CODEWITHSUNDEEP
c
pic_taym
pic_taym

Reputation: 21

How to Move File Pointer Backwards

I am currently working on files and I want to read a character from the file which the file pointer has already passed. Imagine file consists of these characters: 123456789

I want to print 3 characters forward and 1 character from back, output must be like: 123234345456567678789

I tried this but it didnt print anything. I also tried fseek function but still it didn't print anything.

while(c != EOF){
    for(int i=0; i<=2; i++){
            c = fgetc(fp);
            printf("%c", c);
        }
     fp -= 1;
}

Upvotes: 2

Views: 4897

Answers (2)

Aplet123
Aplet123

Reputation: 35560

fseek does work, just make sure you subtract 2, not 1, since calling fgetc will advance the file pointer once more.

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

int main() {
    int c = 0;
    FILE* fp = fopen("test", "r");
    int shouldBreak = 0;
    while (c != EOF) {
        for (int i = 0; i <= 2; i++) {
            c = fgetc(fp);
            if (c == EOF) {
                shouldBreak = 1;
                break;
            }
            printf("%c", c);
        }
        if (shouldBreak) {
            break;
        }
        fseek(fp, -2, SEEK_CUR);
    }
    printf("\n");
    return 0;
}

Upvotes: 0

anastaciu
anastaciu

Reputation: 23832

For the desired output you can use fseek to reset the fp file pointer.

Another thing that should be done is to place a first read ouside the loop otherwise, in the first evaluation, c has yet to have read a character from the file, this can lead to undefined behaviour if c is not yet initialized.

I also inverted the printf and fgetc statements otherwise EOF will be printed.

Running sample

#include <stdio.h>

int main() {

    FILE *fp = fopen("file.txt", "r");

    if(!fp)
       return 1;

    int c;

    c = fgetc(fp);

    while(c != EOF) {
        for (int i = 0; i <= 2; i++) {
            printf("%c", c);
            c = fgetc(fp);         
        }
        fseek(fp, -2, SEEK_CUR);
    }
}

Upvotes: 0

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