when you use double type to count, it will be dangerous. see like that

[codesyntax lang=”c” tab_width=”2”]

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#include<stdio.h>
int main(){
  double i;
  for(i=0; i != 5; i++){
    printf("%lf ",i);
  }
  return 0;
}

it will never stop, because the condition of i != 5 will never satisfied. If you want to count, using int type. When you compare two double type number, you should use fabs(a-b) < 0.001, 0.001 is threshold. Sometimes, double is not precision. macro is a great tool, but it is also dangerous. I often forget the language of define. The correct language is :

define

if defined

if !defined

ifndef

ifdef

endif

compile .c file with math lib in linux. It will be like that: cc test.c -o test -lm

stdio redirect to file use freopen() function. like that:

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#include<stdio.h>
// don't write freopen here, it will be some compile error.
//freopen("test.in","r",stdin);
//freopen("test.out","w",stdout);
int main(){
  int n,i;
  #if defined REDIRECT
  freopen("test.in","r",stdin);
  freopen("test.out","w",stdout);
  #endif
  scanf("%d",&n);
  for(i = 0; i < n; i++){
      printf("%d ",i);
  }
  return 0;
}

if you want redirect to file, use the code and compile it with -D REDIRECT. cc test.c -o test -D REDIRECT

If you want to use struct FILE to do the same thing:

[codesyntax lang=”c” tab_width=”2”]

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#include<stdio.h>
#include<math.h>
int main(){
  FILE * fin, *fout;
  fin = fopen("z.in","rb");
  fout = fopen("z.out","wb");
  int n;
  #ifdef REDIRECT
  #define fin stdin
  #define fout stdout
  #endif

  fscanf(fin,"%d",&n);
  fprintf(fout,"%d,%lf",n,sqrt(n));
  fclose(fin);
  fclose(fout);
  return 0;
}

try cc t3.c -o t3 -lm -D REDIRECT and cc t3.c -o t3 -lm