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

String to Integer atoi

📖 Scenario: You are building a simple calculator app that needs to convert user input from text to numbers. Users type numbers as strings, but your app needs to work with actual integers to do math.

🎯 Goal: Learn how to convert a string containing digits into an integer number step-by-step, handling spaces and signs.

📋 What You'll Learn

Create a string variable with a specific value including spaces and digits

Create a variable to track the current position in the string

Write code to skip leading spaces

Detect if the number is negative or positive

Convert the digit characters into an integer number

Print the final integer value

💡 Why This Matters

🌍 Real World

Converting user input from text fields into numbers is common in calculators, forms, and data processing apps.

💼 Career

Understanding how to parse strings into numbers is a fundamental skill for software developers working with input validation and data conversion.

1

Create the input string

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

DSA C++

// Your code here

Hint

Use std::string to create the variable and assign the exact string including spaces and minus sign.

2

Create index variable to track position

Create an integer variable called index and set it to 0 to track the current position in the string.

DSA C++

std::string input = "   -1234abc";
// Your code here

Hint

Use an integer variable to remember where you are in the string as you read characters.

3

Skip spaces and detect sign

Write code to skip all spaces in input starting from index. Then check if the current character is '-' or '+' to set a boolean variable negative accordingly. Increase index if a sign is found.

DSA C++

std::string input = "   -1234abc";
int index = 0;
// Your code here

Hint

Use a while loop to skip spaces. Then use an if statement to check for sign characters.

4

Convert digits to integer and print result

Create an integer variable result set to 0. Use a while loop to read characters from input starting at index. For each digit character, update result by multiplying by 10 and adding the digit value. Stop when a non-digit is found. If negative is true, multiply result by -1. Finally, print result.

DSA C++

std::string input = "   -1234abc";
int index = 0;

while (index < input.size() && input[index] == ' ') {
    index++;
}

bool negative = false;
if (index < input.size() && (input[index] == '-' || input[index] == '+')) {
    negative = (input[index] == '-');
    index++;
}

// Your code here

Hint

Use a loop to read digits and build the number. Remember to apply the sign at the end. Use std::cout to print.

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