What are Selection Statements in Python? Syntax and Examples
Introduction
Selection statements, also known as decision-making statements, allow a Python program to evaluate conditions and execute specific blocks of code accordingly. These statements are fundamental for controlling the flow of a program, enabling dynamic and interactive behavior. Python’s straightforward syntax and powerful features make implementing selection statements both simple and effective, empowering developers to create robust and maintainable applications.
Types of Selection Statements
Python supports four main types of selection statements:
- if Statement: Executes a block of code if the given condition is True.
- if-else Statement: Executes one block of code if the condition is True and another block if it is False.
- if-elif-else Statement: Allows testing multiple conditions in a sequence.
- Nested if Statement: An if statement inside another if statement for hierarchical decision-making.
Syntax and Examples
1. if Statement
The simplest selection statement, which executes a block of code when the condition is True.
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Example:
age = 20 if age >= 18: print(“You are eligible to vote.”)
Explanation: Here, the condition age >= 18 is evaluated. If it is True, the message “You are eligible to vote.” is printed.
2. if-else Statement
Used to execute one block of code if the condition is True and another block if it is False.
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Example:
age = 16 if age >= 18: print(“You are eligible to vote.”) else: print(“You are not eligible to vote.”)
Explanation: If age >= 18 evaluates to False, the else block executes, printing “You are not eligible to vote.”
3. if-elif-else Statement
This is used for testing multiple conditions sequentially. The first condition that evaluates to True gets executed, and the rest are ignored.
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Example:
marks = 85 if marks >= 90: print(“Grade: A”) elif marks >= 75: print(“Grade: B”) elif marks >= 50: print(“Grade: C”) else: print(“Grade: F”)
Explanation: The code checks the conditions in order. For marks = 85, the elif marks >= 75 condition evaluates to True, so “Grade: B” is printed.
4. Nested if Statement
An if statement inside another if statement allows checking multiple conditions in a hierarchical manner.
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Example:
age = 20 if age >= 18: if age < 21: print(“You are eligible for a student voter card.”) else: print(“You are eligible for a regular voter card.”)
Explanation: The outer if checks if age >= 18. If True, the inner if checks if age < 21, and executes the corresponding block.
Key Points to Remember
- Indentation Matters: Python uses indentation to define blocks of code, so ensure proper alignment. Incorrect indentation will result in syntax errors.
- Condition Evaluation: Conditions in selection statements must evaluate to a boolean (True or False). Use comparison operators (==, !=, >, <, >=, <=) to form conditions.
- Logical Operators: Use and, or, and not for combining multiple conditions. Logical operators enhance the flexibility of decision-making in your programs.
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Example:
age = 19 is_registered = True if age >= 18 and is_registered: print(“You can vote.”)
Explanation: The and operator ensures both conditions (age >= 18 and is_registered) are True before executing the code inside the if block.
Additional Tips for Beginners
- Use Meaningful Variable Names: Choose names that clearly describe the purpose of the variable. For example, use age instead of x.
- Test Edge Cases: When writing selection statements, test conditions like boundary values to ensure accuracy.
- Avoid Redundant Conditions: Optimize your code by avoiding unnecessary checks.
- Debugging: Use print statements to debug and trace the flow of your program when working with complex selection logic.
- Practice Regularly: Familiarize yourself with selection statements by solving problems and building small projects.