atoi() Function in C

In C, atoi stands for ASCII To Integer. The atoi() is a library function in C that converts the numbers in string form to their integer value. To put it simply, the atoi() function accepts a string (which represents an integer) as a parameter and yields an integer value in return.

C atoi() function is defined inside <stdlib.h> header file.

Syntax of atoi() Function

int atoi(const char *strg);

Parameters

• strg: The string that is to be converted to an integer.

Note: The atoi function takes in a string as a constant (Unchangeable) and gives back the converted integer without modifying the original string.

Return Value

The atoi() function returns the following value:

• An equivalent integer value by interpreting the input string as a number only if the input string str is Valid.
• If the conversion is not valid, the function returns 0.

Note: In the case of an overflow the returned value is undefined.

Examples of atoi() in C

Example 1:

This example shows how to convert numbers in String form to their Integer values.

C

// C program to illustrate the use of atoi()

#include <stdio.h>
#include <stdlib.h>

int main()
{

    // string to be converted
    char strToConvert[] = "908475966";

    // converting string using atoi()
    int ConvertedStr = atoi(strToConvert);

    // printing the Converted String
    printf("String to be Converted: %s\n", strToConvert);
    printf("Converted to Integer: %d\n", ConvertedStr);

    return 0;
}

Output

String to be Converted: 908475966
Converted to Integer: 908475966

Explanation: The string “99898989” is converted to the corresponding integer value, and both the original string and the returned integer value are printed.

Example 2:

This example shows if the input string contains non-numeric characters so the function returns 0.

C

// C program to illustrate the use of atoi()

#include <stdio.h>
#include <stdlib.h>

int main()
{
    // string to be converted
    char strToConvert[] = "geeksforgeeks";

    // converting string using atoi()
    int ConvertedStr = atoi(strToConvert);

    // printing the Converted String
    printf("String to be Converted: %s\n", strToConvert);
    printf("Converted to Integer: %d\n", ConvertedStr);

    return 0;
}

Output

String to be Converted: geeksforgeeks
Converted to Integer: 0

Explanation: The string “geeksforgeeks” is attempted to be converted to an integer but the string contains non-numeric characters so the function returns 0.

How to Implement Your Own atoi() in C?

The equivalent of the atoi() function is easy to implement. One of the possible method to implement is shown below:

Approach

1. Initialize res to 0.
2. Iterate through each character of the string and update the res by multiplying it by 10 and keep adding the numeric value of the current digit. (res = res*10+(strg[i]-‘0’))
3. Continue until the end of the string.
4. return res.

Implementation of atoi() in C

C

// C program to Implement Custom atoi()

#include <stdio.h>

int atoi_Conversion(const char* strg)
{

    // Initialize res to 0
    int res = 0;
    int i = 0;

    // Iterate through the string strg and compute res
    while (strg[i] != '\0') {
        res = res * 10 + (strg[i] - '0');
        i++;
    }
    return res;
}

int main()
{
    const char strg[] = "12345";
    int value = atoi_Conversion(strg);

    // print the Converted Value
    printf("String to be Converted: %s\n", strg);
    printf("Converted to Integer: %d\n", value);
    return 0;
}

Output

String to be Converted: 12345
Converted to Integer: 12345

To learn more about different approaches, refer to the article – Write your own atoi()

Properties of atoi() Function

The atoi() function have different output for different input types. The following examples shown the behaviors of the atoi() function for different input values.

1. The atoi() function does not recognize decimal points or exponents.

C

// C program to illustrate the Property 1: Passing the
// decimal point string to atoi()

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    int res_val;
    char inp_str[30];

    // Initialize the input string
    strcpy(inp_str, "12.56");

    // Converting string to integer using atoi()
    res_val = atoi(inp_str);

    // print result
    printf("Input String = %s\nResulting Integer = %d\n",
           inp_str, res_val);
    return 0;
}

Output

Input String = 12.56
Resulting Integer = 12

2. Passing invalid string to atoi() returns 0.

C

// C program to illustrate the Property 2: Passing Invalid
// string to atoi()

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    int res_val;
    char inp_str[30];

    // Initialize the input string
    strcpy(inp_str, "geeksforgeeks");

    // Converting string to integer using atoi()
    res_val = atoi(inp_str);

    // print result
    printf("Input String = %s\nResulting Integer = %d\n",
           inp_str, res_val);
    return 0;
}

Output

Input String = geeksforgeeks
Resulting Integer = 0

3. Passing partially valid string results in conversion of only integer part. (If non-numerical values are at the beginning then it returns 0)

C

// C program to illustrate the Property 3: Passing Partially
// valid String to atoi()

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    int res_val;
    char inp_str[30];

    // Initializing the input string
    strcpy(inp_str, "1234adsnds");

    // Convert string to integer using atoi() and store the
    // result in result_value
    res_val = atoi(inp_str);

    printf("Input String = %s\nResulting Integer = %d\n",
           inp_str, res_val);
    return 0;
}

Output

Input String = 1234adsnds
Resulting Integer = 1234

4. Passing string beginning with the character ‘+’ to atoi() then ‘+’ is ignored and only integer value is returned.

C

// C program to illustrate Property 4: Passing String
// beginning with the character '+' to atoi()

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    int res_val;
    char inp_str[30];

    // Initializing the input string
    strcpy(inp_str, "+23234");

    // Convert string to integer using atoi() and store the
    // result in result_value
    res_val = atoi(inp_str);

    printf("Input String = %s\nResulting Integer = %d\n",
           inp_str, res_val);
    return 0;
}

Output

Input String = +23234
Resulting Integer = 23234

5. Passing string beginning with the character ‘-‘ to atoi() then the result includes ‘-‘ at the beginning.

C

// C program to illustrate Property 5: Passing String
// beginning with the character '-' passed to atoi()

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    int res_val;
    char inp_str[30];

    // Initializing the input string
    strcpy(inp_str, "-23234");

    // Convert string to integer using atoi() and store the
    // result in result_value
    res_val = atoi(inp_str);

    printf("Input String = %s\nResulting Integer = %d\n",
           inp_str, res_val);
    return 0;
}

Output

Input String = -23234
Resulting Integer = -23234

Conclusion

• atoi() function converts numeric string to its corresponding integer value.
• To use the atoi() function in your C program include the stdlib.h header file.
• atoi() accepts only one string parameter as input.
• If an invalid string is passed as parameter, the function returns 0.