in the if statement is true. If the condition is false, then the optional else statement runs which contains some code for the else condition.

When you want to justify one condition while the other condition is not true, then you use Python if else statement.

Python if Statement Syntax:

if expression Statement else Statement

Python if…else Flowchart

Let’s see an example of Python if else Statement:

# #Example file for working with conditional statement # def main(): x,y =2,8 if(x < y): st= "x is less than y" print(st) if __name__ == "__main__": main()

• Code Line 5: We define two variables x, y = 2, 8
• Code Line 7: The if Statement in Python checks for condition xTrue in this case
• Code Line 8: The variable st is set to “x is less than y.”
• Code Line 9: The line print st will output the value of variable st which is “x is less than y”,

What happen when “if condition” does not meet

In this step, we will see what happens when if condition in Python does not meet.

• Code Line 5: We define two variables x, y = 8, 4
• Code Line 7: The if Statement in Python checks for condition xFalse in this case
• Code Line 8: The variable st is NOT set to “x is less than y.”
• Code Line 9: The line print st – is trying to print the value of a variable that was never declared. Hence, we get an error.

How to use “else condition”

The “else condition” is usually used when you have to judge one statement on the basis of other. If one condition goes wrong, then there should be another condition that should justify the statement or logic.

# #Example file for working with conditional statement # def main(): x,y =8,4 if(x < y): st= "x is less than y" else: st= "x is greater than y" print (st) if __name__ == "__main__": main()

• Code Line 5: We define two variables x, y = 8, 4
• Code Line 7: The if Statement in Python checks for condition xFalse in this case
• Code Line 9: The flow of program control goes to else condition
• Code Line 10: The variable st is set to “x is greater than y.”
• Code Line 11: The line print st will output the value of variable st which is “x is greater than y”,

When “else condition” does not work

There might be many instances when your “else condition” won’t give you the desired result. It will print out the wrong result as there is a mistake in program logic. In most cases, this happens when you have to justify more than two statement or condition in a program.

An example will better help you to understand this concept.

Here both the variables are same (8,8) and the program output is “x is greater than y”, which is WRONG. This is because it checks the first condition (if condition in Python), and if it fails, then it prints out the second condition (else condition) as default. In next step, we will see how we can correct this error.

# #Example file for working with conditional statement # def main(): x,y =8,8 if(x < y): st= "x is less than y" else: st= "x is greater than y" print(st) if __name__ == "__main__": main()

How to use “elif” condition

To correct the previous error made by “else condition”, we can use “elif” statement. By using “elif” condition, you are telling the program to print out the third condition or possibility when the other condition goes wrong or incorrect.

# #Example file for working with conditional statement # def main(): x,y =8,8 if(x < y): st= "x is less than y" elif (x == y): st= "x is same as y" else: st="x is greater than y" print(st) if __name__ == "__main__": main()

• Code Line 5: We define two variables x, y = 8, 8
• Code Line 7: The if Statement checks for condition xFalse in this case
• Code Line 10: The flow of program control goes to the elseif condition. It checks whether x==y which is true
• Code Line 11: The variable st is set to “x is same as y.”
• Code Line 15: The flow of program control exits the if Statement (it will not get to the else Statement). And print the variable st. The output is “x is same as y” which is correct

How to execute conditional statement with minimal code

In this step, we will see how we can condense out the conditional statement. Instead of executing code for each condition separately, we can use them with a single code.

def main(): x,y = 10,8 st = "x is less than y" if (x < y) else "x is greater than or equal to y" print(st) if __name__ == "__main__": main()

• Code Line 2: We define two variables x, y = 10, 8
• Code Line 3: Variable st is set to “x is less than y “if xy variable st is set to “x is greater than or equal to y.”
• Code Line 4: Prints the value of st and gives the correct output

Instead of writing long code for conditional statements, Python gives you the freedom to write code in a short and concise way.

Python Nested if Statement

Following example demonstrates nested if Statement Python

total = 100 #country = "US" country = "AU" if country == "US": if total 
Uncomment Line 2 in above code and comment Line 3 and run the code again
Switch Case Statement in Python
What is Switch statement?
A switch statement is a multiway branch statement that compares the value of a variable to the values specified in case statements.
Python language doesn’t have a switch statement.
Python uses dictionary mapping to implement Switch Case in Python
For the above Switch case in Python
def SwitchExample(argument): switcher = < 0: " This is Case Zero ", 1: " This is Case One ", 2: " This is Case Two ", >return switcher.get(argument, "nothing") if __name__ == "__main__": argument = 1 print (SwitchExample(argument))
 
Python 2 Example
 
Above codes are Python 3 examples, If you want to run in Python 2 please consider following code.
 
# If Statement #Example file for working with conditional statement # def main(): x,y =2,8 if(x < y): st= "x is less than y" print st if __name__ == "__main__": main() # How to use "else condition" #Example file for working with conditional statement # def main(): x,y =8,4 if(x < y): st= "x is less than y" else: st= "x is greater than y" print st if __name__ == "__main__": main() # When "else condition" does not work #Example file for working with conditional statement # def main(): x,y =8,8 if(x < y): st= "x is less than y" else: st= "x is greater than y" print st if __name__ == "__main__": main() # How to use "elif" condition #Example file for working with conditional statement # def main(): x,y =8,8 if(x < y): st= "x is less than y" elif (x == y): st= "x is same as y" else: st="x is greater than y" print st if __name__ == "__main__": main() # How to execute conditional statement with minimal code def main(): x,y = 10,8 st = "x is less than y" if (x < y) else "x is greater than or equal to y" print st if __name__ == "__main__": main() # Nested IF Statement total = 100 #country = "US" country = "AU" if country == "US": if total return switcher.get(argument, "nothing") if __name__ == "__main__": argument = 1 print SwitchExample(argument)
 
Summary
 
A conditional statement in Python is handled by if statements and we saw various other ways we can use conditional statements like Python if else over here.
 
 
“if condition” – It is used when you need to print out the result when one of the conditions is true or false.
 
“else condition”- it is used when you want to print out the statement when your one condition fails to meet the requirement
 
“elif condition” – It is used when you have third possibility as the outcome. You can use multiple elif conditions to check for 4 th ,5 th ,6 th possibilities in your code
 
We can use minimal code to execute conditional statements by declaring all condition in single statement to run the code
 
Python If Statement can be nested
 
 
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