String to Integer atoi in DSA C++ - Execution Trace

Concept Flow - String to Integer atoi

Start

↓

Skip leading spaces

↓

Check sign: + or -

↓

Read digits one by one

↓

Convert char to digit and build number

↓

Check overflow

↓

Return final integer

The process starts by ignoring spaces, then checks for a sign, reads digits to build the number, checks for overflow, and finally returns the integer.

Execution Sample

DSA C++

int myAtoi(string s) {
  int i = 0, sign = 1, result = 0;
  while (s[i] == ' ') i++;
  if (s[i] == '+' || s[i] == '-') sign = (s[i++] == '-') ? -1 : 1;
  while (isdigit(s[i])) {
    int digit = s[i++] - '0';
    if (result > (INT_MAX - digit) / 10) return sign == 1 ? INT_MAX : INT_MIN;
    result = result * 10 + digit;
  }
  return sign * result;
}

This code converts a string to an integer by skipping spaces, handling sign, reading digits, checking overflow, and returning the integer.

Execution Table

Step	Operation	Index i	Current Char	Sign	Digit	Result	Overflow Check	Visual State
1	Start	0	' '	1	-	0	No	s=' -42' result=0 sign=1
2	Skip space	1	' '	1	-	0	No	s=' -42' result=0 sign=1
3	Skip space	2	' '	1	-	0	No	s=' -42' result=0 sign=1
4	Check sign	3	'-'	1	-	0	No	s=' -42' result=0 sign=1
5	Set sign to -1	4	'4'	-1	-	0	No	s=' -42' result=0 sign=-1
6	Read digit	4	'4'	-1	4	0	No	s=' -42' result=0 sign=-1
7	Update result	5	'2'	-1	4	4	No	s=' -42' result=4 sign=-1
8	Read digit	5	'2'	-1	2	4	No	s=' -42' result=4 sign=-1
9	Update result	6	End	-1	2	42	No	s=' -42' result=42 sign=-1
10	End digits	6	End	-1	-	42	No	Final result = -42

💡 Reached non-digit or end of string at index 6, returning final result with sign applied.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9	Final
i	0	1	2	3	4	4	5	5	6	6	6
sign	1	1	1	1	1	-1	-1	-1	-1	-1	-1
digit	-	-	-	-	-	-	4	2	-	-	-
result	0	0	0	0	0	0	4	4	42	42	42

Key Moments - 3 Insights

Why do we skip spaces before processing digits?

Because the function ignores leading spaces to find the actual number start, as shown in steps 1-3 in the execution_table where index i moves past spaces.

How does the sign affect the final result?

The sign is set once after skipping spaces (step 5), then multiplied with the result at the end (step 10), changing positive to negative if needed.

Why do we check for overflow before updating the result?

To avoid integer overflow, we check if adding the next digit would exceed INT_MAX or INT_MIN, as seen in step 7 where overflow check prevents invalid results.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'sign' after step 5?

A -1

B 0

C 1

D Undefined

💡 Hint

Check the 'Sign' column at step 5 in the execution_table.

At which step does the function stop reading digits?

A Step 6

B Step 8

C Step 9

D Step 10

💡 Hint

Look at the 'Current Char' and 'Operation' columns to find when non-digit or end is reached.

If the input string started with '+' instead of '-', what would be the 'sign' value after step 5?

A -1

B 1

C 0

D Undefined

💡 Hint

Refer to the sign setting logic in step 5 of the execution_table.

Concept Snapshot

String to Integer (atoi) conversion:
- Skip leading spaces
- Check for optional '+' or '-' sign
- Read digits and convert to number
- Check for overflow before adding digit
- Return number with sign applied

Full Transcript

This visual trace shows how a string is converted to an integer step-by-step. First, spaces are skipped to find the start of the number. Then, the sign is checked and set. Next, digits are read one by one, converted from characters to numbers, and added to the result while checking for overflow. Finally, the result is returned with the correct sign. The variable tracker shows how the index, sign, digit, and result change at each step. Key moments clarify why spaces are skipped, how the sign affects the result, and why overflow checks are important. The quiz tests understanding of sign setting, digit reading end, and sign changes with different inputs.

