Multidimensional array into function

[browser]

I have read a file into an array in my main and it has done so correctly (after help from this board a few threads down). I now need to perform arithmetic with that array in an external function. With the code I have so far it has been successful for all values except array[1][0]. Also, it does not seem to return the final value back into main.

Let me know if you need to see any more code.

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

double finalcalc(double array[][3], int N);

int main()
{
  FILE* input;
  int N;
  char type[1];
  double array[N][3];
  double *ptr;
  int i, j;
  double finalvalue;
 
  
  input=fopen("filename.xyz", "r");

  ptr=malloc(((double)N*3)*sizeof(double));
  i=0;
  for(i=0;i<N;i++)
  {
      fscanf(input, "%s", &type);
      printf("Type %d is %s\n", i, type);
      j=0;
      for(j=0;j<3;j++)
      {
      fscanf(input, "%lf", &array[i][j]);
      printf("array in i = %d and j = %d is %lf\n", i, j, array[i][j]);
      }
   }
  
  fclose(input);
  
  finalvalue=finalcalc(array, N);
  printf("final is %lf\n", finalvalue);
  
  
 
return 0;
}

double finalcalc(double array[][3], int N)
{
  double final;
  double finalvalue;
  int i=0;
  int j=0;

  for(i=0;i<N;i++)
  {
    j=0;
    for(j=0;j<3;j++)
    {
      printf("array in i = %d and j = %d is %lf\n", i, j, array[i][j]);
    }
  }

  final = 5 * 6; /* The actual mathematics is more complex but left it out for ease */
  printf("Final value is %lf\n", final);
  return(finalvalue);
}

edit: Also, would you suggest reading the file into the array using another external function? Should I use global variables for the array and for N (and for final/finalvalue)?

[jimblumberg]

You need to check your compiler settings and insure warnings are being generated. When I compile your code I get these warnings:

main.c||In function ‘main’:|
main.c|25|warning: format ‘%s’ expects type ‘char *’, but argument 3 has type ‘char (*)[1]’|
main.c||In function ‘finalcalc’:|
main.c|63|warning: ‘finalvalue’ is used uninitialized in this function|
main.c||In function ‘main’:|
main.c|13|warning: ‘N’ is used uninitialized in this function|

All of these warnings should be considered errors and need to be fixed.

Jim

[browser]

Thanks Jim, I have fixed those. Still getting the same problem.

[jimblumberg]

Post your modified code.

Jim

[browser]

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
 
double finalcalc(double array[][3], int N);
 
int main()
{
  FILE* input;
  int N;
  char type;
  double array[N][3];
  double *ptr;
  int i, j;
  double finalvalue;
  
   
  input=fopen("filename.xyz", "r");
 /*NEW BIT THAT I JUST ADDED*/
  fscanf(input, "%d", &N);
  printf("N is %d\n", N); //This gives me 4
/*END NEW BIT*/
  ptr=malloc(((double)N*3)*sizeof(double));
  i=0;
  for(i=0;i<N;i++)
  {
      fscanf(input, "%s", &type);
      j=0;
      for(j=0;j<3;j++)
      {
      fscanf(input, "%lf", &array[i][j]);
      printf("array in i = %d and j = %d is %lf\n", i, j, array[i][j]);
      }
   }
   
  fclose(input);
  printf("N is still %d\n", N); //This gives me 0
  finalvalue=finalcalc(array, N);
  printf("final is %lf\n", finalvalue);
   
   
  
return 0;
}
 
double finalcalc(double array[][3], int N)
{
  double final=0;
  double finalvalue=0;
  int i=0;
  int j=0;
 
  for(i=0;i<N;i++)
  {
    j=0;
    for(j=0;j<3;j++)
    {
      printf("array in i = %d and j = %d is %lf\n", i, j, array[i][j]);
    }
  }
 
  final = 5 * 6; /* The actual mathematics is more complex but left it out for ease */
  printf("Final value is %lf\n", final);
  return(finalvalue);
}

filename.xyz looks like this

    4
O   -0.906801    0.569837    0.066005
O    0.956585   -0.258900    0.342064
O   -0.962633    0.494832    0.380356
O   -0.259093    0.292540    0.854958

The problem seems to be in my reading of N. In my first printf it reads perfectly as 4 but it seems to disappear by the second call?

Thanks

[jimblumberg]

In the following snippet:

int main()
{
  FILE* input;
  int N;
  char type;
  double array[N][3];

You are using N before it is initialized. Also unless your compiler supports Variable Length Arrays, N must be a constant.

Jim

[browser]

So if N needs to be read from a file and the array size needs to be set to 3 x N how would I do that without getting this problem?

Thanks

[jimblumberg]

You will need to use dynamic memory, malloc/free, after you read the file.

Jim

[anduril462]

    int N;
    char type;
    double array[N][3];

The C99 standard (the "current version" of C) supports variable length arrays, so your declaration of array is gramatically correct, but N is uninitialized at that point. That means array has an unknown dimension, and N may be 0, 1, 4 or 4,000,000. Reading into array may result in undefined behavior if N is too small. You'll be writing to memory you don't own. You need to move the definition of array below where you read N.

[browser]

Jim - I am using malloc after I have read the file...well, while I am reading the file?

anduril - I moved

double array[N][3];
double *ptr;

to above my malloc line. Still prints "N is still 0"

[MK27]

So if N needs to be read from a file and the array size needs to be set to 3 x N how would I do that without getting this problem?

You can use malloc and a double**, or this is legal under C99 (you may need to set your compiler to accept that):

	int n;
	scanf("%d", &n);

	double array[n];
	printf("%lu\n", sizeof(array));

The issue is that n must be initialized to the proper value before you declare the array.

[browser]

The issue is that n must be initialized to the proper value before you declare the array.

Isn't that what I am doing here;

int main()
{
  FILE* input;
  int N;
  char type;
  int i, j;
  double finalvalue;
   
    
  input=fopen("filename.xyz", "r");

  fscanf(input, "%d", &N);
  printf("N is %d\n", N); //This gives me 4 so N now has a value, correct?
  double array[N][3];
  double *ptr;
  ptr=malloc(((double)N*3)*sizeof(double));
  i=0;
  for(i=0;i<N;i++)
  {
      fscanf(input, "%s", &type);
      j=0;
      for(j=0;j<3;j++)
      {
      fscanf(input, "%lf", &array[i][j]);
      printf("array in i = %d and j = %d is %lf\n", i, j, array[i][j]);
      }
   }
    
  fclose(input);
  printf("N is still %d\n", N); //This gives me 0

[jimblumberg]

The C99 standard (the "current version" of C) supports variable length arrays

This is true but not all compilers support C99 or VLA's.

Jim

[stahta01]

WARNING a single char can NOT hold a string!!
You are destroying your own data here.

char type;

fscanf(input, "%s", &type);

Tim S.

[MK27]

Isn't that what I am doing here;

Yes. That the value of N would change between those printf lines implies some kind of overwrite. Check it inside the for() loop right before and after the fscanf line here:

int N;
char type;
[...]
fscanf(input, "%s", &type);

You have read a string into a single char, and a string with content is at minimum two bytes long, including the null terminator. The value of the null terminator is 0, that could be be overwriting the least significant byte of N.

Also, what the heck is this:

ptr=malloc(((double)N*3)*sizeof(double));

You don't use it, so I'm not sure what you want there, but whatever it is, that is the wrong way to create it. Maybe you meant:

ptr = (double*)malloc(N*3*sizeof(double));

Altho the (double*) cast is unnecessary in vanilla C.