String to Integer atoi in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this atoi implementation for input " -42abc"?

Consider the following C# code that converts a string to an integer similar to atoi. What will be printed?

DSA C#

using System;

class Program {
    static int MyAtoi(string s) {
        int i = 0, sign = 1, result = 0;
        while (i < s.Length && s[i] == ' ') i++;
        if (i < s.Length && (s[i] == '+' || s[i] == '-')) {
            sign = s[i] == '-' ? -1 : 1;
            i++;
        }
        while (i < s.Length && Char.IsDigit(s[i])) {
            int digit = s[i] - '0';
            if (result > (Int32.MaxValue - digit) / 10) {
                return sign == 1 ? Int32.MaxValue : Int32.MinValue;
            }
            result = result * 10 + digit;
            i++;
        }
        return sign * result;
    }

    static void Main() {
        Console.WriteLine(MyAtoi("  -42abc"));
    }
}

A -42

B 0

C 42

D -420

💡 Hint

Focus on how the code skips spaces, handles sign, and stops reading digits at the first non-digit.

The code skips leading spaces, detects the '-' sign, then reads digits '4' and '2'. It stops at 'a' which is not a digit. So the result is -42.

❓ Predict Output

intermediate

What does this atoi code return for input "4193 with words"?

Analyze the following C# atoi-like function and determine the output when input is "4193 with words".

DSA C#

using System;

class Program {
    static int MyAtoi(string s) {
        int i = 0, sign = 1, result = 0;
        while (i < s.Length && s[i] == ' ') i++;
        if (i < s.Length && (s[i] == '+' || s[i] == '-')) {
            sign = s[i] == '-' ? -1 : 1;
            i++;
        }
        while (i < s.Length && Char.IsDigit(s[i])) {
            int digit = s[i] - '0';
            if (result > (Int32.MaxValue - digit) / 10) {
                return sign == 1 ? Int32.MaxValue : Int32.MinValue;
            }
            result = result * 10 + digit;
            i++;
        }
        return sign * result;
    }

    static void Main() {
        Console.WriteLine(MyAtoi("4193 with words"));
    }
}

A 41930

B 419

C 4193

D 0

💡 Hint

The function reads digits until a non-digit character is found.

The function reads '4', '1', '9', '3' then stops at the space before 'with'. So it returns 4193.

❓ Predict Output

advanced

What is the output when input is "-91283472332" (overflow case)?

Given this atoi implementation, what will be the output for input string "-91283472332"?

DSA C#

using System;

class Program {
    static int MyAtoi(string s) {
        int i = 0, sign = 1, result = 0;
        while (i < s.Length && s[i] == ' ') i++;
        if (i < s.Length && (s[i] == '+' || s[i] == '-')) {
            sign = s[i] == '-' ? -1 : 1;
            i++;
        }
        while (i < s.Length && Char.IsDigit(s[i])) {
            int digit = s[i] - '0';
            if (result > (Int32.MaxValue - digit) / 10) {
                return sign == 1 ? Int32.MaxValue : Int32.MinValue;
            }
            result = result * 10 + digit;
            i++;
        }
        return sign * result;
    }

    static void Main() {
        Console.WriteLine(MyAtoi("-91283472332"));
    }
}

A -2147483648

B -91283472332

C 2147483647

D 0

💡 Hint

Check how the code handles integer overflow for negative numbers.

The number is less than Int32.MinValue, so the function returns Int32.MinValue (-2147483648).

❓ Predict Output

advanced

What is the output for input "+-12"?

Analyze the atoi code below. What will it output for the input string "+-12"?

DSA C#

using System;

class Program {
    static int MyAtoi(string s) {
        int i = 0, sign = 1, result = 0;
        while (i < s.Length && s[i] == ' ') i++;
        if (i < s.Length && (s[i] == '+' || s[i] == '-')) {
            sign = s[i] == '-' ? -1 : 1;
            i++;
        }
        while (i < s.Length && Char.IsDigit(s[i])) {
            int digit = s[i] - '0';
            if (result > (Int32.MaxValue - digit) / 10) {
                return sign == 1 ? Int32.MaxValue : Int32.MinValue;
            }
            result = result * 10 + digit;
            i++;
        }
        return sign * result;
    }

    static void Main() {
        Console.WriteLine(MyAtoi("+-12"));
    }
}

A 12

B 0

C -12

D Exception

💡 Hint

The code only accepts one optional sign character before digits.

The code reads '+' as sign, then sees '-' which is not a digit, so it stops and returns 0.

🧠 Conceptual

expert

Why does the atoi implementation check overflow with condition 'result > (Int32.MaxValue - digit) / 10'?

In the atoi code, before adding a new digit, it checks if 'result > (Int32.MaxValue - digit) / 10'. Why is this check necessary?

