How will be the c program for reversing first n characters in a file?

Answer 1

#include <stdio.h>

#include <conio.h>

#include <string.h>

#include <process.h>

void main(int argc, char *argv[])

{

char a[15];

char s[20];

char n;

int k;

int j=0;

int i;

int len;

FILE *fp;

if(argc!=3)

{

puts("Improper number of arguments.");

exit(0);

}

fp = fopen(argv[1],"r");

if(fp == NULL)

{

puts("File cannot be opened.");

exit(0);

}

k=*argv[2]-48;

n = fread(a,1,k,fp);

a[n]='\0';

len=strlen(a);

for(i=len-1;i>=0;i--)

{

s[j]=a[i];

printf("%c",s[j]);

j=j+1;

}

s[j+1]='\0';

getch();

}

Answer 2

#include<stdio.h>

#include<assert.h>

int main (void) {

FILE* pf;

int i, j, count;

char file[255];

char* buf;

while (true) {

printf ("File name: ");

scanf ("%s", file);

if (pf=fopen (file, "br+")) break;

printf ("Invalid file name\n");

}

while (true) {

printf ("Count of bytes to swap: ");

scanf ("%d", &count);

if (count<1) {

printf ("Count must be greater than 0\n");

continue;

}

buf = malloc (count * sizeof (char));

if (fread (buf, 1, count, pf)==count) break;

free (buf);

fseek (pf, 0, SEEK_SET);

printf ("The file doesn't contain enough bytes to swap\n");

}

assert (pf);

assert (buf);

i=-1, j=count;

while (--i<--j) swap (buf[i], buf[j]);

fseek (pf, 0, SEEK_SET);

fwrite (buf, 1, count, pf);

fclose (f);

free (buf);

return 0;

}