Sometimes we want to play pubg on our phone if the day is Sunday.
Sometimes we order Ice-cream online if the day is sunny.
Sometimes we go hiking if our parents allow.
All these are decisions that depend on the condition being met.
In python programming too, we must be able to execute instructions on a condition(s) being met. This is what conditions are for!
Decisions in a program are used when the program has conditional choices to execute a code block. Let’s take an example of traffic lights, where different colors of lights lit up in different situations based on the conditions of the road or any specific rule.
Python provides various types of conditional statements:
|if Statements||It consists of a Boolean expression which results are either TRUE or FALSE, followed by one or more statements.|
|if else Statements||It also contains a Boolean expression. The if the statement is followed by an optional else statement & if the expression results in FALSE, then else statement gets executed. It is also called alternative execution in which there are two possibilities of the condition determined in which any one of them will get executed.|
|Nested Statements||We can implement if statement and or if-else statement inside another if or if – else statement. Here more than one if conditions are applied & there can be more than one if within elif.|
The decision-making structures can be recognized and understood using flowcharts.
Python If Statement
if expression: #execute your code
a = 15 if a > 10: print("a is greater")
a is greater
Python if else Statement
if expression: #execute your code else: #execute your code
a = 15 b = 20 if a > b: print("a is greater") else: print("b is greater")
b is greater
Python elif Statements
elif – is a keyword used in Python replacement of else if to place another condition in the program. This is called chained conditional.
if expression: #execute your code elif expression: #execute your code else: #execute your code
a = 15 b = 15 if a > b: print("a is greater") elif a == b: print("both are equal") else: print("b is greater")
both are equal
If else and elif in Python
If else and elif statements are a multiway decision taken by our program due to certain conditions in our code.
''' if (condition1): // if condition 1 is true print(“yes”) elif (condition2): // if condition 2 is true print(“No”) else: // otherwise print(“May be”) '''
a = 22 if (a>9): print(“Greater”) else: print(“lesser”)
Quick Quiz: Write a program to print yes when the age entered by the user is greater than or equal to 18.
Single Statement Condition
If the block of an executable statement of if – clause contains only a single line, programmers can write it on the same line as a header statement.Example:
a = 15 if (a == 15): print("The value of a is 15")
Relational operators are used to evaluate conditions inside if statements. Some examples of relational operators are:
= = -> equals >= -> greater than/equal to <=, etc.
In python, logical operators operate on conditional statements. Example:
and -> true if both operands are true else false or -> true if at least one operand is true else false not -> inverts true to false and false to true
elif in python means [else if]. If statement can be chained together with a lot of these elif statements followed by an else statement.
Python elif clause
''' if (condition1): #code elif (condition 2): #code elif (condition 2): #code …. else: #code '''
- The above ladder will stop once a condition in an if or elif is met.
- There can be any number of elif statements.
- Last else is executed only if all the conditions inside elifs fail.
Exercise On If Else
- Write a program to find the greatest of four numbers entered by the user.
- Write a program to find out whether a student is pass or fail if it requires a total of 40% and at least 33% in each subject to pass. Assume 3 subjects and take marks as an input from the user.
- A spam comment is defined as a text containing the following keywords:
“make a lot of money”, “buy now”, “subscribe this”, “click this”. Write a program to detect these spams.
- Write a program to find whether a given username contains less than 10 characters or not.
- Write a program that finds out whether a given name is present in a list or not.
- Write a program to calculate the grade of a student from his marks from the following scheme: