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.