Write your own atoi() - GeeksforGeeks

The atoi() function in C takes a string (which represents an integer) as an argument and returns its value of type int. So basically the function is used to convert a string argument to an integer.

Syntax:

int atoi(const char strn)

Parameters: The function accepts one parameter strn which refers to the string argument that is needed to be converted into its integer equivalent.

Return Value: If strn is a valid input, then the function returns the equivalent integer number for the passed string number. If no valid conversion takes place, then the function returns zero.

Example:

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
  
int main() 
{ 
    int val; 
    char strn1[] = "12546"; 
  
    val = atoi(strn1); 
    printf("String value = %s\n", strn1); 
    printf("Integer value = %d\n", val); 
  
    char strn2[] = "GeeksforGeeks"; 
    val = atoi(strn2); 
    printf("String value = %s\n", strn2); 
    printf("Integer value = %d\n", val); 
  
    return (0); 
} 

Output:

String value = 12546
Integer value = 12546
String value = GeeksforGeeks
Integer value = 0

Now let’s understand various ways in which one can create there own atoi() function supported by various conditions:

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Following is a simple implementation of conversion without considering any special case. We initialize result as 0. We start from the first character and update result for every character.

C

// Program to implement atoi() in C 
#include <stdio.h> 
  
// A simple atoi() function 
int myAtoi(char* str) 
{ 
    int res = 0; // Initialize result 
  
    // Iterate through all characters of input string and 
    // update result 
    for (int i = 0; str[i] != '\0'; ++i) 
        res = res * 10 + str[i] - '0'; 
  
    // return result. 
    return res; 
} 
  
// Driver program to test above function 
int main() 
{ 
    char str[] = "89789"; 
    int val = myAtoi(str); 
    printf("%d ", val); 
    return 0; 
} 

C++

// A simple C++ program for implementation of atoi 
#include <bits/stdc++.h> 
using namespace std; 
  
// A simple atoi() function 
int myAtoi(char* str) 
{ 
    int res = 0; // Initialize result 
  
    // Iterate through all characters of input string and 
    // update result 
    for (int i = 0; str[i] != '\0'; ++i) 
        res = res * 10 + str[i] - '0'; 
  
    // return result. 
    return res; 
} 
  
// Driver code 
int main() 
{ 
    char str[] = "89789"; 
    int val = myAtoi(str); 
    cout << val; 
    return 0; 
} 
  
// This is code is contributed by rathbhupendra 

Java

// A simple Java program for implementation 
// of atoi 
class GFG 
{ 
  
    // A simple atoi() function 
    static int myAtoi(String str) 
    { 
        int res = 0; // Initialize result 
  
        // Iterate through all characters of 
        // input string and update result 
        for (int i = 0; i < str.length(); ++i) 
            res = res * 10 + str.charAt(i) - '0'; 
  
        // return result. 
        return res; 
    } 
  
    // Driver code 
    public static void main(String[] args) 
    { 
        String str = "89789"; 
        int val = myAtoi(str); 
        System.out.println(val); 
    } 
} 
  
// This code is contributed by PrinciRaj1992 

Python

# Python program for implementation of atoi 
  
# A simple atoi() function 
def myAtoi(string): 
    res = 0
  
    # Iterate through all characters of input string and  
    # update result 
    for i in xrange(len(string)): 
        res = res * 10 + (ord(string[i]) - ord('0')) 
  
    return res 
  
# Driver program 
string = "89789"
print myAtoi(string) 
  
# This code is contributed by BHAVYA JAIN 

C#

// A simple C# program for implementation 
// of atoi 
using System; 
  
class GFG { 
  
    // A simple atoi() function 
    static int myAtoi(string str) 
    { 
        int res = 0; // Initialize result 
  
        // Iterate through all characters of 
        // input string and update result 
        for (int i = 0; i < str.Length; ++i) 
            res = res * 10 + str[i] - '0'; 
  
        // return result. 
        return res; 
    } 
  
    // Driver program to test above function 
    public static void Main() 
    { 
        string str = "89789"; 
        int val = myAtoi(str); 
        Console.Write(val); 
    } 
} 
  
// This code is contributed by Sam007. 

Output:

In the below implementation negative numbers have been handled.

