Atoi() Function in C++

Syntax of atoi() in C++

The syntax of the atoi() function in C++ is as follows:

int atoi(const char *str);

• str is a pointer to a null-terminated string, which represents the integer you want to convert.
• The function returns the integer value represented by the string. If the string is not a valid integer, it returns 0.

How to Convert String to Integer Using atoi() in C++?

Here’s an example of how to use the atoi() function to convert a string into an integer:

#include <iostream>
#include <cstdlib>  // For atoi()

int main() {
    const char *str = "12345";
    int num = atoi(str);  // Convert string to integer
    std::cout << "Converted integer: " << num << std::endl;
    return 0;
}

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Explanation:

• The atoi() function takes the string "12345" and converts it into the integer 12345.
• The integer result is then printed to the console.

What are the Other Methods to Convert String to Integer?

Besides atoi(), there are several other methods to convert a string to an integer in C++:

1. Using stringstream: C++ provides the stringstream class in the <sstream> library, which can be used to perform string-to-integer conversion.

2. Using stoi(): The stoi() function (introduced in C++11) is another safe and efficient method for converting strings to integers.

3. Using strtol(): The strtol() function (from <cstdlib>) provides more control and error handling compared to atoi().

Using stoi() Function

The stoi() function in C++ is another convenient way to convert a string to an integer, introduced in C++11. It offers better error handling than atoi() and throws exceptions like std::invalid_argument or std::out_of_range when the input is invalid.

Syntax:

int stoi(const std::string& str, size_t* idx = 0, int base = 10);

• str is the string to be converted.
• idx is an optional argument that will store the position of the first invalid character (if any).
• base specifies the numerical base (defaults to 10 for decimal).

Example:

#include <iostream>
#include <string>  // For stoi()

int main() {
    std::string str = "12345";
    int num = std::stoi(str);  // Convert string to integer
    std::cout << "Converted integer: " << num << std::endl;
    return 0;
}

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Explanation:

• The stoi() function converts the string "12345" into the integer 12345. If the string contains non-numeric characters, it throws a std::invalid_argument exception.

How to Write Your Own atoi() Function in C++?

If you want to write your own version of the atoi() function, here’s a step-by-step approach:

• Initialize an integer variable to store the result.
• Iterate through each character in the string.
• Check if the character is a digit (using isdigit()).
• Convert the character to an integer and update the result.
• Handle negative numbers (by checking for a minus sign at the beginning).

Steps:

• Initialize an integer variable (result = 0).
• Loop through each character of the string.
• For each character, check if it’s a digit:
  • Convert the digit ('0' to '9') by subtracting '0'.
  • Multiply the current result by 10 and add the digit.
• If a negative sign is found at the beginning, multiply the result by -1 at the end.
• Return the result.

Sample Examples 

Example 1:

Input : “123456” 
Output: 123456 
Explanation: 
In this example, we have passed "123456" as a string, and the output is 123456, which is of integer type.  

Example 2:

Input: “CodingNinjas” 
Output: 0
Explanation: 
In this example, we have passed "CodingNinjas" as a string, and the output is 0 because the string input is not valid. Therefore the function returns 0.

We will discuss different approaches to implementing atoi() function. 

Approach 1 

Initially, we will implement the atoi() function without considering any special case. 

• Initialize the ans = 0. 
• Start traversing the string or character array and update the ans for each character one by one. 
• For every character update the answer as ans = ans *10 + (s[i] - ‘0’).  
• Finally, return the ans after traversing the complete string.

Implementation in C++

#include<bits/stdc++.h>

using namespace std;
int atoi(string s) {
  // initialise ans = 0
  int ans = 0;
  // traverse the complete string
  // take ascii value of numeric string
  // and update the result
  for (int i = 0; i < s.size(); i++) {
    ans = ans * 10 + (s[i] - '0');
  }
  // finally return the ans.
  return ans;
}

int main() {
  string s = "123456";
  cout << atoi(s) << endl;
}

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Output: 

123456 

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Approach 2 

We will now handle the negative integer strings as well. If the input string is negative, store this in a flag variable and convert the remaining integer string using the previous approach. When returning the ans, check if the string was negative, and multiply the result with -1. Otherwise, return the answer. 

