Find the maximum number

Question

I need to find the maximum number from file in.txt with content: 12 7 -14 3 -8 10 and then write it to file out.txt. Here is my full code:

#include <conio.h>
#include <stdio.h>

main() {
  int N, max;
  FILE*F;
  F = fopen("in.txt", "r");
  FILE*G;
  G = fopen("out.txt", "w");
  fscanf(F, "%d", &max);
  while (feof(F)) {
    fscanf(F, "%d", &N);
    if (max < N)
      max = N;
  }
  printf("max=%d", max);
  fprintf(G, "%d", max);
  fclose(F);
  fclose(G);
}

**Format your code.** so, C or C++? In C++, you could use `` and `std::max_element`. — The Paramagnetic Croissant, Nov 09 '14 at 19:09
@TheParamagneticCroissant, Unfortunately, `std::max_element` requires storing everything and then iterating through it again. — chris, Nov 09 '14 at 19:15
possible duplicate of [Finding Max and Min floating numbers from a file in C](http://stackoverflow.com/questions/15377214/finding-max-and-min-floating-numbers-from-a-file-in-c) — gsamaras, Nov 09 '14 at 19:15
@TheParamagneticCroissant, It needs a ForwardIterator because it returns the iterator to the element, which doesn't work so well in the case of input on demand. — chris, Nov 09 '14 at 19:23
@chris I've tried it and it worked just fine. [Link](http://ideone.com/sMm8am) — The Paramagnetic Croissant, Nov 09 '14 at 19:26
@TheParamagneticCroissant, I have a striking feeling it's undefined behaviour, but compiles because all of the operations are syntactically valid (as opposed to, say, needing `--it` and giving something with just `++it`). — chris, Nov 09 '14 at 19:30
@chris if that's the case, it means that `std::max_element` can't be used directly with files, which is horrible. — The Paramagnetic Croissant, Nov 09 '14 at 19:35
@TheParamagneticCroissant, [Well, I asked the lounge.](http://chat.stackoverflow.com/transcript/message/19863381#19863381) — chris, Nov 09 '14 at 19:38

The Paramagnetic Croissant · Answer 1 · 2014-11-09T19:42:16.307

In C++14, you could use std::max_element directly with std::istream_iterator:

std::ifstream inf("in.txt");
auto it = std::max_element(std::istream_iterator<int>(inf), std::istream_iterator<int>());

if (it != std::istream_iterator<int>()) {
    std::ofstream outf("out.txt");
    outf << *it << std::endl;
}

In C++11 and earlier, this seems to be undefined behavior (because istream_iterator is an InputIterator, but max_element expects a ForwardIterator), so in those variations of the language you will need to read the file in a container (most like std::vector) before using max_element.

score 0 · Accepted Answer · answered Nov 09 '14 at 22:30

Incorrect use of feof().

Do not use feof() to check if additional input is available. feof() reports that the previous I/O operation resulted in the end-of-file condition.

IOW, check the results of your input operations like fscanf().

Suggest:

int MaxFound = 0;
while (fscanf(F, "%d", &N) == 1) {
  if (MaxFound) {
    if (N > Max) {
      Max = N;
    }
  } else {
    MaxFound = 1;
    Max = N;
  }
}
if (MaxFound) {
  printf("max=%d\n", max);
} else {
  puts("max not found");
}

Find the maximum number

2 Answers2