C++

// A C++ program for implementation of atoi 
#include <bits/stdc++.h> 
using namespace std; 
  
// A simple atoi() function 
int myAtoi(char* str) 
{ 
    int res = 0; // Initialize result 
    int sign = 1; // Initialize sign as positive 
    int i = 0; // Initialize index of first digit 
  
    // If number is negative, then update sign 
    if (str[0] == '-') { 
        sign = -1; 
        i++; // Also update index of first digit 
    } 
  
    // Iterate through all digits and update the result 
    for (; str[i] != '\0'; ++i) 
        res = res * 10 + str[i] - '0'; 
  
    // Return result with sign 
    return sign * res; 
} 
  
// Driver code 
int main() 
{ 
    char str[] = "-123"; 
    int val = myAtoi(str); 
    cout << val; 
    return 0; 
} 
  
// This is code is contributed by rathbhupendra 

C

// A C program for implementation of atoi 
#include <stdio.h> 
  
// A simple atoi() function 
int myAtoi(char* str) 
{ 
    int res = 0; // Initialize result 
    int sign = 1; // Initialize sign as positive 
    int i = 0; // Initialize index of first digit 
  
    // If number is negative, then update sign 
    if (str[0] == '-') { 
        sign = -1; 
        i++; // Also update index of first digit 
    } 
  
    // Iterate through all digits and update the result 
    for (; str[i] != '\0'; ++i) 
        res = res * 10 + str[i] - '0'; 
  
    // Return result with sign 
    return sign * res; 
} 
  
// Driver program to test above function 
int main() 
{ 
    char str[] = "-123"; 
    int val = myAtoi(str); 
    printf("%d ", val); 
    return 0; 
}

Java

// Java program for implementation of atoi 
class GFG  
{ 
  
// A simple atoi() function 
static int myAtoi(char []str) 
{ 
  
    // Initialize result 
    int res = 0; 
  
    // Initialize sign as positive 
    int sign = 1; 
  
    // Initialize index of first digit 
    int i = 0; 
  
    // If number is negative, then 
    // update sign 
    if (str[0] == '-') 
    { 
        sign = -1; 
  
        // Also update index of first 
        // digit 
        i++; 
    } 
  
    // Iterate through all digits 
    // and update the result 
    for (; i < str.length; ++i) 
        res = res * 10 + str[i] - '0'; 
  
    // Return result with sign 
    return sign * res; 
} 
  
// Driver code 
public static void main(String[] args) 
{ 
    char []str = "-123".toCharArray(); 
    int val = myAtoi(str); 
    System.out.println(val); 
} 
} 
  
// This code is contributed by 29AjayKumar 

Python

# Python program for implementation of atoi 
  
# A simple atoi() function 
def myAtoi(string): 
    res = 0
    # initialize sign as positive 
    sign = 1
    i = 0
  
    # if number is negative then update sign 
    if string[0] == '-': 
        sign = -1
        i+= 1
  
    # Iterate through all characters of input string and update result 
    for j in xrange(i, len(string)): 
        res = res * 10 + (ord(string[j]) - ord('0')) 
  
    return sign * res 
  
# Driver program 
string = "-123"
print myAtoi(string) 
  
# This code is contributed by BHAVYA JAIN 

C#

// C# program for implementation of atoi 
using System; 
  
class GFG { 
  
    // A simple atoi() function 
    static int myAtoi(string str) 
    { 
  
        // Initialize result 
        int res = 0; 
  
        // Initialize sign as positive 
        int sign = 1; 
  
        // Initialize index of first digit 
        int i = 0; 
  
        // If number is negative, then 
        // update sign 
        if (str[0] == '-') { 
            sign = -1; 
  
            // Also update index of first 
            // digit 
            i++; 
        } 
  
        // Iterate through all digits 
        // and update the result 
        for (; i < str.Length; ++i) 
            res = res * 10 + str[i] - '0'; 
  
        // Return result with sign 
        return sign * res; 
    } 
  
    // Driver program to test above function 
    public static void Main() 
    { 
        string str = "-123"; 
        int val = myAtoi(str); 
        Console.Write(val); 
    } 
} 
  
// This code is contributed by Sam007. 

Output:

-123

This implementation handles various type of errors. What if str is NULL or str contains non-numeric characters. Following implementation handles errors.

