C - United output to screen and text file

Question

As my term project to school, I wrote simple C application which calculates functional value of polynom, using Horner algoritm. One of the requests was to print the output to txt file. I solved it by some easy for cycle and fprintf functions.

My professor wants me to edit it to have united output to screen and to file via using library stdarg.h. I'm propably slightly retardet, becouse I have been trying to do it for three days without any result.

I have read most of the topics with similar theme, tried to implement some kinds of tee macro into it, but i was not capabble to make it work. Please, help me someone, or I will never pass this class by myself.

Whole code below

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

#define OUT "./out.txt"

double horner(double * koe, int level, double x) {
  int i;
  for (i = level - 1; i >= 0; --i) {
    koe[i] += x * koe[i + 1]; //
  }
  return koe[0];
} //poly[0]x(n-1) + poly[1]x(n-2) + .. + poly[n-1]

int scanINT() {
  int number;
  while (1) {
    int LoadResult = scanf("%d", & number);
    fflush(stdin);

    if (LoadResult != 1) {
      printf("Value is not number.\n");
    } else {
      return number;
    }
  }
  return 0;
}

double ScanDouble() {
  double number;

  while (1) {
    int LoadResult = scanf("%lf", & number);
    fflush(stdin);

    if (LoadResult != 1) {
      printf("Value is not double.\n");
    } else {
      return number;
    }
  }
  return 0;
}

int ScanPositiveNumber() {
  while (1) {
    int number = scanINT();

    if (number < 0) {
      printf("Value is negative.\n");
    } else {
      return number;
    }
  }
  return 0;
}

int main(void) {
  int level, i;
  double x;
  double * koe;

  FILE * f = fopen(OUT, "w");
  if (f == NULL) {
    printf("Couldnt open file out.txt \n");
    exit(1);
  }

  while (1) {
    int option;

    printf("********************************\n");
    printf("Option 1: Functional value calc\n");
    printf("Option 2: Save result and end\n");
    printf("********************************\n\n");

    option = scanINT();

    if (option == 1) {
      printf("Insert level of polynom: ");
      level = ScanPositiveNumber();

      koe = (double * ) malloc((level + 1) * sizeof(double));
      if (koe == NULL) printf("Memory allocation failed.\n");
      for (i = level; i >= 0; --i) {
        printf("x^%d: ", i);
        koe[i] = ScanDouble();
      }

      printf("Insert point x: ");
      x = ScanDouble();

      double hornerVal = horner(koe, level, x);

      printf("f(%0.2f) = %0.2f\n", x, hornerVal);

      for (i = level; i >= 0; --i) {
        if (i != level) {
          if (koe[i] < 0) {
            fprintf(f, " ");
          } else {
            fprintf(f, " +");
          }
        }
        fprintf(f, "%0.2fx^%d", koe[i], i);
      }

      fprintf(f, "\nf(%0.2f) = %0.2f\n", x, hornerVal);
      free(koe);
    } else if (option == 2) {
      break;
    }
  }
  if (fclose(f) != 0) {
    printf("Couldnt save file out.txt \n");
  }

  return 0;
}

**/

Unrelated to your question, but note that calling [`fflush`](https://en.cppreference.com/w/c/io/fflush) on an input-only stream (like e.g. `stdin`) is explicitly mentioned in the C specification as *undefined behavior*. Some systems implement it as a non-standard extension, but it isn't portable and you should really try to avoid such constructs. — Some programmer dude, May 02 '19 at 13:18
As for your question, perhaps your teacher wants you to create a custom `printf`-like function which uses `printf` to print to the console and `fprintf` to print to a file? Perhaps you should ask your teacher for clarifications first? — Some programmer dude, May 02 '19 at 13:20
If he asks you to use stdarg.h he probably wants you to write a function which takes a format string as printf or fprintf followed by an arbitrary amount of parameters (matching the amount in your format string of course). The following side provides some basic info about your capabilities with a variable-length argument list: http://www.cplusplus.com/reference/cstdarg/ — Trickzter, May 02 '19 at 13:27
In practice, no modifications to your programme are necessary; this would be how you'd do it, https://stackoverflow.com/questions/418896/how-to-redirect-output-to-a-file-and-stdout. However, your professor probably doesn't want this. — Neil, May 02 '19 at 22:07

pmg · Answer 1 · 2019-05-02T19:07:48.627

I think you want something like this

#include <stdarg.h>
#include <stdio.h>

#define OUT "./out.txt"

void printtwice(const char *fmt, ...) {
    va_list args;

    va_start(args, fmt);
    vprintf(fmt, args);
    va_end(args);

    va_start(args, fmt);
    FILE *h = fopen(OUT, "a"); // assume it worked
    vfprintf(h, fmt, args);
    fclose(h);
    va_end(args);
}

int main(void) {
    printtwice("%d-->%f\n", 42, 2.7182818);
}

See slightly different code running on ideone

C - United output to screen and text file

1 Answers1