String to Integer atoi in DSA C# - Execution Trace

Concept Flow - String to Integer atoi

Start

↓

Skip leading spaces

↓

Check sign (+/-)?

↓

Read digits one by one

↓

Convert char to digit and build number

↓

Check overflow

↓

Apply sign

↓

Return integer result

The process reads the string from left to right, skipping spaces, detecting sign, converting digits to number, checking overflow, and returning the final integer.

Execution Sample

DSA C#

string s = "  -42";
int i = 0, sign = 1, result = 0;
// Skip spaces
// Check sign
// Convert digits
// Return result

This code converts the string " -42" to the integer -42 following the atoi rules.

Execution Table

Step	Operation	Index i	Current Char	Sign	Result	Action	Visual State
1	Start parsing	0	' '	1	0	Skip space	Result=0, i=1
2	Skip space	1	' '	1	0	Skip space	Result=0, i=2
3	Skip space	2	'-'	1	0	Detect sign '-'	Sign=-1, i=3
4	Read digit	3	'4'	-1	0	Convert '4' to 4	Result=4, i=4
5	Read digit	4	'2'	-1	4	Convert '2' to 42	Result=42, i=5
6	End digits	5	End of string	-1	42	Apply sign	Result=-42
7	Return	5	End	-1	-42	Return final integer	Final result = -42

💡 Reached end of digits or string, conversion complete.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
i	0	1	2	3	4	5	5
sign	1	1	1	-1	-1	-1	-1
result	0	0	0	0	4	42	-42

Key Moments - 3 Insights

Why do we skip spaces before processing digits?

Because atoi ignores leading spaces to find the actual number start, as shown in steps 1 and 2 where 'i' moves past spaces before detecting sign or digits.

How do we handle the sign of the number?

At step 3, we check if the current character is '+' or '-'. If '-', sign is set to -1. This sign is applied at the end (step 6) to the result.

What happens if a non-digit character appears after digits?

Conversion stops at the first non-digit after digits. In this example, the string ends after digits, but if a non-digit appeared, we would stop reading digits and return the current result.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'sign' at step 4?

A 1

B 0

C -1

D Undefined

💡 Hint

Check the 'Sign' column at step 4 in the execution_table.

At which step does the result first become 42?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look at the 'Result' column in the execution_table to see when it changes to 42.

If the input string was " +123abc", at which step would conversion stop?

A After reading '3'

B After reading '+'

C After reading 'a'

D At the start

💡 Hint

Conversion stops when a non-digit is found after digits; check how digits are processed in the execution_table.

Concept Snapshot

String to Integer (atoi) conversion:
- Skip leading spaces
- Detect optional '+' or '-' sign
- Convert consecutive digits to number
- Stop at first non-digit after digits
- Handle overflow by clamping
- Return signed integer result

Full Transcript

The atoi function converts a string to an integer by first skipping any leading spaces. Then it checks if the next character is a sign '+' or '-'. It reads digits one by one, converting each character to its numeric value and building the result number. If a non-digit character appears after digits, it stops reading further. The sign is applied at the end. Overflow checks ensure the result fits in integer limits. Finally, the integer is returned.

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