Control Flow Statements: Find and Fix the Mistakes

To learn about Common Errors in Python, see Common Errors in Python

1. Mistaken Use of elif Instead of if [Python Quiz #43]

Code:

x = 7

if x > 5:
    print("x is greater than 5")
elif x > 0:
    print("x is positive")

• Watch the Solution Now ✨:https://youtu.be/oQAvH8N1uPk

1. Infinite while Loop Due to Missing Update Statement [Python Quiz #44]

Problem Statement: Write a program that starts with a given positive integer and repeatedly prints its value, decreasing it by 1 in each iteration until it reaches 0. However, the initial code runs into an issue: the loop does not terminate because the value of the integer is never updated, leading to an infinite loop.

Code:

n = 10

while n > 0:
    print(n)

Mistake:
The loop will run infinitely because n is never updated, so n > 0 is always true.

• Watch the Solution Now ✨

1. continue Misused in while Loop [Python Quiz #45]

Scenario: You are tasked with debugging a piece of Python code that uses a while loop. The goal of the loop is to print numbers from 0 to 4, skipping the number 2. However, the code as written enters an infinite loop when the value of i becomes 2.

Given Code:

i = 0

while i < 5:
    if i == 2:
        continue
    print(i)
    i += 1

Problem: The continue statement is misused in this code. When i == 2, the continue statement is executed, causing the loop to skip the i += 1 statement. As a result, i remains 2 indefinitely, leading to an infinite loop.

Task:

1. Identify the mistake in the code that causes the infinite loop.
2. Correct the code so that it successfully prints all numbers from 0 to 4, except for 2.

Expected Output:

0
1
3
4

• Watch the Solution Now ✨

1. Incorrect Use of Multiple elif Conditions

Code:

score = 75

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")
elif score < 70:
    print("F")

Mistake:
The last elif block (elif score < 70) is redundant because the elif score >= 70 block already covers it.

Improved Code:

score = 75

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")
else:
    print("F")

• The else block now correctly handles any score below 60.

1. Syntax Error: Missing Colon [Python Quiz #49]

if x > 5  # Missing colon here
  print("X is greater than 5")

Here’s another question:

Question:

The following code is intended to print the even numbers from 0 to 10, but it contains an error. Identify and fix the error.

for i in range(0, 11):
if i % 2 == 0:
    print(i)

Answer:

The code has an indentation error. The if statement needs to be indented to be part of the for loop.

Fixed Code:

for i in range(0, 11):
    if i % 2 == 0:
        print(i)

Explanation:

In Python, all statements inside a for loop and any nested statements (like if) need to be indented consistently. This indicates that they are part of the respective block.

Which of the following will NOT cause a syntax error in Python?

A)

1st_variable = 10

B)

if x == 10:
print("x is 10")

C)

print("Hello World!"

D)

print "Hello World!)

E) None

1. Incorrect if Statement Syntax

Code:

x = 10

if x > 5
    print("x is greater than 5")

Mistake:
The if statement is missing a colon (:) at the end.

Corrected Code:

x = 10

if x > 5:
    print("x is greater than 5")

1. Incorrect Indentation in if-else Statement

Code:

y = -3

if y > 0:
    print("y is positive")
    print("This is always printed")
else:
print("y is not positive")

Mistake:
The else block is not properly indented.

Corrected Code:

y = -3

if y > 0:
    print("y is positive")
    print("This is always printed")
else:
    print("y is not positive")

1. while Loop with Incorrect Condition

Code:

count = 10

while count > 10:
    print("This will never print")
    count -= 1

Mistake:
The loop condition count > 10 is false at the start, so the loop will never run.

Corrected Code:

count = 10

while count >= 0:
    print("Count:", count)
    count -= 1

1. for Loop with Incorrect Range

Code:

for i in range(1, 10, -1):
    print(i)

Mistake:
The step value in range(1, 10, -1) is negative, but the start value is less than the stop value, so this loop will not run.

Corrected Code:

for i in range(10, 0, -1):
    print(i)

• This prints numbers from 10 to 1.

1. Using break Incorrectly in a Loop

Code:

for i in range(5):
    if i == 2:
        break
    else:
        print(i)

Mistake:
The else block is not correctly used here. It runs as part of each iteration, not after the loop.

Corrected Code:

for i in range(5):
    if i == 2:
        break
    print(i)
else:
    print("Loop completed without a break")

• The else block will only run if the loop completes without a break.

1. try Block with Unhandled Exception

Code:

try:
    print(5 / 0)
except TypeError:
    print("A TypeError occurred")

Mistake:
The code raises a ZeroDivisionError, but the except block is only catching a TypeError.

Corrected Code:

try:
    print(5 / 0)
except ZeroDivisionError:
    print("You can't divide by zero!")

1. Misuse of continue Statement

Code:

for i in range(5):
    if i == 3:
        continue
        print("This will never be printed")
    print(i)

Mistake:
The print statement after continue will never be executed.

Corrected Code:

for i in range(5):
    if i == 3:
        continue
    print(i)

• The code correctly skips printing 3 and continues with the next iteration.

1. Logical Error with if-elif Statements

Code:

z = 20

if z > 30:
    print("z is greater than 30")
elif z > 10:
    print("z is greater than 10")
else:
    print("z is less than 10")

Mistake:
The code will print "z is greater than 10" even though z is also greater than 30. This is logically correct, but not what might be intended if you want to check for ranges.

Corrected Code:

z = 20

if z > 30:
    print("z is greater than 30")
elif z > 20:
    print("z is greater than 20 but less than 30")
elif z > 10:
    print("z is greater than 10")
else:
    print("z is less than or equal to 10")

• Now, this checks for more specific conditions.

1. What will be the output of the following code?

for i in range(5):
    print(i)
else:
    break

1. Incorrect Usage of pass in Control Flow

Code:

for i in range(5):
    if i == 2:
        pass
        print("This should not print")
    print(i)

Mistake:
The pass statement does nothing, but the print("This should not print") will still run.

Corrected Code:

for i in range(5):
    if i == 2:
        pass
    print(i)

• Now, the loop will simply skip doing anything when i is 2, without any unintended behavior.

1. Improper Use of finally in Exception Handling

Code:

try:
    result = 10 / 0
finally:
    print("This will run no matter what.")
except ZeroDivisionError:
    print("You can't divide by zero!")

Mistake:
The finally block must come after except, not before.

Corrected Code:

try:
    result = 10 / 0
except ZeroDivisionError:
    print("You can't divide by zero!")
finally:
    print("This will run no matter what.")

These challenges are designed to highlight common mistakes that occur when working with control flow statements in Python. Working through these examples will help reinforce proper syntax and logical structure.