File I/O and Command Line Arguments #

Commandline Argument #

An easier way to take input from the user in shell.

Commandline Argument #

#include <stdio.h>
int main(int argc, char* argv[]) {
    printf("The number of arguments is %d\n", argc);

    for (int i = 0; i < argc; i++) {
        printf("%d Argument: %s\n", i, argv[i]);    
    }
    return 0;
}

Problem #

Write a program that takes the First Name Last Name Age as commandline arguments and prints it as follows First Name:
Last name :
Age :

Solution #

#include "stdio.h"

int main(int argc, char* argv[]) {
    
    if (argc != 4) {
        printf("Incorrect number of arguments provided.\n");
        return 0;
    }

    printf("First Name:\t%s\n", argv[1]);
    printf("Last Name :\t%s\n", argv[2]);
    printf("Age       :\t%s\n", argv[3]);
    return 0;
 }

Why store data in files? #

Much larger data storage than RAM.
Persist accross different executions of the program.
Work with other programs.

Opening/Closing a file #

#include <stdlib.h>
#include <stdio.h>
/* File pointer to hold reference to our file */
FILE * fPtr;
// Open file in w (write) mode. "data/file1.txt"  is complete path to create file
fPtr = fopen("data/file1.txt", "w");
/* fopen() return NULL if last operation  was unsuccessful */
if(fPtr == NULL)
{
    /* File not created hence exit */
    printf("Unable to create file.\n");
    exit(0);
}
/* Done with this file, close file to release resource */
fclose(fPtr);

Reading from file #

fgetc() – Used to read single character from file.
fgets() – Used to read string from file.
fscanf() – Use this to read formatted input from file.
fread() – Read block of raw bytes from file. Used to read binary files.

Reading #

Open a file using fopen() function and store its reference in a FILE pointer say fPtr.
You must open file in r(read) mode or atleast mode that support read access.
Read content from file using any of these functions fgetc(), fgets(), fscanf() or fread(). Finally, close the file using fclose(fPtr).

Reading from file, char by char #

do {
    /* Read single character from file */
    ch = fgetc(fPtr);
    /* Print character read on console */
    putchar(ch);
} while(ch != EOF); /* Repeat this 
if last read character is not EOF */