Reading a file.dat, applying calculations

Hello, this is my first post on these boards, I have seen alot of helpfull responses, this website tends to be my first call when i have problems.

I have a file created by another program called sin.dat, the file is an array of 60 numbers that have been saved:

0.00
3.09
5.88
8.09
9.51
10.00
9.51
8.09
5.88
3.09
-0.00
-3.09
-5.88
-8.09
-9.51
-10.00
-9.51
-8.09
-5.88
-3.09
0.00
3.09
5.88
8.09
9.51
10.00
9.51
8.09
5.88
3.09
-0.00
-3.09
-5.88
-8.09
-9.51
-10.00
-9.51
-8.09
-5.88
-3.09
0.00
3.09
5.88
8.09
9.51
10.00
9.51
8.09
5.88
3.09
-0.00
-3.09
-5.88
-8.09
-9.51
-10.00
-9.51
-8.09
-5.88
-3.09

Plotting this in excell gives a sin wave of amplitude 10.

Now my problem doesn't lie here, it is the program I wrote to load this file and use it.

My code:

Code:

---

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

#define pi 3.1416

float array[60], ary[2];
int index=0, i;
char infilename[20];
float vpk, a1, a2;
float h=1e-3, w;


int main()
{
 FILE *infile;
 printf("Please enter a filename\n");
 gets(infilename);
 if((infile=fopen(infilename,"r"))==NULL); //I believe the problem lies here//
 {
  printf("\nThere has been a problem locating or reading the specified file,\nPlease ensure you have typed the filename correctly.\n");
 }
 
 while(!feof(infile)||index<60)
 {
  fscanf(infile,"%f\n",&array[index]);
  printf("%f\n",array[index]);
  index++;
 }
 
 
 printf("Now calculating the peak voltage...\n");
 
 for(i=0;i<59;i++)
 {
  ary[i+2]=ary[i+1];
  ary[i]=ary[i+1];
  ary[i]=array[index];
 
  a1=(ary[i+1]*ary[i+1]);
  a2=((ary[i+2]-ary[i])/2*h);
  w=(2*pi*50);
 
  vpk=a1+((a2*a2)/(w*w));
 }
  printf("%f",vpk);
 
 fclose(infile);
}

---

When I compile, build then execute, I allways get "There has been a problem locating or reading the specified file, Please ensure you have typed the filename correctly" and then it displays all 60 numbers from the saved array.

I can't seem to get my head round why it shows this messege. As there is no problem with opening the array, it displays the contents of it on screen. Feel free to copy and paste this.

I know there are a few problems with the working after "printf("Now calculating the peak voltage...\n");", these problems I plan to resolve after. In this part I am trying to find the peak amplitude of the sine wave, using the equation: Vpk^2=(Vt+1)^2+[(Vt+2-Vt)/2*h]^2/w^2.

I may not have explained this very well, but any advice would be appreciated.

Many Thanks.

> gets(infilename);
Nah, this is your first problem - see the FAQ

> //I believe the problem lies here//
Your belief is correct - now study the line in detail to see which character should not be there.

> while(!feof(infile)||index<60)
1. You should not use feof() in control loops - see the FAQ
2. it should be &&, not ||

Thaks for the swift reply,

Ah, so fgets is secure and limits the reading.
Would I have to include it and define it like this?

char *fgets(char *s, int size, FILE *stream);

then later... fgets(array,60,infilename)
This has thrown me slightly...

HAHA! I guess it's the ";" thrown on the end! ~ Resolved

I see how feof can be a problem now, it would be better to use fgets() and check the code returned.
Although I am a little unsure on how to use fgets(), (this ones new to me).

Can you take a look through this?

Code:

---

printf("Now calculating the peak voltage...\n");
 
 for(i=0;i<59;i++)
 {
  ary[i+2]=ary[i+1];
  ary[i]=ary[i+1];
  ary[i]=array[index];
 
  a1=(ary[i+1]*ary[i+1]);
  a2=((ary[i+2]-ary[i])/2*h);
  w=(2*pi*50);
 
  vpk=a1+((a2*a2)/(w*w));
 }
  printf("%f",vpk);

---

What I have attemted to do is take the loaded array, create another array of 3 slots and take the first 3 values from the loaded array and put them into the array of 3. Ughh, after reading that back to myself I'd be suprised if you can make sense of it! I'm trying to do this,

Code:

---

  ary[i+2]=ary[i+1];
  ary[i]=ary[i+1];
  ary[i]=array[index];

---

But it may not be right as when I execute the program, it outputs 0. It also outputs 0 for a1 and a2, if this helps.

don't you see that ary[i] is initialized twice, and ary[i+1] never?
also, where is the local array of 3?

so i should change it to:
ary[i+2]=ary[i+1];
ary[i+1]=ary[i];
ary[i]=array[index];
This isn't what you mean is it?

The array of 3 is: ary[2]
float array[60], ary[2];
Vt should go into 0, Vt+1 into 1 and Vt+2 into 2...

if ary is declared in that way the possible indexes are from 0 to 1
you cannot access any other member of the array

Quote:

---

Originally Posted by vart

if ary is declared in that way the possible indexes are from 0 to 1
you cannot access any other member of the array

---

So ary[2] should infact be ary[3] for starters?
Can you follow that snipit of code, does it make sense to you?

Code:

---

for(i=0;i<58;i++)
{
  float *ary = array+i;
 
  a1=(ary[1]*ary[1]);
  a2=((ary[2]-ary[0])/2*h);
  w=(2*pi*50);
 
  vpk=a1+((a2*a2)/(w*w));
}

---

And of course w*w should be calculated outside the loop

Thanks, That makes more sense.
I have a few adjustments and corrections to the program to make.
I will update with the outcome.

3 more hours of typing and de-bugging and i'm done :)
Thanks for all your advice.