C++

// A simple C++ program for implementation of atoi 
  
#include <stdio.h> 
  
// A utility function to check whether x is numeric 
bool isNumericChar(char x) 
{ 
    return (x >= '0' && x <= '9') ? true : false; 
} 
  
// A simple atoi() function. If the given string contains 
// any invalid character, then this function returns 0 
int myAtoi(char* str) 
{ 
    if (*str == '\0') 
        return 0; 
  
    int res = 0; // Initialize result 
    int sign = 1; // Initialize sign as positive 
    int i = 0; // Initialize index of first digit 
  
    // If number is negative, then update sign 
    if (str[0] == '-') { 
        sign = -1; 
        i++; // Also update index of first digit 
    } 
  
    // Iterate through all digits of input string and update result 
    for (; str[i] != '\0'; ++i) { 
        if (isNumericChar(str[i]) == false) 
            return 0; // You may add some lines to write error message 
        // to error stream 
        res = res * 10 + str[i] - '0'; 
    } 
  
    // Return result with sign 
    return sign * res; 
} 
  
// Driver program to test above function 
int main() 
{ 
    char str[] = "-134"; 
    int val = myAtoi(str); 
    printf("%d ", val); 
    return 0; 
} 

Java

// A simple Java program for implementation of atoi 
class Atoi 
{ 
  
    // A simple atoi() function. If the given string contains 
    // any invalid character, then this function returns 0 
    public static int atoi(String str) 
    { 
          
        if(str.length() == 0) 
        { 
            return -1; 
        } 
        int sign = 1; // Initialize sign as positive 
        int res = 0; // Initialize result 
        int i = 0; // Initialize index of first digit 
          
        // If number is negative, then update sign 
        if(str.charAt(i)=='-') 
        { 
            sign = -1; 
            i++; // Also update index of first digit 
        } 
          
        // Iterate through all digits of input 
        // string and update result 
        for(; i < str.length(); i++) 
        { 
            if(Character.isDigit(str.charAt(i)) == false) 
            { 
                return -1; 
            } 
            res = res * 10 + Character.getNumericValue(str.charAt(i)); 
        } 
          
        return sign * res; 
          
    } 
      
    // Driver code 
    public static void main(String[] args) 
    { 
        String str = "-134"; 
        int num = atoi(str); 
        System.out.println(num); 
    } 
} 
  
// This code is contributed by Anonymous_Geek 

Python

# Python program for implementation of atoi 
  
# A utility function to check whether x is numeric 
def isNumericChar(x): 
    if (x >= '0' and x <= '9'): 
        return True
    return False
  
# A simple atoi() function. If the given string contains 
# any invalid character, then this function returns 0 
def myAtoi(string): 
    if len(string) == 0: 
        return 0
  
    res = 0
    # initialize sign as positive 
    sign = 1
    i = 0
  
    # if number is negative then update sign 
    if string[0] == '-': 
        sign = -1
        i+= 1
  
    # Iterate through all characters of input string and update result 
    for j in xrange(i, len(string)): 
        # You may add some lines to write error message to error stream 
        if isNumericChar(string[j] == False): 
            return 0
  
        res = res * 10 + (ord(string[j]) - ord('0')) 
  
    return sign * res 
  
# Driver program 
string = "-134"
print myAtoi(string) 
  
# This code is contributed by BHAVYA JAIN 

C#

// A simple C# program for implementation of atoi 
using System; 
  
class Atoi 
{ 
  
    // A simple atoi() function. If the given string contains 
    // any invalid character, then this function returns 0 
    public static int atoi(String str) 
    { 
          
        if(str.Length == 0) 
        { 
            return -1; 
        } 
        int sign = 1; // Initialize sign as positive 
        int res = 0; // Initialize result 
        int i = 0; // Initialize index of first digit 
          
        // If number is negative, then update sign 
        if(str[i]=='-') 
        { 
            sign = -1; 
            i++; // Also update index of first digit 
        } 
          
        // Iterate through all digits of input 
        // string and update result 
        for(; i < str.Length; i++) 
        { 
            if(char.IsDigit(str[i]) == false) 
            { 
                return -1; 
            } 
            res = res * 10 + int.Parse(str[i].ToString()); 
        } 
          
        return sign * res; 
          
    } 
      
    // Driver code 
    public static void Main(String[] args) 
    { 
        String str = "-134"; 
        int num = atoi(str); 
        Console.WriteLine(num); 
    } 
} 
  
// This code is contributed by 29AjayKumar 

Time Complexity: O(n) where n is the number of characters in the input string.

