String to Integer atoi in DSA C# - Build from Scratch

String to Integer atoi

📖 Scenario: Imagine you are building a simple calculator app. Users enter numbers as text, but your program needs to convert these text numbers into actual numbers to do math.

🎯 Goal: Build a program that converts a string containing digits into an integer number, similar to the atoi function.

📋 What You'll Learn

Create a string variable with a numeric value

Create an integer variable to store the result

Write code to convert the string to an integer without using built-in conversion methods

Print the integer result

💡 Why This Matters

🌍 Real World

Converting user input from text to numbers is common in calculators, form inputs, and data parsing.

💼 Career

Understanding how to manually convert strings to integers helps in debugging, working with low-level data, and improving algorithmic thinking.

1

Create the input string

Create a string variable called input and set it to the value "1234".

DSA C#

# Your code here

Hint

Use string input = "1234"; to create the input string.

2

Create the integer result variable

Create an integer variable called result and set it to 0.

DSA C#

string input = "1234";
# Your code here

Hint

Use int result = 0; to create the result variable.

3

Convert string to integer

Use a for loop with variable i to iterate over each character in input. Inside the loop, update result by multiplying it by 10 and adding the integer value of the current character (use input[i] - '0').

DSA C#

string input = "1234";
int result = 0;
# Your code here

Hint

Loop through each character and convert it to a number by subtracting '0'. Then update result.

4

Print the integer result

Write Console.WriteLine(result); to print the integer value stored in result.

DSA C#

string input = "1234";
int result = 0;
for (int i = 0; i < input.Length; i++)
{
    result = result * 10 + (input[i] - '0');
}
# Your code here

Hint

Use Console.WriteLine(result); to display the number.

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