Implementation in C++

#include<bits/stdc++.h>

using namespace std;
int atoi(string s) {
  // initialise ans = 0
  int ans = 0;
  // check for negative string
  bool flag = false;
  // declaring i, to traverse the string
  int i = 0;
  
  // if the number is negative, mark flag as true
  // and move the pointer i, by one.
  if (s[i] == '-') {
    flag = true;
    i++;
  }

  // traverse the complete string
  // take ascii value of numeric string
  // and update the result
  for (i; i < s.size(); i++) {
    ans = ans * 10 + (s[i] - '0');
  }
  // finally return the ans.
  return flag == true ? ans * -1 : ans;
}
int main() {
  string s = "-123456";
  cout << atoi(s) << endl;
}

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Output:

-123456

Also read, Application of Oops

Approach 3 

After implementing negative integers, we will take more corner cases while implementing the atoi() function. Let's look at the different corner cases. 

1. Discarding all leading Whitespaces: It may be possible that strings contain leading white spaces in input. We should remove them, as they do not have any role in output. 
2. Overflow: We will check if the input string is in the range or not because the range of the integers is from INT_MIN to INT_MAX. If the input is greater than INT_MAX/10, return INT_MAX; if the sign is positive, otherwise return INT_MIN. 
3. Invalid Input: If the input string is invalid, then simply returns 0. 

Implementation in C++

#include<bits/stdc++.h>

using namespace std;
int atoi(string str) {
  // declaring the i to traverse the string
  // ans variable to store the final answer  
  int i = 0, ans = 0;
  // declaring the flag variable
  // to check if the input is negative or not
  // if input is negative then isNegative variable is true
  // otherwise false
  bool isNegative = false;
  
  // removing the white spaces
  if (str[i] == ' ') {
    i++;
  }
  
  // checking the sign of the input
  if (str[i] == '-' || str[i] == '+') {
    if (str[i] == '-') {
      isNegative = true;
    }
    i++;
  }
  
  for (i; i < str.size(); i++) {
    // if the number is out of range, then return INT_MAX if the input is positive
    // otherwise return INT_MIN if the input is negative.
    if (ans > INT_MAX / 10) {
      if (isNegative) {
        return INT_MIN;
      } else {
        return INT_MAX;
      }
    }
    
    // checking for validity of the input
    // if input is something other than numbers
    // then simply return 0
    if (str[i] < '0' || str[i] > '9') {
      return 0;
    }
    
    // finally if the input is correct
    // add it to the ans
    ans = ans * 10 + str[i] - '0';
  }
  return isNegative ? ans * -1 : ans;
}
int main() {
  string s = "CodingNinjas";
  cout << atoi(s) << endl;
}

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Output: 

0 

Frequently Asked Questions 

Which sorting algorithm is used when we sort using STL? 

The sorting algorithm used in STL sort() is IntroSort. Intersport is a hybrid sorting algorithm that uses three sorting algorithms to minimize the running time. These algorithms are quicksort, Heapsort, and Insertion Sort.

What atoi() function in C language is present in which library? 

The C programming language's atoi() function translates a string to an integer numerical representation. ASCII to integer is abbreviated as atoi. It's in the 'stdlib' header file of the C standard library.

What is the range of long long int? 

The range of long long int is -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807. 

Write some functions similar to atoi() functions? 

Some functions similar to atoi() function are:  
→ atof function <stdlib.h>
→ atol function <stdlib.h>
→ strtol function <stdlib.h>
→ Cstrtoul function <stdlib.h>

Conclusion

In this article, we discussed the introduction of the atoi() function, discussed some sample examples, and approaches to implementing the atoi() function. 

Recommended Readings:

• Dynamic Binding in C++
• C++ Interview Questions
• Introduction to C++
• Decimal to Binary c++