Iterating over strings in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of iterating and modifying a string

What is the output of this Python code?

text = "hello"
result = ""
for ch in text:
    if ch == 'l':
        result += 'L'
    else:
        result += ch
print(result)

Python

text = "hello"
result = ""
for ch in text:
    if ch == 'l':
        result += 'L'
    else:
        result += ch
print(result)

A heLLo

B hello

C heLlo

D heLLoL

💡 Hint

Remember that the loop checks each character and replaces 'l' with 'L'.

The loop goes through each character in 'hello'. When it finds 'l', it adds 'L' to result. Otherwise, it adds the original character. So 'l' becomes 'L', resulting in 'heLLo'.

❓ Predict Output

intermediate

Counting vowels in a string

What is the output of this code?

text = "Programming"
vowels = 'aeiouAEIOU'
count = 0
for char in text:
    if char in vowels:
        count += 1
print(count)

Python

text = "Programming"
vowels = 'aeiouAEIOU'
count = 0
for char in text:
    if char in vowels:
        count += 1
print(count)

A 5

B 4

C 3

D 2

💡 Hint

Check each letter if it is a vowel.

The vowels in 'Programming' are 'o', 'a', and 'i', so the count is 3.

❓ Predict Output

advanced

Output of nested iteration over string

What does this code print?

word = "abc"
result = ""
for i in word:
    for j in word:
        if i == j:
            result += i.upper()
        else:
            result += j
print(result)

Python

word = "abc"
result = ""
for i in word:
    for j in word:
        if i == j:
            result += i.upper()
        else:
            result += j
print(result)

A AbcBaCcaB

B AbcaBcabC

C AbcBacCab

D ABCabcabc

💡 Hint

Look at how the inner loop builds the string for each outer loop character.

For each character i in 'abc', the inner loop goes through 'abc'. When i equals j, it adds uppercase i, else adds j. So for i='a', it adds 'A', 'b', 'c' → 'Abc'. For i='b', adds 'a', 'B', 'c' → 'aBc'. For i='c', adds 'a', 'b', 'C' → 'abC'. Combined: 'AbcaBcabC'. Option B matches this.

❓ Predict Output

advanced

Result of modifying string characters conditionally

What is the output of this code?

text = "Data123"
result = ""
for ch in text:
    if ch.isalpha():
        result += ch.lower()
    else:
        result += '*'
print(result)

Python

text = "Data123"
result = ""
for ch in text:
    if ch.isalpha():
        result += ch.lower()
    else:
        result += '*'
print(result)

A data***

B DATA123

C data123

D Data***

💡 Hint

Check how letters and digits are handled differently.

Letters are converted to lowercase, digits replaced by '*'. So 'Data123' becomes 'data***'.

🧠 Conceptual

expert

Why strings are immutable in Python?

Which of the following is the main reason strings are immutable in Python?

A To prevent strings from being garbage collected

B To make string concatenation faster

C Because strings are stored in a special memory area called stack

D To allow strings to be used as dictionary keys and set elements safely

💡 Hint

Think about what properties dictionary keys need.

Strings are immutable so they can be hashed and used safely as dictionary keys and set elements without risk of changing their value.