String to Integer atoi in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this atoi implementation with leading spaces and sign?

Consider the following Python function that converts a string to an integer. What will be printed when calling my_atoi(" -42")?

DSA Python

def my_atoi(s: str) -> int:
    i = 0
    n = len(s)
    while i < n and s[i] == ' ':
        i += 1
    sign = 1
    if i < n and (s[i] == '+' or s[i] == '-'):
        sign = -1 if s[i] == '-' else 1
        i += 1
    num = 0
    while i < n and s[i].isdigit():
        num = num * 10 + int(s[i])
        i += 1
    return sign * num

print(my_atoi("   -42"))

A -42

B 42

C 0

D SyntaxError

💡 Hint

Check how the function handles spaces and the sign before digits.

The function skips leading spaces, detects the '-' sign, then parses digits to form the number 42, finally applying the negative sign.

❓ Predict Output

intermediate

What is the output when the string contains non-digit characters after number?

What will my_atoi("4193 with words") print using the same function as before?

DSA Python

def my_atoi(s: str) -> int:
    i = 0
    n = len(s)
    while i < n and s[i] == ' ':
        i += 1
    sign = 1
    if i < n and (s[i] == '+' or s[i] == '-'):
        sign = -1 if s[i] == '-' else 1
        i += 1
    num = 0
    while i < n and s[i].isdigit():
        num = num * 10 + int(s[i])
        i += 1
    return sign * num

print(my_atoi("4193 with words"))

A 4193 with words

B 0

C 4193

D TypeError

💡 Hint

The function stops parsing when it encounters a non-digit character.

The function parses digits until it hits a space, then stops and returns the number parsed so far.

🔧 Debug

advanced

What error does this atoi variant raise?

Consider this modified atoi function. What error will it raise when called with my_atoi("+")?

DSA Python

def my_atoi(s: str) -> int:
    i = 0
    n = len(s)
    while i < n and s[i] == ' ':
        i += 1
    sign = 1
    if i < n and (s[i] == '+' or s[i] == '-'):
        sign = -1 if s[i] == '-' else 1
        i += 1
    num = 0
    while i < n and s[i].isdigit():
        num = num * 10 + int(s[i])
        i += 1
    return sign * num

print(my_atoi("+"))

A ValueError

B IndexError

C 0

D No error, returns 0

💡 Hint

Check what happens if no digits follow the sign.

The function returns 0 because no digits are found after the '+' sign, so num remains 0.

❓ Predict Output

advanced

What is the output when input string is empty?

What will my_atoi("") print using the same function?

DSA Python

def my_atoi(s: str) -> int:
    i = 0
    n = len(s)
    while i < n and s[i] == ' ':
        i += 1
    sign = 1
    if i < n and (s[i] == '+' or s[i] == '-'):
        sign = -1 if s[i] == '-' else 1
        i += 1
    num = 0
    while i < n and s[i].isdigit():
        num = num * 10 + int(s[i])
        i += 1
    return sign * num

print(my_atoi(""))

A 0

B IndexError

C ValueError

D None

💡 Hint

Empty string means no digits to parse.

The function returns 0 because no characters are parsed and num stays 0.

🧠 Conceptual

expert

What is the main reason to check for integer overflow in atoi implementations?

Why do atoi implementations often check if the parsed integer exceeds 32-bit signed integer limits during conversion?

A To convert the string faster using bitwise operations

B To ensure the returned integer fits within the allowed range and avoid incorrect values

C To speed up the parsing process by stopping early

D To prevent the program from crashing due to memory overflow

💡 Hint

Think about what happens if the number is too big for the data type.

Checking overflow ensures the function returns a valid integer within the allowed range, preventing wrong results.

Practice

1. What does the atoi function do when given a string input?

easy

A. Converts the string to an integer number

B. Converts the string to a floating-point number

C. Reverses the string characters

D. Counts the number of characters in the string

Solution

1. Step 1: Understand the purpose of atoi

   The atoi function is designed to convert a string that represents a number into an integer type.

2. Step 2: Compare options with function behavior

   Only Converts the string to an integer number matches the behavior of converting string to integer. Others describe different operations.

