C language how to scan file with integers in reverse order?

Question

I have a small file containing numbers, separated by new lines/spaces. I am looking for a way to scan the file , but in reverse.

Input: 1 2 3 4 5 1025
Output: 1025 5 4 3 2 1

Attention! in the following code, I need to modify the scanf. I need to leave printf unchanged! I don't need to transform 1024 into 4201, the integers remain unchanged. Only their order changes.

Code:

while (fscanf(input,"%d", &b) && !feof(input)) {printf("%d",b);}

Read the numbers into an array and print the array in the reverse order. — DYZ, Dec 04 '18 at 16:35
Either you do difficult back-seeking or you read it into an array and print that in reverse order. — Paul Ogilvie, Dec 04 '18 at 16:36
Your loop condition is wrong. If an input error occurs, your code goes into an infinite loop. — melpomene, Dec 04 '18 at 16:37
[Possible dupe](https://stackoverflow.com/questions/8394155/how-to-print-lines-from-a-file-in-reverse-order). — DYZ, Dec 04 '18 at 16:37
[`while (!feof(file))` is always wrong](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — pmg, Dec 04 '18 at 16:38
Testing the return value of `fscanf` like a sane person: `while (fscanf(input, "%d", &b) == 1)` — melpomene, Dec 04 '18 at 16:43

score 2 · Accepted Answer · answered Dec 04 '18 at 16:42

2

Do you have a big enough stack?

void printreverse(FILE *f) {
    int n;
    if (fscanf(f, "%d", &n) != 1) return;
    printreverse(f); // recursive call
    printf("%d", n);
}

https://ideone.com/1jejpM

answered Dec 04 '18 at 16:42

pmg

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I was going to suggests reversing through recursion, and here it is – Tim Randall Dec 04 '18 at 17:02
1

`printf("%d", n);` -> `printf("%d ", n);`, output will look better. – Jabberwocky Dec 04 '18 at 17:02
@Jabberwocky OP said: "I need to leave printf unchanged!" – pmg Dec 04 '18 at 17:08
@pmg yes, but in his sample output there _are_ spaces between numbers. OK, not the first time requirements in questions are contradictory. – Jabberwocky Dec 04 '18 at 17:12

C language how to scan file with integers in reverse order?

1 Answers1