Reverse printing from txt file using recursion in C

Question

I need to write out .txt file backwards in terminal using recursion, but it seems I'm stuck. This is my code so far, but it creates an infinite loop. Also, the procedure write() should have only 1 parameter - pointer to file.

#include <stdio.h>

void write(FILE **f)
{
    char cur;

    fseek(*f,-1,SEEK_CUR);
    cur = fgetc(*f);

    printf("%c",cur);
    write(f);
}

int main(void)
{
    FILE *f;
    f = fopen("text.txt","r");
    fseek(f, 0, SEEK_END);
    write(&f);
    
    fclose(f);
    return 0;
}

I wanted to use fseek and ftell functions, they seem to be best way to do this. My expected output would be: FILE Hello world OUTPUT dlrow olleH

Considering they have a infinite call stack, they won't even be able to see the print outputs. — Rohan Asokan, Jan 02 '21 at 13:50
@Cassie can you please provide the outputs in the file, if any? — Rohan Asokan, Jan 02 '21 at 13:51
Your recursive function has no termination criteria. It always makes the recursive call, so it goes on forever until stack overflow. You need logic in a recursive function that decides whether to make the recursive call or not. — lurker, Jan 02 '21 at 13:56
@RohanAsokan Right now my output is *dddddddddddddd* until I stop the program. — Cassie, Jan 02 '21 at 13:57

0___________ · Answer 1 · 2021-01-02T14:29:16.730

0

This task is actually not very suitable for the recursion but ..

EDIT I actually misunderstood your question but it is same trivial.

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

void writeRecursive(FILE *file)
{
    int ch;
    if(fread(&ch, 1, 1, file) == 1) 
    { 
        writeRecursive(file);
        fwrite(&ch, 1, 1, file);
    }
    else
    {
        rewind(file);
    }

}

int main(void)
{
    FILE *fi = fopen("a.txt", "w+");
    char c;

    fprintf(fi, "123456789");
    rewind(fi);

    writeRecursive(fi);
    rewind(fi);
    while(fread(&c, 1, 1, fi) == 1)
    {
        printf("%c", c);
    }
    printf("\n");
}

https://godbolt.org/z/edGfKa

or

void writeRecursive(FILE *file)
{
    int ch;
    if((ch = fgetc(file)) != EOF) 
    { 
        writeRecursive(file);
        fputc(ch, file);
    }
    else
    {
        rewind(file);
    }

}

Old answer

void printRecursive(FILE *fi, FILE *fo)
{
    char ch;
    if(fread(&ch, 1, 1, fi) == 1) 
    { 
        printRecursive(fi,fo);
        fwrite(&ch, 1, 1, fo);
    }

}

int main(void)
{
    FILE *fi = stdin, *fo = stdout;

    printRecursive(fi, fo);
}

https://godbolt.org/z/YjfvcP

edited Jan 02 '21 at 14:29

answered Jan 02 '21 at 13:58

0___________

60,014
4
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The question is clearly to reverse the file and write it to the same file. And hence the recursion. – Rohan Asokan Jan 02 '21 at 14:02
1

@RohanAsokan The question is literally "I need to print .txt file backwards using recursion". Nowhere is "write it to the same file" even implied. – Andrew Henle Jan 02 '21 at 14:08
@RohanAsokan I just need to wrtite it out in terminal, not in file – Cassie Jan 02 '21 at 14:11
@Cassie Cool. I have made some progress, it now prints the content. Just figuring wth is the end clause, hehe. – Rohan Asokan Jan 02 '21 at 14:13
@Cassie here you have a recursive function which reverses the file – 0___________ Jan 02 '21 at 14:17
From certain size of the input file the code won't work any more, because the program would have run out of stack space. – alk Jan 02 '21 at 14:46

Rohan Asokan · Accepted Answer · 2021-01-02T14:29:23.217

0

From what you have given, the way to do the problem recursively using fseek and ftell can be written as follows,

#include <stdio.h>

void write(FILE **f)
{
    char cur;
    cur = fgetc(*f);
    printf("%c", cur);
    if (ftell(*f) == 1) return;
    fseek(*f,-2,SEEK_CUR);
    write(f);
}

int main(void)
{
    FILE *f;
    f = fopen("text.txt","r");
    fseek(f, -1, SEEK_END);
    write(&f);
    
    fclose(f);
    return 0;
}

Considering the file is read in reverse, the stop condition requires a bit of thinking. The position of a pointer is returned by ftell and using this we can detect if the current position of the pointer is at the start of the file or not (as in if it is equal to 1).

There were tiny mistakes in your code - the recursion never stopped (as pointed out in the comments), make sure you always have a stop inside your recursive functions.

The order of your fseek needs changing so that you read the first character in properly.

And you need to use a -2 inside your fseek to move to the previous character. This is because of the fact that fgetc will advance the pointer by 1 and you will have to go back by 1 character to reach the character that was read just now and you then you want to move it backwards by one more, so that you get to the character before the one that was just read. Thanks to p-j-supports-women-in-poland for pointing this out.

Otherwise the above code will make it clear for you, I hope. if you still got doubts ping me in the comments. Happy to help :)

edited Jan 02 '21 at 14:29

answered Jan 02 '21 at 14:21

Rohan Asokan

634
4
22

what is the `fseek` for? It shows lack of understanding how the recursion works. – 0___________ Jan 02 '21 at 14:23
Sorry what? Can you be more clear? Thanks – Rohan Asokan Jan 02 '21 at 14:23
It is not needed at all. Type for cur is wrong. Vary bad code. No double `*` pointers needed. – 0___________ Jan 02 '21 at 14:24
Umm..it is as the fgetc increments the pointer by 1 and you want to move it two positions back to read the previous character - if that makes sense to you. And yes I agree with the wrong type for cur. I am sorry. But you do not have to be so rude. – Rohan Asokan Jan 02 '21 at 14:27
1

@RohanAsokan Thank you! Code works for me and does exactly what I need. – Cassie Jan 02 '21 at 14:29
Glad that it worked. Please mark it as the answer. Would help others moving through SO. – Rohan Asokan Jan 02 '21 at 14:29

Reverse printing from txt file using recursion in C

2 Answers2