Question

Reading and Writing Complete Files in C:

The first part of the lab is to write a program to read the complete contents of a file to a string. This code will be used in subsequent coding problems. You will need 3 functions: main(), read_file() and write_file(). The main function contains the driver code. The read_file() function reads the complete contents of a file to a string. The write_file() writes the complete contents of a string to a file. The read_file() will require:

Open the file for reading
Calculating the size of a file
Allocating memory to read the file to a string
Rewinding the file to the beginning
Reading the files contents to the allocated string
Close the file

There are a few ways to calculate the length of a file. Beware that some of these methods actually get the last position of the input buffer, which may not be the file length for very large files. I suggest using getc(file_name) until EOF is read with a counter. getc() reads one char at a time from a data stream. EOF will be read at the end of the text file. The rewind(file_name) function returns the file pointer to the beginning of the file. You can use getc() or fgets() to read the file into the string. Writing the file will require:

Open the file for writing
Write the string to file
Close the file

You can use putc() or fputs().

The main function should free() allocated memory before returning. Test your program by reading and writing a few files. Make sure to give the written file different names than the read files so you can compare them.

Answer 1

The above problem can be solved in the following steps-

STEP 1- First we will code the read_file() method. In this function, we will first read a file using file pointer.

We will use read permission to open the file. Then traverse the entire file to compute the length of the file.

Then using dynamic memory allocation, create an array of characters of size=length of file.

Use rewind() function to get to the beginning of file. And again start traversing the file and store each character of file in character array declared above.

Finally, print the contents of character array.

STEP 2- Here we will code the write_file() method. We will use file pointer to open a file with write permission.

Then declare a character array. And ask the user to enter a sentence.

Then using fputs() method, put this sentence in the file opened. And finally close the file pointer.

Check the file if the sentence was written properly.

STEP 3- In the main method, we will call the read_file() and write_file() methods.

C CODE-

#include <stdio.h>
#include <stdlib.h>
//fuction to read file
void read_file(){
   //create a FILE pointer
   FILE *fptr;
   //character variable
   char ch;
   //to store the length of file
int lengthFile = 0;
//open a file, here file name is scores.txt and open it with read permission
fptr = fopen( "read.txt" , "r" );
  
//check if fptr is NULL, means there was error in opening file
   if(fptr == NULL){
       printf("Error opening file");
       //exit the code
       exit(0);
   }
  
//run a loop till the file ptr eaches EOF
    while(1) {
        //store the character
   ch = fgetc(fptr);
   //if we've reached End of File, break the loop
   if( ch == EOF ) {
      break ;
       }
       //increment the length of file
       lengthFile++;
   }
   //char array declared using dynamic allocation
char *s = (char *)malloc(lengthFile * sizeof(char));
//rewind the file ptr to point to beginning of file
rewind(fptr);
printf("\n");
//counter for string
int i = 0;
//again run a loop to traverse the file
while(1) {
        //store the character
        ch = fgetc(fptr);
        //check if we've reached End of file
   if( ch == EOF ) {
   break ;
   }
   //store the character to string
   s[i] = ch;
   i++; //increment counter
}
//display the file string
for(int k=0;k<lengthFile;k++){
    printf("%c",s[k]);
}
//close the file pointer
fclose(fptr);
}
//function to write to a file
void write_file(){
   //file pointer
   FILE *fptr;
   //character variable
   char ch;
   //open a file with write permission
   fptr = fopen("write.txt","w");
   //declare a character array
   char str[10000];
   //check if fptr is NULL, means there was error in opening file
   if(fptr == NULL){
       printf("Error opening file");
       //exit the code
       exit(0);
   }
   //prompt user to enter a sentence
   printf("\n\nEnter sentence to be written : ");
  
   fgets(str, sizeof(str), stdin);   //store the user input
  
   //put the string onto the file
   fputs(str, fptr);
   //close the file
   fclose(fptr);
}
//main method that calls the read_file and write_file method
int main()
{
read_file();
write_file();
return 0;
}

Image of C CODE-

OUTPUT-

read.txt

output on console-

write.txt

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