Thread: C Programming question about file manipulation

Good evening!

I have a C program that I have been working on for most of the week. I have gotten around 75% of it working. I'm stuck on the last part.

The question:

How do you read a file into memory, all into one buffer, reverse the contents of the buffer and then overwrite the buffer contents? I have a sample text file which I select in one of my menu options. I then use it in my option 4.

I have as my 4th option:

Code:

else if (option == 4)
 
      { 
       fp = fopen(filename, "r");     //open file 
        
       //some other stuff inbetween 
        
       fclose(fp);                    //close file 
            
      }

I believe I need to create and define the size of a buffer (max of 20,000 bytes), dump the contents of the chosen file into the buffer and then reverse contents. I close the file afterwards. If I choose to view the contents of the file I reversed (another option in my menu) I will see everything backwards. At least that is the plan.

On top of that, if no file is selected an error is displayed.

If anyone has a few suggestions, directions, examples or links to some really good tutorials on the subject of file manipulation I would greatly appreciate it.

Eternal Thanks!

If you want to overwrite the buffer with the reversed string you can use the following code

Code:

#include <stdio.h>
#include <string.h>
main()
{
        char s2[100],temp;
        FILE *fp;
        fp = fopen("file","r");
        int i,len;
        fgets(s2,sizeof(s2),fp);
        printf("s2 length  : %d\n",strlen(s2));
        printf("contents from file : %s\n",s2);
        len=strlen(s2)-1;
        for(i=0;i<strlen(s2)/2;i++)
        {
                temp =s2[i];
                s2[i]= s2[len];
                s2[len--]= temp;
        }
        printf("Reversed string in the buffer:%s\n",s2);
}

Use the following code

Code:

#include<stdio.h>
#include<string.h>
main()
{
void reverse(char[]);
char s[30];
int c;
int i;
FILE *fp;
fp = fopen("file","r");
while(!feof(fp))
{
fgets(s,sizeof(s),fp);
reverse(s);
}
}

void reverse(char s1[])
{
int c,i,j;
for(i=0,j=strlen(s1)-1;i<j;i++,j--)
{
c=s1[i],s1[i]=s1[j],s1[j]=c;
}
printf("The reverse string is:%s\n",s1);
                                              
}

Code:

#include <stdio.h>
#include <string.h>
#include<malloc.h>
void reverse(char s[]);
main()
{
        char *buf=malloc(10000);
        FILE *fp;
        fp = fopen("test","r");
        int i=0;
        int c;
       while((c=fgetc(fp))!=EOF) # To read the entire file
       {
              buf[i++]=c;
       }
       printf("File contents :\n %s",buf);

       printf("Reversed contents:\n");
       reverse(buf);
}



void reverse(char s[])
{
        int c,i,j;

        for(i=0,j=strlen(s)-1;i<j;i++,j--)
        {
                c=s[i];
        s[i]=s[j];
        s[j]=c;
        }
        printf("%s",s);
}

My thanks to everyone who posted a reply! All were excellent suggestions. I now have a better understanding of the process.

This is what I went with:

Code:

else if (option == 4)
      {
       fp = fopen(filename, "r");     //open file
       if((fp == NULL))
       {
        printf("No file was selected!\n");
        exit(1);       
       }
              
        char *buffer = malloc(20000);
        char buf[20000];
        
        n = fread(buffer, 20000, 1, fp);
        n = strlen(buffer);
        printf("buffer :%s == %d\n"  ,buffer,n);

        for ( ; n>=0 ;n--)
        {
         if ( (c=*(buffer+n)) !='\0'  )
         {
          buf[i++]= c ;
         }
        }
        printf("reversed contents :%s\n" , buf);
              
       fclose(fp);                    //close file
      }

The program now runs! After a file is selected and Option 4 is chosen, the screen will say "buffer :" and display the contents of the text file. Right below it, it says "reversed contents :" and then displays the text file's contents, but backwards.

Now, what I'm trying to do is take the reversed contents in the buffer and overwrite the text file. In other words, replace the contents of the selected text file with the reversed content. If you were to open the text file, everything would be backwards.

So, I believe I need to write the reversed buffer contents to the file before fclose(fp);? that way, when I open the text file again, I'll see everything backwards.

In theory, anyway... ;0)