Converting String to Long in C

Here, we will see how to build a C Program For String to Long Conversion using strtol() function.

Syntax:

long int strtol(char *string, char **ptr, int base)

1. The first argument is given as a string
2. The second argument is a reference to an object of type char*
3. The third argument denotes the base in which the number is represented. To know more about visit strtol() function.

Note: We don’t have to use long int in the case of strtoul() because the range of unsigned long is greater than long on the positive front. 
[long : -2147483648 to 2147483647 and unsigned long : 0 to 4294967295]

Syntax:
strtoul(char *string, char **ptr, int base) // no long int need in strtoul()

C

// C program to demonstrate working of strol()
#include <stdio.h>
#include <stdlib.h>

int main()
{
    // To store Number in String form.
    char string[10] = "1234567890";
    long integer;

    // Base is 10 because we are converting to integer.
    integer = strtol(string, NULL, 10);
    printf("Number is %lu", integer);
}

Output

Number is 1234567890

C

// C program to demonstrate working of strol()
#include <stdio.h>
#include <stdlib.h>

int main()
{
    char string[40] = "100 GeeksforGeeks";
    long integer;
    char* ptr;

    // strtol function to convert number in string form to
    // long integer with base 10
    integer = strtol(string, &ptr, 10);
    printf("Integer part is %lu\n", integer);
    printf("String part is %s\n", ptr);

    return 0;
}

Output

Integer part is 100
String part is  GeeksforGeeks

Method: Using atol() function 

C

#include <stdlib.h>
#include <stdio.h>
int main()
{
    long l;
    char *str;
    str = "349639 geeksforgeeks";
    l = atol(str);
printf("l = %.ld\n",l);
}

Output

l = 349639

Method: Using ltoa() 

C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
    long i = 1234;
    char buffer[100];

    ltoa(i, buffer, 10);
    printf("string is %s\n", buffer);
    return 0;
}

Output

string is 1234

Method: Without Inbuilt Function

C

// C program to demonstrate working of strol()
#include <stdio.h>
#include <stdlib.h>

// Driver Code
int main()
{
    // To store Number in String form.
    char string[10] = "123456789";
    long integer = 0;

    int i = 0;

    // Until null character is encountered
    while (string[i] != '\0') {
        integer = (integer * 10) + (string[i] - '0');
        i++;
    }

    // Printing the number
    printf("Number is %lu", integer);
}

Output

Number is 123456789

Approach name: String to Long Conversion using Custom Algorithm

Steps:

1. Initialize a variable ‘result’ to zero.
2. Iterate through each character of the input string:
   a. Multiply the result by 10.
   b. Convert the current character to a digit by subtracting the ASCII value of ‘0’.
   c. Add the digit to the result.
3. Return the result.

C

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

long stringToLong(char *str) {
    long result = 0;
    int sign = 1, i = 0;
    if (str[0] == '-') {
        sign = -1;
        i++;
    }
    for (; str[i] != '\0'; i++) {
        result = result * 10 + str[i] - '0';
    }
    return sign * result;
}

int main() {
    char str[] = "123456789";
    long result = stringToLong(str);
    printf("%ld\n", result);
    return 0;
}

Output

123456789

Time complexity: O(n), where n is the length of the input string.
Auxiliary space: O(1)

Method: Using sscanf() function

Here’s another approach to convert a string to a long integer in C:

• Include the “stdlib.h” and “stdio.h” header files in your program.
• Define a string variable to hold the input string.
• Use the “sscanf()” function to read the long integer from the input string
• Print the long integer using the “printf()” function.

C

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

int main() {
    char input[100] = "123456789";
    long num;
    sscanf(input, "%ld", &num);
    printf("%ld\n", num);
    return 0;
}

Output

123456789

Time complexity: The time complexity of the “sscanf()” function is O(n), where n is the length of the input string.

Auxiliary space: The space complexity of the program is O(1), as we are not using any additional data structures to perform the conversion.