How to Create Your Own scanf() in C?

The scanf() function is used to take input from the console. It is defined in <stdio.h> header file and takes const char* and variable argument list as parameters.

In this article, we will learn how to implement your own custom scanf() function in C language. For this, we need to have a firm grasp of the following concepts:

Prerequisites: Variadic Functions in C, Strings in C, Format Specifiers in C

Problem Statement: Write your own scanf() in C

Example

Statement 1: scanf("%s %d", str, &in);
Input: Hello_world 10
str = Hello_world, in = 10
 
Statement 2: scanf("%d %s %f", &in, str, &fl);
Input: 10 Geeks 1.1
in = 10, str = Geeks, fl = 1.1

Algorithm:

1. Create a function int myscanf() with const char* and ‘…’ as its parameters. [Here ‘…’ enables the function to receive any number of arguments]
2. Initialize a pointer of type va_list to be able to work with variable arguments.
3. Run a loop through the received string str. Repeat Steps 4 and 5 till we reach the end of str.
4. Store the token when delimiter ‘%’ is found.
5. Check if the second character of the token is a format specifier of scanf(). If true then retrieve its pointer using va_arg and pass it to fscanf().
6. End the va_list pointer.
7. Return 0.

Code:

C

// C program to illustrate the your own scanf() function
// using variable argument list
#include <stdarg.h>
#include <stdio.h>

// custon scanf()
int myscanf(const char* str, ...)
{
    char token[100];
    int k = 0;

    // initializing list pointer
    va_list ptr;
    va_start(ptr, str);

    // parsing the formatted string
    for (int i = 0; str[i] != '\0'; i++) {

        // copying str to token
        token[k++] = str[i];

        // When a format specifier of null character is
        // found
        if (str[i + 1] == '%' || str[i + 1] == '\0') {
            token[k] = '\0';
            k = 0;

            // processing token
            char ch1 = token[1];

            // for integers
            if (ch1 == 'i' || ch1 == 'd' || ch1 == 'u') {
                fscanf(stdin, "%i", va_arg(ptr, int*));
            }

            // for short ubt
            else if (ch1 == 'h') {
                fscanf(stdin, "%hi", va_arg(ptr, short*));
            }

            // for characters
            else if (ch1 == 'c') {
                char c;

                // using this loop to ignore some chars
                while ((c = fgetc(stdin)) == '\n'
                       || c == ' ' || c == EOF) {
                }
                *va_arg(ptr, char*) = c;
            }
            // for float
            else if (ch1 == 'f') {
                fscanf(stdin, "%f", va_arg(ptr, float*));
            }
            else if (ch1 == 'l') {
                char ch2 = token[2];

                // for long int
                if (ch2 == 'u' || ch2 == 'd'
                    || ch2 == 'i') {
                    fscanf(stdin, "%li",
                           va_arg(ptr, long*));
                }

                // for double
                else if (ch2 == 'f') {
                    fscanf(stdin, "%lf",
                           va_arg(ptr, double*));
                }
            }
            else if (ch1 == 'L') {
                char ch2 = token[2];

                // for long int
                if (ch2 == 'u' || ch2 == 'd'
                    || ch2 == 'i') {
                    fscanf(stdin, "%Li",
                           va_arg(ptr, long long*));
                }

                // for long double
                else if (ch2 == 'f') {
                    fscanf(stdin, "%Lf",
                           va_arg(ptr, long double*));
                }
            }

            // for string
            else if (ch1 == 's') {
                fscanf(stdin, "%s", va_arg(ptr, char*));
            }
        }
    }

    // closing va_list
    va_end(ptr);
    return 0;
}

// driver code
int main()
{
    int a;
    float b;
    char c;
    char s[20];
    myscanf(" %i %c %s %f", &a, &c, s, &b);
    printf("%i %c %s %f", a, c, s, b);
    return 0;
}

Input

2023 g Article 4.04

Output

2023 g Article 4.040000

Time Complexity

The time complexity of the following function is: O(n), where,

• N: Length of the formatted string.