How to Write Your Own printf() in C?
Last Updated :
14 Jul, 2023
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In this article, we will learn how to implement your own custom printf() function in C language.
The printf() function is present in <stdio.h> header and uses a const char* and variable length arguments as its parameters. Hence we would be using <stdarg.h> header file which provides operations available on variable arguments passed to a function.
Prerequisites: Variadic Functions in C, Strings in C
Problem Statement: Write your own printf() function in C
Example Outputs:
Code 1: myprintf("Hello world %i",404);
Output: Hello world 404.
Code 2: char s[]="Multiverse is real";
myprintf("%s Hello %.3f",s,2.3);
Output: Multiverse is real Hello 2.30
Algorithm:
- Create a function int myprintf() with const char* and ‘…’ as its parameters. (‘. . .’ enables the function to receive any number of arguments).
- Initialize a pointer of type va_list to work with variable arguments. Pass the pointer to va_start.
- Process the passed string with a delimiter as ‘%’ or ‘\0’, and store it in a token string.
- For every token starting with ‘%’ check if its next characters are a format specifier defined in C. It can be ‘i’,’s’,’lf’.
- Pass the token string and value retrieved from va_arg() to fprintf() in order to print the required value to the console.
- Once the whole string is processed, end the va_arg pointer.
- Return 0.
Code:
C
// C program to implement your own printf() function #include <stdarg.h> #include <stdio.h> // your own printf int myprintf(const char* str, ...) { // initializing list pointer va_list ptr; va_start(ptr, str); // char array to store token char token[1000]; // index of where to store the characters of str in // token int k = 0; // parsing the formatted string for (int i = 0; str[i] != '\0'; i++) { token[k++] = str[i]; if (str[i + 1] == '%' || str[i + 1] == '\0') { token[k] = '\0'; k = 0; if (token[0] != '%') { fprintf( stdout, "%s", token); // printing the whole token if // it is not a format specifier } else { int j = 1; char ch1 = 0; // this loop is required when printing // formatted value like 0.2f, when ch1='f' // loop ends while ((ch1 = token[j++]) < 58) { } // for integers if (ch1 == 'i' || ch1 == 'd' || ch1 == 'u' || ch1 == 'h') { fprintf(stdout, token, va_arg(ptr, int)); } // for characters else if (ch1 == 'c') { fprintf(stdout, token, va_arg(ptr, int)); } // for float values else if (ch1 == 'f') { fprintf(stdout, token, va_arg(ptr, double)); } else if (ch1 == 'l') { char ch2 = token[2]; // for long int if (ch2 == 'u' || ch2 == 'd' || ch2 == 'i') { fprintf(stdout, token, va_arg(ptr, long)); } // for double else if (ch2 == 'f') { fprintf(stdout, token, va_arg(ptr, double)); } } else if (ch1 == 'L') { char ch2 = token[2]; // for long long int if (ch2 == 'u' || ch2 == 'd' || ch2 == 'i') { fprintf(stdout, token, va_arg(ptr, long long)); } // for long double else if (ch2 == 'f') { fprintf(stdout, token, va_arg(ptr, long double)); } } // for strings else if (ch1 == 's') { fprintf(stdout, token, va_arg(ptr, char*)); } // print the whole token // if no case is matched else { fprintf(stdout, "%s", token); } } } } // ending traversal va_end(ptr); return 0; } // driver code int main() { myprintf("Integer: %d\nString: %s\nFloat: %0.2f\n", 10, "GeeksforGeeks", 12.2548); return 0; } |
Output
Integer: 10 String: GeeksforGeeks Float: 12.25
Time Complexity:
The time complexity of the above code is: O(n * L), where,
- n: length of format specifier. For example, If %0.5f then n=3.
- L: Length of the output.
In most practical cases, the time complexity is O(1).
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