3. Final Answer:

   Converts the string to an integer number -> Option A

4. Quick Check:

   atoi = string to integer [OK]

Hint: Remember atoi means ASCII to integer conversion [OK]

Common Mistakes:

• Thinking atoi converts to float
• Confusing atoi with string reversal
• Assuming atoi counts characters

2. Which of the following Python code snippets correctly converts a string s to an integer similar to atoi?

easy

A. len(s)

B. int(s)

C. float(s)

D. str(s)

Solution

1. Step 1: Identify Python function for string to integer

   Python's built-in int() function converts a string representing a number into an integer.

2. Step 2: Check other options

   str() converts to string, float() converts to float, and len() returns length, so they are incorrect.

3. Final Answer:

   int(s) -> Option B

4. Quick Check:

   int() converts string to integer [OK]

Hint: Use int() to convert string to integer in Python [OK]

Common Mistakes:

• Using str() instead of int()
• Using float() when integer is needed
• Using len() expecting number conversion

3. What will be the output of the following Python code?

def my_atoi(s):
    s = s.strip()
    sign = 1
    if s and s[0] == '-':
        sign = -1
        s = s[1:]
    num = 0
    for char in s:
        if not char.isdigit():
            break
        num = num * 10 + (ord(char) - ord('0'))
    return sign * num

print(my_atoi('  -123abc'))

medium

A. 0

B. 123

C. -123

D. Error

Solution

1. Step 1: Analyze string trimming and sign detection

   The string ' -123abc' is stripped to '-123abc'. The sign is detected as negative and removed from string, leaving '123abc'.

2. Step 2: Convert digits until non-digit character

   Loop reads '1', '2', '3' converting to number 123. Stops at 'a' which is not digit.

3. Final Answer:

   -123 -> Option C

4. Quick Check:

   Parsed number with sign = -123 [OK]

Hint: Stop parsing at first non-digit character [OK]

Common Mistakes:

• Ignoring sign and returning positive number
• Parsing entire string including letters
• Raising error on non-digit characters

4. Find the bug in this atoi implementation and choose the correct fix:

def atoi_buggy(s):
    s = s.strip()
    sign = 1
    if s[0] == '-':
        sign = -1
        s = s[1:]
    num = 0
    for char in s:
        if not char.isdigit():
            return 0
        num = num * 10 + int(char)
    return sign * num

print(atoi_buggy('  -42abc'))

medium

A. Add check if string is empty before accessing s[0]

B. Change return 0 to break inside loop

C. Remove sign variable and always return positive

D. Replace int(char) with ord(char)

Solution

1. Step 1: Identify potential error with empty string

   If input is empty or only spaces, s[0] access causes IndexError. Need to check if s is not empty before accessing.

2. Step 2: Evaluate other fixes

   Changing return 0 to break is good but does not fix crash on empty string. Removing sign loses negative support. Using ord(char) is incorrect for digit conversion.

3. Final Answer:

   Add check if string is empty before accessing s[0] -> Option A

4. Quick Check:

   Check empty string before indexing [OK]

Hint: Always check string length before indexing [OK]

Common Mistakes:

• Not checking empty string before s[0]
• Returning 0 instead of stopping parse
• Ignoring negative sign handling

5. You want to implement atoi that handles leading spaces, optional sign, digits, and clamps output to 32-bit signed integer range [-2^31, 2^31-1]. Which approach correctly applies this?

hard

A. Convert entire string to int directly and then clamp

B. Return 0 if string contains any non-digit characters

C. Ignore sign and clamp only positive values

D. Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647

Solution

1. Step 1: Understand requirements for atoi with clamping

   We must parse string carefully: skip spaces, detect sign, convert digits until non-digit, then clamp to 32-bit integer range.

2. Step 2: Evaluate options for correctness

   Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647 correctly describes parsing and clamping. Convert entire string to int directly and then clamp risks error if string has invalid chars. Ignore sign and clamp only positive values ignores sign which is wrong. Return 0 if string contains any non-digit characters returns 0 too aggressively.

3. Final Answer:

   Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647 -> Option D

4. Quick Check:

   Parse, sign, clamp = correct atoi [OK]

Hint: Clamp after parsing signed integer to avoid overflow [OK]

Common Mistakes:

• Clamping before applying sign
• Not handling non-digit characters properly
• Ignoring integer overflow limits