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In our last article, we have introduced you to some important disk input/output operations we could perform. In this tutorial, we are going to explain how to read the content of an existing file. But before we read the content of an existing file present on the disk, we must open it. For this, the C language provides us a structure called FILE, which is defined in the header file <conio.h>. The FILE structure allows us to create, read, write/modify the file on disk.

The FILE structure provides us a function named fopen(), using which we could provide location of the file stored on the disk and in return(if the file is found), the fopen() function open the file and gives us a pointer which is pointed at the either the first character position in the file or the last character position in the file to be read, depending on the mode in which we open this file. Let's read the file mode to use to read the content of a file.

• Syntax of fopen() function




FILE *fopen(const char *filename, const char *mode)


where filename is the name of the file being searched and mode defines the mode in which the file is opened. As, we have to open a file for reading, hence this mode will take a string value "r".



File Mode	Description
"r"	Searches for the file and opens it in read mode(if the file is found). If the file is not found, NULL is returned and no new file is created.



Using this mode, if this file does not exist on the disk then a NULL is returned and if it already exists then this function returns the pointer to the first character position in the file.






• Reading a file present on Disk


Let us see a code to search for a particular file in the current directory(the same directory where we are going to store the upcoming C program.)



#include<stdio.h>

int main()
{

/* this will find the file in the directory where this C program file is stored */
FILE *fp = fopen("File1.txt", "r"); /*  r = read-only mode */

char ch;

int tab = 0;
int space = 0;
int character = 0;
int newline = 0;

while(2)
{
ch = fgetc(fp);

	if(ch == 't')
	{
		tab = tab + 1;	
	}


	if(ch == '\n')
	{
		newline = newline + 1;
	}

	if(ch == ' ')
	{
		space = space + 1;
	}

	if(ch == EOF)
	{
		break;
	}

	printf("ch : %c \n", ch);
	character = character + 1;
}

/*Closing the file after reading is completed */
fclose(fp);

printf("The total number of characters in a file : %d ", character);
printf("The total number of tabs in a file : %d ", tab);
printf("The total number of newline in a file : %d ", newline);
printf("The total number of spaces in a file : %d ", space);
}




Output




The total number of characters in a file : 95
The total number of tabs in a file : 4
The total number of newline in a file : 3
The total number of spaces in a file : 14






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Program Analysis


As you may see in the output of the code, the program looks for a file named File1.txt in the directory where this C program is stored and if the file is found, it calculates the total of all characters, tabs , newline and spaces in it. To do this -


• We have used a function fgetc(), which takes a FILE pointer and returns the char value in the first position pointed by this pointer and on its next call it returns the char value in the next position pointed by pointer and so on.


• Just after we have concluded reading the file, we have used another function named fclose(), which closes the file being opened for reading and thereby frees the memory that was used for working with this file and flushes the buffer associated with this file(if we were writing into the file - this is for later on).

Searching for a file in any directory on disk

But we could even search for a file in any directory on our disk by specifying its full path with an extra backslash \ in fopen() function, such as -

FILE *fp = fopen("C:\\Users\\Admin\\Desktop\\C Programs\\File2.txt", "r");

• Trying to read a file which does not exist on disk?


Now what happens when we are trying to use FILE structure to read a file which does not exist on disk. In this case, the fopen() function of FILE structure would return a pointer that points to NULL(defined in <conio.h>). The word NULL is capital and it means that the fopen() function is unable to find the file being searched.



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

int main()
{

/* this will find the file in the directory where this C file is stored */
FILE *fp = fopen("File2.txt", "r"); /*Let's say this file doesn't exist on disk */

char ch;

int tab = 0;
int space = 0;
int character = 0;
int newline = 0;



/* This if block will be executed, if the searched file is not found */
if(fp == NULL)
{
	printf("Sorry, the file that you are looking for, does not exist.");	
	exit(1);
}
/* If the file is found, this else block will be executed */
else
{
	while(1)
	{
	ch = getc(fp);

		if(ch == 't')
		{
			tab = tab + 1;	
		}	


		if(ch == '\n')
		{
			newline = newline + 1;
		}
	
		if(ch == ' ')
		{
			space = space + 1;
		}

		if(ch == EOF)
		{
			break;
		}

		printf("ch : %c \n", ch);
		character = character + 1;
	}
}
	/* Closing the file being opened for reading */
	fclose(fp);

	printf("The total number of characters in a file : %d ", character);
	printf("\n");
	printf("The total number of tabs in a file : %d ", tab);
	printf("\n");
	printf("The total number of newline in a file : %d ", newline);
	printf("\n");
	printf("The total number of spaces in a file : %d ", space);
	
return 0;
}




Output




Sorry, the file that you are looking for, does not exist.



The program couldn't find the file named File2.txt in the directory where this program is stored, hence on searching the file, the fopen() function returns NULL, the program displays an appropriate message and exits.

In the next tutorial, we are going to explain how to write the content in a file using the FILE structure.






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