We only need to handle four cases:

Discards all leading whitespaces
sign of the number
overflow
invalid input

Below image is a dry run of the above approach:

Below is the implementation of the above approach:

C++

// A simple C++ program for implementation of atoi 
#include <bits/stdc++.h> 
  
using namespace std; 
int myAtoi(const char* str) 
{ 
    int sign = 1, base = 0, i = 0; 
    // if whitespaces then ignore. 
    while (str[i] == ' ') { 
        i++; 
    } 
    // sign of number 
    if (str[i] == '-' || str[i] == '+') { 
        sign = 1 - 2 * (str[i++] == '-'); 
    } 
    // checking for valid input 
    while (str[i] >= '0' && str[i] <= '9') { 
        // handling overflow test case 
        if (base > INT_MAX / 10 || (base == INT_MAX / 10 && str[i] - '0' > 7)) { 
            if (sign == 1) 
                return INT_MAX; 
            else
                return INT_MIN; 
        } 
        base = 10 * base + (str[i++] - '0'); 
    } 
    return base * sign; 
} 
  
int main() 
{ 
    char str[] = "  -123"; 
    int val = myAtoi(str); 
    printf("%d ", val); 
    return 0; 
} 
// This code is contributed by Yogesh shukla. 

Java

// A simple Java program for implementation of atoi 
class GFG 
{ 
    static int myAtoi(char[] str) 
    { 
        int sign = 1, base = 0, i = 0; 
          
        // if whitespaces then ignore. 
        while (str[i] == ' ')  
        { 
            i++; 
        } 
          
        // sign of number 
        if (str[i] == '-' || str[i] == '+')  
        { 
            sign = 1 - 2 * (str[i++] == '-' ? 1 : 0); 
        } 
          
        // checking for valid input 
        while (i < str.length && str[i] >= '0' && str[i] <= '9')  
        { 
              
            // handling overflow test case 
            if (base > Integer.MAX_VALUE / 10 ||  
                    (base == Integer.MAX_VALUE / 10 &&  
                     str[i] - '0' > 7))  
            { 
                if (sign == 1) 
                    return Integer.MAX_VALUE; 
                else
                    return Integer.MIN_VALUE; 
            } 
            base = 10 * base + (str[i++] - '0'); 
        } 
        return base * sign; 
    } 
  
    // Driver code 
    public static void main(String[] args) 
    { 
        char str[] = " -123".toCharArray(); 
        int val = myAtoi(str); 
        System.out.printf("%d ", val); 
    } 
} 
  
// This code is contributed by 29AjayKumar 

C#

// A simple C# program for implementation of atoi 
using System; 
  
class GFG 
{ 
    static int myAtoi(char[] str) 
    { 
        int sign = 1, Base = 0, i = 0; 
          
        // if whitespaces then ignore. 
        while (str[i] == ' ')  
        { 
            i++; 
        } 
          
        // sign of number 
        if (str[i] == '-' || str[i] == '+')  
        { 
            sign = 1 - 2 * (str[i++] == '-' ? 1 : 0); 
        } 
          
        // checking for valid input 
        while (i < str.Length && str[i] >= '0' && str[i] <= '9')  
        { 
              
            // handling overflow test case 
            if (Base > int.MaxValue / 10 ||  
                    (Base == int.MaxValue / 10 &&  
                    str[i] - '0' > 7))  
            { 
                if (sign == 1) 
                    return int.MaxValue; 
                else
                    return int.MinValue; 
            } 
            Base = 10 * Base + (str[i++] - '0'); 
        } 
        return Base * sign; 
    } 
  
    // Driver code 
    public static void Main(String[] args) 
    { 
        char []str = " -123".ToCharArray(); 
        int val = myAtoi(str); 
        Console.Write("{0} ", val); 
    } 
} 
  
// This code is contributed by 29AjayKumar 

Output :

-123

Time Complexity O(n), where n is length of string.

Recursive program for atoi().

Exercise
Write your won atof() that takes a string (which represents an floating point value) as an argument and returns its value as double.

This article is compiled by Abhay Rathi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
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My Personal Notes

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

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Python

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