Question

I wrote a program to calculate the sum of a harmonic series (1 + 1/2 + 1/3 .. + 1/n), but I’m having trouble compiling it. I went over the code a bunch of times but I don’t see any syntax errors [2 are showing up when I try to compile]. Is my logic wrong, or is it syntactical?

#include <stdio.h>
int main( void ) {

 int v,p,i;
 double x=0;

 printf("Enter a value to calculate the value of this harmonic series: \n");
 scanf("%d",&v);

 if (v<=0) {
   printf("Please enter a POSITIVE number: \n");
   scanf("%d",&p);
   while (i=1; i<=v; i++) {
     x=x+(1/i);  }
     printf("The value for the series is %lf", x);
  }
    else {
      while (i=1; i<=v; i++) {
        x=x+(1/i);
      }
      printf("The value for the series is %lf", x);
    }
  return 0;
}

Thanks for any help

Answer 1

All those while should be for, and that 1/i should be either 1./i or 1/(double)i, because otherwise an integer division is performed. Also, you should restructure your program flow to avoid that code duplication.

But, there’s a subtle but more important mistake: due to how floating point arithmetic works, you should start to sum from smaller numbers to big ones, otherwise you may reach the point were each new addend is smaller than the current precision of that double, and the addition will have no effect¹. So, your for should be reversed:

for (i=v; i>=1; i--)

---

Also: you should check the return value of the scanf to be 1, to make sure that the user actually inserted some valid numeric input. And, the printf specifier for doubles is just %f, without the l.

Taking all this in account, you could rewrite the program as this:

#include <stdio.h>

int main( void )
{
    int v=0,i,ch;
    double x=0.;

    printf("Enter a value to calculate the value of this harmonic series: ");

    /* The loop calls the scanf, and is repeated as far as the user continues to
       write garbage */
    while(scanf("%d",&v)==0 || v<=0)
    {
        printf("Please enter a POSITIVE number: ");

        /* Empty the input buffer to remove the eventual garbage; the logic is
           a bit convoluted to handle the case where the user enters EOF */
        while((ch=getchar())!='\n')
            if(ch==EOF)
                return 1;
    }

    /* perform the actual sum - done from smallest to biggest term */
    for (i=v; i>=1; i--)
        x+=1./i;

    printf("The value for the series is %f\n", x);
    return 0;
}

---

1. For an harmonic series this is actually almost impossible – you would need to go to really big numbers to notice some difference – but with other series (whose terms get very small very fast) this suggestion can be really important.