Practice

1. What does the atoi function do in C++?

easy

A. Converts a string representing a number into an integer

B. Converts an integer into a string

C. Sorts characters in a string alphabetically

D. Calculates the length of a string

Solution

1. Step 1: Understand the purpose of atoi

   The function atoi stands for ASCII to integer and converts a numeric string to an integer value.

2. Step 2: Differentiate from other string functions

   It does not convert integers to strings, sort strings, or calculate string length.

3. Final Answer:

   Converts a string representing a number into an integer -> Option A

4. Quick Check:

   atoi = string to int conversion [OK]

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

Common Mistakes:

• Confusing atoi with itoa (integer to string)
• Thinking atoi sorts or measures strings
• Assuming atoi handles non-numeric strings safely

2. Which of the following is the correct syntax to use atoi in C++?

easy

A. int num = atoi("1234");

B. int num = atoi(1234);

C. int num = atoi('1234');

D. int num = atoi(\"1234\");

Solution

1. Step 1: Check the parameter type for atoi

   The atoi function expects a C-style string (const char*), so the argument must be a string literal like "1234".

2. Step 2: Analyze each option

   int num = atoi("1234"); passes a string literal correctly. int num = atoi(1234); passes an integer, which is invalid. int num = atoi('1234'); uses single quotes, which denote a char, not a string. int num = atoi(\"1234\"); has escaped quotes incorrectly.

3. Final Answer:

   int num = atoi("1234"); -> Option A

4. Quick Check:

   atoi needs a string literal argument [OK]

Hint: atoi requires a string literal, not a number or char [OK]

Common Mistakes:

• Passing integer instead of string
• Using single quotes instead of double quotes
• Incorrect escaping of quotes

3. What will be the output of the following C++ code?

char str[] = "-42abc";
int val = atoi(str);
std::cout << val << std::endl;

medium

A. 0

B. Runtime error

C. 42

D. -42

Solution

1. Step 1: Understand atoi behavior with non-digit characters

   atoi converts the initial part of the string that forms a valid integer. It stops reading when it encounters a non-digit after the number.

2. Step 2: Analyze the string "-42abc"

   It starts with '-', then '4', '2' which form -42. The characters 'a', 'b', 'c' are ignored after the number.

3. Final Answer:

   -42 -> Option D

4. Quick Check:

   atoi stops at first non-digit after number [OK]

Hint: atoi reads number until non-digit character [OK]

Common Mistakes:

• Expecting atoi to parse entire string including letters
• Assuming atoi returns 0 for strings with trailing letters
• Thinking it throws runtime error on letters

4. Identify the error in the following code snippet that uses atoi:

std::string s = "123";
int num = atoi(s);
std::cout << num << std::endl;

medium

A. Missing header file for atoi

B. atoi requires integer argument

C. Cannot pass std::string directly to atoi

D. No error, code runs fine

Solution

1. Step 1: Check parameter type for atoi

   atoi expects a C-style string (const char*), but s is a std::string object.

2. Step 2: Correct usage

   To use atoi with std::string, call s.c_str() to get a C-style string.

3. Final Answer:

   Cannot pass std::string directly to atoi -> Option C

4. Quick Check:

   atoi needs const char*, not std::string [OK]

Hint: Use s.c_str() to convert std::string for atoi [OK]

Common Mistakes:

• Passing std::string directly to atoi
• Ignoring need for c_str() method
• Assuming atoi accepts std::string

5. You want to convert a string to integer but also handle invalid inputs safely without crashing. Which approach is best in C++?

hard

A. Use atoi and check if result is zero to detect errors

B. Use stoi inside try-catch block to catch exceptions

C. Use atoi and ignore invalid inputs

D. Use printf to print the string and guess the number

Solution

1. Step 1: Understand limitations of atoi

   atoi returns 0 for invalid inputs but cannot distinguish between actual zero and error, so it is unsafe for error handling.

2. Step 2: Use stoi with exception handling

   stoi throws exceptions on invalid input, allowing safe detection and handling of errors using try-catch blocks.

3. Final Answer:

   Use stoi inside try-catch block to catch exceptions -> Option B

4. Quick Check:

   stoi with try-catch is safe for invalid input [OK]

Hint: Use stoi with try-catch for safe string to int conversion [OK]

Common Mistakes:

• Relying on atoi to detect invalid input
• Ignoring exceptions from stoi
• Using printf to guess numbers