String Reversal Approaches in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse the string using slicing.

DSA Python

reversed_str = original_str[1]

Drag options to blanks, or click blank then click option'

A[1:]

B[:]

C[::-1]

D[::-2]

Attempts:

3 left

💡 Hint

Common Mistakes

Using [:] returns the original string without reversing.

Using [1:] skips the first character but does not reverse.

Using [::-2] reverses but skips characters.

Using slicing with [::-1] reverses the string by stepping backwards one character at a time.

2fill in blank

medium

Complete the code to reverse the string using the reversed() function and join.

DSA Python

reversed_str = ''.[1](reversed(original_str))

Drag options to blanks, or click blank then click option'

Asplit

Bjoin

Cappend

Dreplace

Attempts:

3 left

💡 Hint

Common Mistakes

Using append() is for lists, not strings.

Using split() breaks strings into lists, not reverse.

Using replace() changes characters, not order.

The join() method combines characters from reversed() into a new reversed string.

3fill in blank

hard

Fix the error in the code to reverse the string using a for loop.

DSA Python

reversed_str = ''
for char in original_str:
    reversed_str = char [1] reversed_str

Drag options to blanks, or click blank then click option'

A/

B*

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using * or - operators causes errors or wrong results.

Appending char after reversed_str does not reverse.

Concatenating the current character before the reversed string builds the reversed string step by step.

4fill in blank

hard

Fill both blanks to reverse the string using a list and the reverse() method.

DSA Python

char_list = list(original_str)
char_list.[1]()
reversed_str = ''.[2](char_list)

Drag options to blanks, or click blank then click option'

Areverse

Bjoin

Cappend

Dsort

Attempts:

3 left

💡 Hint

Common Mistakes

Using sort() changes order but sorts alphabetically, not reverses.

Using append() is for adding elements, not reversing.

Using join() before reverse() will not reverse the string.

reverse() reverses the list in place, and join() combines the list into a string.

5fill in blank

hard

Fill all three blanks to reverse the string using recursion.

DSA Python

def reverse_str(s):
    if len(s) [1] 0:
        return ''
    else:
        return reverse_str(s[[3]]) + s[[2]]

Drag options to blanks, or click blank then click option'

A==

B0

C1:

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using != in base case causes infinite recursion.

Using -1 as index returns last char, but recursion needs first char then rest.

Incorrect slicing causes errors or wrong output.

Base case checks if string is empty (length 0). Then return recursive call on rest plus first char.

Practice

1. Which of the following methods correctly reverses a string s in Python?

easy

A. Using reversed(s) without converting to string

B. Using slicing: s[::-1]

C. Using s.reverse() method

D. Using s.append() method

Solution

1. Step 1: Understand string slicing

   The slicing syntax s[::-1] creates a new string with characters in reverse order.

2. Step 2: Check other methods

   s.reverse() and s.append() are list methods, not valid for strings. reversed(s) returns an iterator, not a string, so it needs conversion.

3. Final Answer:

   Using slicing: s[::-1] -> Option B

4. Quick Check:

   String slicing with negative step reverses string [OK]

Hint: Use s[::-1] to reverse strings quickly [OK]

Common Mistakes:

• Trying to use list methods on strings
• Forgetting to convert reversed() iterator to string
• Using append() on strings

2. Which of the following code snippets correctly reverses the string text = 'hello' and prints it?

easy

A. print(text.append('o'))

B. print(text.reverse())

C. print(text[::-2])

D. print(''.join(reversed(text)))

Solution

1. Step 1: Analyze each option

   text.reverse() is invalid for strings. text[::-2] reverses with step 2, skipping characters. text.append() is invalid for strings.

2. Step 2: Confirm correct syntax

   reversed(text) returns an iterator of characters reversed. ''.join() converts it back to string. This prints 'olleh'.

3. Final Answer:

   print(''.join(reversed(text))) -> Option D

4. Quick Check:

   reversed() + join() prints reversed string [OK]

Hint: Use ''.join(reversed(s)) to reverse strings [OK]

Common Mistakes:

• Using reverse() method on strings
• Using slicing with wrong step
• Trying to append to strings

3. What is the output of the following code?

def reverse_string(s):
    result = ''
    for char in s:
        result = char + result
    return result

print(reverse_string('abc'))

medium

A. 'cba'

B. 'abc'

C. 'bac'

D. Error

Solution

1. Step 1: Trace the loop for s='abc'

   Initially, result = ''. Iteration 1: char='a', result='a' + '' = 'a'. Iteration 2: char='b', result='b' + 'a' = 'ba'. Iteration 3: char='c', result='c' + 'ba' = 'cba'.

2. Step 2: Return the reversed string

   The function returns 'cba', which is the reverse of 'abc'.

3. Final Answer:

   'cba' -> Option A

4. Quick Check:

   Prepending chars reverses string [OK]

Hint: Add each char before result to reverse string [OK]

Common Mistakes:

• Appending char instead of prepending
• Returning original string
• Confusing order of concatenation

4. The following code is intended to reverse a string but has an error. What is the error?

def reverse_str(s):
    s.reverse()
    return s

print(reverse_str('hello'))

medium

A. Missing return statement

B. Function returns None

C. Strings do not have reverse() method

D. Syntax error in function definition

Solution

1. Step 1: Check method usage on string

   The reverse() method exists for lists, not strings. Calling it on a string causes AttributeError.

2. Step 2: Understand the error cause

   Since strings are immutable, they do not support reverse(). The code will raise an error at runtime.

3. Final Answer:

   Strings do not have reverse() method -> Option C

4. Quick Check:

   reverse() is list method, not string method [OK]

Hint: Use slicing or reversed() for strings, not reverse() [OK]

Common Mistakes:

• Trying to use reverse() on strings
• Expecting reverse() to modify string in place
• Ignoring immutability of strings

5. You want to reverse only the vowels in a string s = 'hello world' while keeping other characters in place. Which approach below correctly achieves this?

hard

A. Extract vowels, reverse them, then replace vowels in original positions

B. Reverse entire string using s[::-1]

C. Use reversed(s) and join all characters

D. Replace vowels with '*' and then reverse string

Solution

1. Step 1: Understand the problem

   We want to reverse only vowels, keeping consonants and spaces fixed in their places.

2. Step 2: Analyze each option

   Reverse entire string using s[::-1] reverses whole string, changing all characters. Use reversed(s) and join all characters reverses all characters, not just vowels. Replace vowels with '*' and then reverse string replaces vowels with '*', losing original vowels. Extract vowels, reverse them, then replace vowels in original positions extracts vowels, reverses them, then puts them back in original vowel positions, preserving other characters.

3. Final Answer:

   Extract vowels, reverse them, then replace vowels in original positions -> Option A

4. Quick Check:

   Reverse vowels only by extracting and replacing [OK]

Hint: Extract vowels, reverse, then put back in place [OK]

Common Mistakes:

• Reversing entire string instead of vowels only
• Replacing vowels with placeholders without restoring
• Ignoring character positions