Introduction to if … else Statements
Basic Structure
The basic structure of conditional statements in Python is:
if condition: # Block of code executed if the condition is true else: # Block of code executed if the condition is false
Explanation of the Structure
- if: The if keyword introduces the condition. The condition must be an expression that evaluates to True or False.
- condition: This is a boolean expression. Common conditions include comparisons (==, !=, >, <, >=, <=), arithmetic expressions, or function calls that return a boolean.
- Block of code: The code indented under if will execute only if the condition is true.
- else: The else keyword is optional and defines a block of code that runs when the condition is false.
Example:
x = 8
if x > 5:
print("x is greater than 5")
else:
print("x is 5 or less")
Values of Condition
In Python, any expression that returns a boolean (True or False) can be used as a condition. Here are some values considered False in Python:
- None
- False
- 0 (zero, of any numeric type, like 0.0, 0j, etc.)
- ” (empty string)
- [] (empty list)
- {} (empty dictionary or set)
- () (empty tuple)
All other values are considered True.
Example:
value = []
if value:
print("The value is not empty")
else:
print("The value is empty")
Comparison with Other Languages
Conditional structures in Python are similar to those in other programming languages, but with notable syntactic differences. For example:
- Python uses indentation to delimit blocks of code.
- C/C++/Java use curly braces {} to define blocks of code.
Python Example:
if x > 10:
print("x is greater than 10")
else:
print("x is 10 or less")
Example:
if (x > 10) {
printf("x is greater than 10");
} else {
printf("x is 10 or less");
}
Common Errors
Incorrect Indentation: In Python, indentation is crucial. Incorrect indentation can lead to syntax errors or logical errors.
Example of an Error:
x = 10
if x > 5:
print("x is greater than 5")
# Error: code is not correctly indented
Correction:
x = 10
if x > 5:
print("x is greater than 5")
Malformed Conditions: Ensure conditions are properly formed and return a boolean.
Example of an Error:
x = 10
if x + 5: # x + 5 is 15, so the condition is always True
print("x + 5 is non-zero")
Correction:
x = 10
if x + 5 > 10:
print("x + 5 is greater than 10")
Using == Instead of =: == is for comparison, while = is for assignment.
Example of an Error:
x = 10
if x = 10: # Error: assignment instead of comparison
print("x is 10")
Correction:
x = 10
if x == 10:
print("x is 10")
Advanced Examples
Example 1: Checking User Input Validity
age = int(input("Enter your age: "))
if age < 0:
print("Age cannot be negative")
elif age < 18:
print("You are a minor")
else:
print("You are an adult")
Example 2: Password Validation
password = input("Enter your password: ")
if password == "secret123":
print("Password is correct")
else:
print("Password is incorrect")