Write your own atoi()

22 Jun, 2022
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: 
C++
C
#include <bits/stdc++.h>using namespace std; int main(){    int val;    char strn1[] = "12546";     val = atoi(strn1);    cout <<"String value = " << strn1 << endl;    cout <<"Integer value = " << val << endl;     char strn2[] = "GeeksforGeeks";    val = atoi(strn2);    cout <<"String value = " << strn2 << endl;    cout <<"Integer value = " << val <<endl;     return (0);} // This code is contributed by shivanisinghss2110
#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);}
String value = 12546
Integer value = 12546
String value = GeeksforGeeks
Integer value = 0
Time Complexity: O(1)
Auxiliary Space: O(1)
Now let’s understand various ways in which one can create there own atoi() function supported by various conditions:
Approach 1: Following is a simple implementation of conversion without considering any special case. 
Initialize the result as 0.
Start from the first character and update result for every character.
For every character update the answer as result = result * 10 + (s[i] – ‘0’)
C++
C
Java
Python3
C#
Javascript
// A simple C++ program for// implementation of atoi#include <bits/stdc++.h>using namespace std; // A simple atoi() functionint myAtoi(char* str){    // Initialize result    int res = 0;     // Iterate through all characters    // of input string and update result    // take ASCII character of corresponding digit and    // subtract the code from '0' to get numerical    // value and multiply res by 10 to shuffle    // digits left to update running total    for (int i = 0; str[i] != '\0'; ++i)        res = res * 10 + str[i] - '0';     // return result.    return res;} // Driver codeint main(){    char str[] = "89789";       // Function call    int val = myAtoi(str);    cout << val;    return 0;} // This is code is contributed by rathbhupendra
// Program to implement atoi() in C#include <stdio.h> // A simple atoi() functionint myAtoi(char* str){    // Initialize result    int res = 0;     // Iterate through all characters    // of input string and update result    // take ASCII character of corresponding digit and    // subtract the code from '0' to get numerical    // value and multiply res by 10 to shuffle    // digits left to update running total    for (int i = 0; str[i] != '\0'; ++i)        res = res * 10 + str[i] - '0';     // return result.    return res;} // Driver Codeint main(){    char str[] = "89789";       // Function call    int val = myAtoi(str);    printf("%d ", val);    return 0;}
// A simple Java program for// implementation of atoiclass GFG {     // A simple atoi() function    static int myAtoi(String str)    {        // Initialize result        int res = 0;         // Iterate through all characters        // of input string and update result        // take ASCII character of corresponding digit and        // subtract the code from '0' to get numerical        // value and multiply res by 10 to shuffle        // digits left to update running total        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";                        // Function call        int val = myAtoi(str);        System.out.println(val);    }} // This code is contributed by PrinciRaj1992
# 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 range(len(string)):        res = res * 10 + (ord(string[i]) - ord('0'))     return res  # Driver programstring = "89789" # Function callprint (myAtoi(string)) # This code is contributed by BHAVYA JAIN
// A simple C# program for implementation// of atoiusing 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        // take ASCII character of corresponding digit and        // subtract the code from '0' to get numerical        // value and multiply res by 10 to shuffle        // digits left to update running total        for (int i = 0; i < str.Length; ++i)            res = res * 10 + str[i] - '0';         // return result.        return res;    }     // Driver code    public static void Main()    {        string str = "89789";               // Function call        int val = myAtoi(str);        Console.Write(val);    }} // This code is contributed by Sam007.
<script>// A simple Javascript program for// implementation of atoi // A simple atoi() functionfunction myAtoi(str){    // Initialize result        let res = 0;          // Iterate through all characters        // of input string and update result        // take ASCII character of corresponding digit and        // subtract the code from '0' to get numerical        // value and multiply res by 10 to shuffle        // digits left to update running total        for (let i = 0; i < str.length; ++i)            res = res * 10 + str[i].charCodeAt(0) - '0'.charCodeAt(0);          // return result.        return res;} // Driver codelet str = "89789";                  // Function calllet val = myAtoi(str);document.write(val);  // This code is contributed by rag2127</script>
89789
Time Complexity  :  O(n) where n  is the length of string str.
Approach 2: This implementation handles the negative numbers. If the first character is ‘-‘ then store the sign as negative and then convert the rest of the string to number using the previous approach while multiplying sign with it. 