A To skip leading whitespace characters

B To ensure the input string contains only digits

C To handle negative numbers correctly

D To prevent integer overflow when multiplying result by 10 and adding digit

💡 Hint

Think about what happens if you multiply and add digits beyond the max integer limit.

Multiplying result by 10 and adding digit can exceed Int32.MaxValue, causing overflow. The check prevents this by returning max/min values.

Practice

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

easy

A. Reverses the string characters

B. Converts the string to a floating-point number

C. Converts the string to an integer number

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: Differentiate from other string operations

   It does not reverse or count characters, nor convert to floating-point; it specifically converts to integer.

3. Final Answer:

   Converts the string to an integer number -> Option C

4. Quick Check:

   atoi converts string to integer [OK]

Hint: atoi always converts string digits to integer number [OK]

Common Mistakes:

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

2. Which of the following C# code snippets correctly converts a string "1234" to an integer using atoi logic?

easy

A. int num = Convert.ToString(1234);

B. int num = "1234".ToInt();

C. int num = int.ToString("1234");

D. int num = int.Parse("1234");

Solution

1. Step 1: Identify correct C# method for string to int

   int.Parse(string) converts a numeric string to an integer correctly.

2. Step 2: Check other options for syntax errors

   "1234".ToInt() and int.ToString("1234") are invalid methods; Convert.ToString(1234) converts int to string, not string to int.

3. Final Answer:

   int num = int.Parse("1234"); -> Option D

4. Quick Check:

   int.Parse converts string to int [OK]

Hint: Use int.Parse to convert string digits to int in C# [OK]

Common Mistakes:

• Using invalid method names like ToInt()
• Confusing Convert.ToString with conversion to int
• Trying to call ToString on string input

3. What is the output of the following C# code?

string s = "  -42abc";
int result = MyAtoi(s);
Console.WriteLine(result);

int MyAtoi(string str) {
    int i = 0, sign = 1, num = 0;
    while (i < str.Length && str[i] == ' ') i++;
    if (i < str.Length && (str[i] == '+' || str[i] == '-')) {
        sign = (str[i] == '-') ? -1 : 1;
        i++;
    }
    while (i < str.Length && char.IsDigit(str[i])) {
        num = num * 10 + (str[i] - '0');
        i++;
    }
    return sign * num;
}

medium

A. Exception thrown

B. -42

C. 42

D. 0

Solution

1. Step 1: Skip leading spaces and detect sign

   The code skips spaces, finds '-', sets sign to -1, then moves to digits.

2. Step 2: Parse digits until non-digit

   It reads '4' and '2', stops at 'a', so number is 42, multiplied by sign -1 gives -42.

3. Final Answer:

   -42 -> Option B

4. Quick Check:

   Parsed number with sign = -42 [OK]

Hint: Skip spaces, check sign, parse digits until non-digit [OK]

Common Mistakes:

• Not skipping spaces before sign
• Parsing non-digit characters
• Ignoring sign character

4. Identify the bug in this MyAtoi function and select the fix:

int MyAtoi(string str) {
    int i = 0, sign = 1, num = 0;
    while (str[i] == ' ') i++;
    if (str[i] == '+' || str[i] == '-') {
        sign = (str[i] == '-') ? -1 : 1;
        i++;
    }
    while (char.IsDigit(str[i])) {
        num = num * 10 + (str[i] - '0');
        i++;
    }
    return sign * num;
}

medium

A. Add boundary checks for index i before accessing str[i]

B. Remove the sign check to simplify code

C. Change char.IsDigit to char.IsLetter

D. Initialize num to -1 instead of 0

Solution

1. Step 1: Check for index out of range errors

   The code accesses str[i] without checking if i < str.Length, risking runtime error.

2. Step 2: Fix by adding boundary checks in all loops

   Adding conditions like i < str.Length in while loops prevents out-of-range exceptions.

3. Final Answer:

   Add boundary checks for index i before accessing str[i] -> Option A

4. Quick Check:

   Always check index bounds before accessing string [OK]

Hint: Always check string length before accessing characters [OK]

Common Mistakes:

• Ignoring string length checks causing exceptions
• Removing sign check breaks negative numbers
• Using char.IsLetter instead of char.IsDigit

5. Given the input string "+-12", what should a robust atoi implementation return according to standard rules?

hard

A. 0

B. -12

C. 12

D. Exception thrown

Solution

1. Step 1: Analyze sign characters in input

   The string starts with '+' followed immediately by '-', which is invalid for atoi parsing.

2. Step 2: According to atoi rules, invalid sign sequence returns 0

   Parsing stops at invalid sequence, so result is 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Invalid sign sequence returns 0 [OK]

Hint: Invalid sign sequences return 0 in atoi [OK]

Common Mistakes:

• Parsing multiple signs as valid
• Returning partial number ignoring invalid signs
• Throwing exceptions instead of returning 0