Loops and Conditionals

Introduction

Python is a high-level programming language widely used in science, engineering, and many other fields. It is user-friendly and allows users to interact with a computer using simple, readable syntax instead of complex assembly or binary commands.

This guide covers loops (for and while) and conditionals (if, elif, else), which are fundamental for controlling the flow of your programs.

Loops

Loops allow you to repeat a block of code multiple times. Python provides two primary types of loops: for and while.

For Loops

A **for loop** is used to iterate over a sequence (e.g., a list, tuple, string, or range). The range() function is commonly used to generate a sequence of numbers.

The range() Function

The range() function generates a sequence of numbers. It can take up to three arguments:

• range(stop): Generates numbers from 0 to stop-1.
• range(start, stop): Generates numbers from start to stop-1.
• range(start, stop, step): Generates numbers from start to stop-1, incrementing by step.

Examples

Print numbers from 0 to 4:

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for x in range(5):  
    print(x)  

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0  
1  
2  
3  
4  

Print numbers from 3 to 5:

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for x in range(3, 6):  
    print(x)  

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3  
4  
5  

Print numbers from 3 to 7, stepping by 2:

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for x in range(3, 8, 2):  
    print(x)  

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3  
5  
7  

Iterating Over Sequences

You can also iterate directly over sequences like lists, tuples, or strings.

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fruits = ["apple", "banana", "cherry"]  
for fruit in fruits:  
    print(fruit)  

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apple  
banana  
cherry  

While Loops

A **while loop** repeats a block of code **as long as a condition is True**. It is often used for infinite loops or loops where the number of iterations is not known in advance.

Example

Print numbers from 0 to 4:

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count = 0  
while count < 5:  
    print(count)  
    count += 1  # Increment count by 1  

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0  
1  
2  
3  
4  

Infinite Loop Example

A while loop with a condition that is always True will run indefinitely unless broken.

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while True:  
    user_input = input("Enter 'quit' to exit: ")  
    if user_input == "quit":  
        break  
    print(f"You entered: {user_input}")  

Break and Continue

• **break**: Exits the loop immediately.
• **continue**: Skips the current iteration and moves to the next one.

Example with break

Exit the loop when count reaches 5:

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count = 0  
while True:  
    print(count)  
    count += 1  
    if count >= 5:  
        break  

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0  
1  
2  
3  
4  

Example with continue

Skip even numbers:

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for i in range(10):  
    if i % 2 == 0:  
        continue  
    print(i)  

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1  
3  
5  
7  
9  

The else Clause in Loops

In Python, loops can have an **else clause** that executes after the loop completes normally (i.e., not terminated by a break statement).

Example with for and else

Print a message after the loop completes:

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for i in range(5):  
    print(i)  
else:  
    print("Loop completed successfully!")  

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0  
1  
2  
3  
4  
Loop completed successfully!  

Example with while and else

Print a message when count reaches 5:

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count = 0  
while count < 5:  
    print(count)  
    count += 1  
else:  
    print(f"count value reached {count}")  

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0  
1  
2  
3  
4  
count value reached 5  

Example with break and else

The else block is skipped if the loop is terminated by break:

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for i in range(1, 10):  
    if i % 5 == 0:  
        break  
    print(i)  
else:  
    print("This is not printed because the loop was terminated by 'break'.")  

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1  
2  
3  
4  

Conditionals

Conditionals allow you to **execute different blocks of code based on whether a condition is True or False**. Python uses the keywords if, elif (short for “else if”), and else for conditionals.

If, Elif, and Else

• **if**: Executes a block of code if a condition is True.
• **elif**: Checks another condition if the previous if or elif condition was False.
• **else**: Executes a block of code if all previous conditions were False.

Example

Assign a grade based on a score:

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score = 85

if score == 100:  
    grade = "A++"  
elif score >= 95:  
    grade = "A+"  
elif score >= 90:  
    grade = "A"  
elif score >= 80:  
    grade = "B"  
elif score >= 70:  
    grade = "C"  
elif score >= 60:  
    grade = "D"  
else:  
    grade = "F"

print("Your grade is:", grade)  

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Your grade is: B  

Boolean Logic in Conditionals

You can use boolean operators (and, or, not) to build complex conditions.

Example

Check if a number is between 10 and 20:

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number = 15

if number >= 10 and number <= 20:  
    print(f"{number} is between 10 and 20.")  
else:  
    print(f"{number} is not between 10 and 20.")  

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15 is between 10 and 20.  

The in Operator

The in operator checks if a value exists in a sequence (e.g., list, tuple, string, set, dictionary).

Example

Check if a fruit is in a list:

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fruits = ["apple", "banana", "cherry"]

if "banana" in fruits:  
    print("Banana is in the list!")  
else:  
    print("Banana is not in the list.")  

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Banana is in the list!  

The is Operator

The is operator checks if two variables refer to the same object in memory, not just if their values are equal.

Example

Compare two variables:

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a = [1, 2, 3]  
b = a  
c = [1, 2, 3]

if a is b:  
    print("a and b refer to the same object.")  
else:  
    print("a and b do not refer to the same object.")

if a is c:  
    print("a and c refer to the same object.")  
else:  
    print("a and c do not refer to the same object.")  

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a and b refer to the same object.  
a and c do not refer to the same object.  

Indentation in Python

Python uses indentation (whitespace at the beginning of a line) to define code blocks, instead of brackets or braces. The standard indentation is 4 spaces, but you can use tabs or any consistent number of spaces.

Example

Correct indentation:

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if 5 > 2:  
    print("Five is greater than two!")  
    print("This is also part of the if block.")  
print("This is not part of the if block.")  

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Five is greater than two!  
This is also part of the if block.  
This is not part of the if block.  

Nested Loops and Conditionals

You can nest loops and conditionals to create more complex logic.

Example

Print even numbers from 0 to 9:

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for i in range(10):  
    if i % 2 == 0:  
        print(i)  

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5
6

  
0  
2  
4  
6  
8  

Conclusion

Loops (for and while) and conditionals (if, elif, else) are essential for controlling the flow of your Python programs. They allow you to:

• Repeat actions with loops.
• Make decisions with conditionals.
• Combine them for complex logic.

Understanding these concepts is key to writing efficient and readable Python code.