Iterating over strings in Python

Introduction

Iterating over strings lets you look at each letter one by one. This helps when you want to check or change parts of a word or sentence.

Counting how many times a letter appears in a word.

Changing all letters in a sentence to uppercase or lowercase.

Finding if a certain letter or symbol is inside a word.

Printing each letter of a word on a new line.

Syntax

Python

for letter in string:
    # do something with letter

The word 'string' is the name of your text variable.

Each 'letter' is one character from the string, one at a time.

Examples

Prints each letter of the word 'hello' on its own line.

Python

word = "hello"
for letter in word:
    print(letter)

Prints each letter in uppercase.

Python

text = "Python"
for char in text:
    print(char.upper())

Counts how many times the letter 'a' appears, ignoring case.

Python

name = "Anna"
count_a = 0
for ch in name:
    if ch == 'a' or ch == 'A':
        count_a += 1
print(count_a)

Sample Program

This program prints each letter in the sentence except spaces.

Python

sentence = "Hello World"
for letter in sentence:
    if letter != ' ':
        print(letter)

Important Notes

Strings are like a list of letters, so you can use a loop to go through them.

Spaces and punctuation are also characters and will be included unless you skip them.

You cannot change letters directly in a string because strings are fixed, but you can build a new string.

Summary

Use a for loop to look at each letter in a string one by one.

You can check, count, or print letters inside the loop.

Remember that strings cannot be changed directly, but you can create new strings from letters.

Practice

1. What does the following code do?
for letter in "hello":
print(letter)

easy

A. Prints each letter of the word 'hello' on a new line

B. Prints the whole word 'hello' once

C. Prints the length of the word 'hello'

D. Causes an error because strings can't be looped

Solution

1. Step 1: Understand the for loop over a string

   The loop goes through each character in the string "hello" one by one.

2. Step 2: Print each character

   Inside the loop, each letter is printed on its own line.

3. Final Answer:

   Prints each letter of the word 'hello' on a new line -> Option A

4. Quick Check:

   Loop over string prints letters individually [OK]

Hint: For loops over strings print letters one by one [OK]

Common Mistakes:

• Thinking the whole string prints at once
• Confusing string length with letters
• Believing strings can't be looped

2. Which of the following is the correct syntax to loop over each character in the string text?

easy

A. for char to text: print(char)

B. for char in range(text): print(char)

C. for char in text: print(char)

D. for i in text.length: print(i)

Solution

1. Step 1: Identify correct for loop syntax for strings

   In Python, to loop over characters in a string, use for variable in string:.

2. Step 2: Check each option

   for char in text:
print(char) uses correct syntax. The other options use invalid syntax like 'to' instead of 'in', range() on a string, or non-existent .length attribute.

3. Final Answer:

   for char in text:
   print(char) -> Option C

4. Quick Check:

   Correct for loop syntax over string is for char in text:
   print(char) [OK]

Hint: Use 'for char in string:' to loop letters [OK]

Common Mistakes:

• Using range() on a string directly
• Trying to use .length instead of len()
• Using incorrect loop keywords

3. What is the output of this code?

word = "code"
result = ""
for ch in word:
    result += ch.upper()
print(result)

medium

A. code

B. CODE

C. Code

D. cODe

Solution

1. Step 1: Loop through each letter in 'code'

   The loop takes each character: 'c', 'o', 'd', 'e'.

2. Step 2: Convert each letter to uppercase and add to result

   Each letter is changed to uppercase ('C', 'O', 'D', 'E') and added to the empty string result.

3. Final Answer:

   CODE -> Option B

4. Quick Check:

   Uppercase each letter and join = CODE [OK]

Hint: Uppercase letters inside loop to build new string [OK]

Common Mistakes:

• Not converting letters to uppercase
• Printing original string instead of result
• Using += without initializing result

4. Find the error in this code:

text = "hello"
for i in text:
    print(text[i])

medium

A. Variable 'text' is not defined

B. Missing colon after for loop

C. Loop should be 'for i in range(text):'

D. Using 'i' as index causes TypeError

Solution

1. Step 1: Understand the loop variable 'i'

   Here, 'i' takes each character from 'text', so 'i' is a letter, not an index.

2. Step 2: Using 'text[i]' causes error

   Since 'i' is a letter, using it as an index causes a TypeError because string indices must be integers.

3. Final Answer:

   Using 'i' as index causes TypeError -> Option D

4. Quick Check:

   Loop variable is letter, not index, so indexing fails [OK]

Hint: Loop variable is letter, not index; use range(len(text)) for indices [OK]

Common Mistakes:

• Assuming loop variable is index
• Trying to index string with a character
• Confusing loop variable with range

5. Write code to count how many vowels are in the string sentence. Which code correctly does this?

hard

A. count = 0
for ch in sentence:
if ch in 'aeiouAEIOU':
count += 1
print(count)

B. count = 0
for i in range(len(sentence)):
if sentence[i] == 'aeiou':
count += 1
print(count)

C. count = 0
for ch in sentence:
if ch == 'a' or 'e' or 'i' or 'o' or 'u':
count += 1
print(count)

D. count = 0
for ch in sentence:
if ch in ['a','e','i','o','u']:
count = count + 1
print(count)

Solution

1. Step 1: Check vowel membership correctly

   count = 0 for ch in sentence: if ch in 'aeiouAEIOU': count += 1 print(count) checks if each character is in the string of vowels (both lowercase and uppercase), which is correct.

2. Step 2: Verify counting logic

   count = 0 for ch in sentence: if ch in 'aeiouAEIOU': count += 1 print(count) increments count correctly when a vowel is found and prints the total count.

3. Final Answer:

   count = 0 for ch in sentence: if ch in 'aeiouAEIOU': count += 1 print(count) -> Option A

4. Quick Check:

   Use 'in' with vowel string and increment count [OK]

Hint: Use 'if ch in "aeiouAEIOU"' to check vowels [OK]

Common Mistakes:

• Using '==' to compare with multiple vowels
• Checking membership incorrectly with list without uppercase
• Not incrementing count properly