C++
C
Java
Python3
C#
Javascript
// A C++ program for// implementation of atoi#include <bits/stdc++.h>using namespace std; // A simple atoi() functionint 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 (; str[i] != '\0'; i++)        res = res * 10 + str[i] - '0';     // Return result with sign    return sign * res;} // Driver codeint main(){    char str[] = "-123";     // Function call    int val = myAtoi(str);    cout << val;    return 0;} // This is code is contributed by rathbhupendra
// A C program for// implementation of atoi#include <stdio.h> // A simple atoi() functionint 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 (; str[i] != '\0'; ++i)        res = res * 10 + str[i] - '0';     // Return result with sign    return sign * res;} // Driver codeint main(){    char str[] = "-123";       // Function call    int val = myAtoi(str);    printf("%d ", val);    return 0;}
// Java program for// implementation of atoiclass 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();               // Function call        int val = myAtoi(str);        System.out.println(val);    }} // This code is contributed by 29AjayKumar
# 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 range(i, len(string)):        res = res*10+(ord(string[j])-ord('0'))     return sign * res  # Driver codestring = "-123" # Function callprint (myAtoi(string)) # This code is contributed by BHAVYA JAIN
// C# program for implementation of atoiusing 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 code    public static void Main()    {        string str = "-123";               // Function call        int val = myAtoi(str);        Console.Write(val);    }} // This code is contributed by Sam007.
<script>     // JavaScript program for implementation of atoi     // A simple atoi() function    function myAtoi(str)    {          // Initialize result        var res = 0;          // Initialize sign as positive        var sign = 1;          // Initialize index of first digit        var 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].charCodeAt(0) - '0'.charCodeAt(0);          // Return result with sign        return sign * res;    }        // Driver code          var str = "-129";          var val=myAtoi(str);        document.write(val); </script> <! --This code is contributed by nirajgusain5 -->
-123
Time Complexity  :  O(n) where n  is the length of string str.
Approach 3: Four corner cases needs to be handled: 
Discards all leading whitespaces
Sign of the number
Overflow
Invalid input
To remove the leading whitespaces run a loop until a character of the digit is reached. If the number is greater than or equal to INT_MAX/10. Then return INT_MAX if the sign is positive and return INT_MIN if the sign is negative. The other cases are handled in previous approaches. 
Dry Run: 
Below is the implementation of the above approach: 
C++
C
Java
Python3
C#
Javascript
// 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;}  // Driver Codeint main(){    char str[] = "  -123";       // Functional Code    int val = myAtoi(str);    cout <<" "<< val;    return 0;} // This code is contributed by shivanisinghss2110
// A simple C++ program for// implementation of atoi#include <stdio.h>#include <limits.h> 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;}  // Driver Codeint main(){    char str[] = "  -123";       // Functional Code    int val = myAtoi(str);    printf("%d ", val);    return 0;}// This code is contributed by Yogesh shukla.
// A simple Java program for// implementation of atoiclass 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();               // Function call        int val = myAtoi(str);        System.out.printf("%d ", val);    }} // This code is contributed by 29AjayKumar
# A simple Python3 program for# implementation of atoiimport sys def myAtoi(Str):     sign, base, i = 1, 0, 0         # If whitespaces then ignore.    while (Str[i] == ' '):        i += 1         # Sign of number    if (Str[i] == '-' or Str[i] == '+'):        sign = 1 - 2 * (Str[i] == '-')        i += 1     # Checking for valid input    while (i < len(Str) and          Str[i] >= '0' and Str[i] <= '9'):                       # Handling overflow test case        if (base > (sys.maxsize // 10) or           (base == (sys.maxsize // 10) and           (Str[i] - '0') > 7)):            if (sign == 1):                return sys.maxsize            else:                return -(sys.maxsize)                 base = 10 * base + (ord(Str[i]) - ord('0'))        i += 1         return base * sign # Driver CodeStr = list(" -123") # Functional Codeval = myAtoi(Str) print(val) # This code is contributed by divyeshrabadiya07
// A simple C# program for implementation of atoiusing 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
<script>// A simple JavaScript program for// implementation of atoi    function myAtoi(str)   {    var 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 > Number.MAX_VALUE/ 10            || (base == Number.MAX_VALUE / 10            && str[i] - '0' > 7))        {            if (sign == 1)                return Number.MAX_VALUE;            else                return Number.MAX_VALUE;        }        base = 10 * base + (str[i++] - '0');    }    return base * sign;}     // Driver code        var str = " -123";               // Function call        var val = myAtoi(str);        document.write(" ", val);     // This code is contributed by shivanisinghss2110</script>
 -123
Complexity Analysis for all the above Approaches: 
Time Complexity: O(n). Only one traversal of string is needed.
Space Complexity: O(1). As no extra space is required.
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.
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