About struct and arrays (C language) - c

Question

I'm trying to create code using a struct and a pointer, for example:
struct Vector()
{ double x,y,z;};
struct vector *p[][][]. I need a 3d parameter which have x,y and z coordinates for a molecular simulation.
The problem is: I want to run in Ubuntu's terminal and give the values of sample size using argc argv on terminal.
So I ask you guys: Can I create a struct array? I believe this will solve my problem.

Answer 1

You could take a different approach instead of using command line arguments, as suggested in the previous answer.
You can take user input using file input indirection to stdin, which can be a convenient way of processing data. Your command line call looks like:
./your_prog < data
Where data is a data file with a set of three space separated values on each line:
1 1 1
2.1 2.2 2.3
3.1 3.2 3.3
You could then process the data using something like this:
#include <stdio.h>
#include <stdlib.h>
typedef struct vector {
double x;
double y;
double z;
} Vector;
Vector *createVector(double x, double y, double z);
int main()
{
// Initialize an array of pointers to Vector
Vector **array = calloc(5, sizeof(*array));
double x = 0, y = 0, z = 0;
size_t lineCount = 0;
while(1) {
// Take input from stdin
ssize_t r = scanf("%lf%lf%lf", &x, &y, &z);
if (r == EOF) {
// No more data so finish
break;
}
// Note that you need to check the size of the array
// and resize using realloc() as necessary within this loop.
// The array needs to have space for at least lineCount + 1 elements.
// Check this and reallocate before trying to add a
// new element to the array.
array[lineCount] = createVector(x, y, z);
lineCount++;
}
for (size_t i = 0; i < lineCount; i++) {
// Do something with each element of the array
printf("Vector %zu: (%lf, %lf, %lf)\n", i, array[i]->x, array[i]->y, array[i]->z);
}
// You have dynamically allocated memory, so it should be freed.
for (size_t i = 0; i < lineCount; i++) {
free(array[i]);
}
free(array);
return 0;
}
// Create a pointer to a Vector, dynamically allocating memory and returning
// a pointer to the new array.
Vector *createVector(double x, double y, double z)
{
Vector *v = calloc(1, sizeof(*v));
v->x = x;
v->y = y;
v->z = z;
return v;
}
Hope this helps.

Answer 2

I've just solved the problem.
typedef struct
{
double x,y,z;
}vetor;
vetor ***p;
void alloc(int lx, int lz)
{ int i,j=0;
int ly=lx;
p=calloc(lx,sizeof(vetor**));
for(i=0;i<lx;i++)
{
p[i]=calloc(lx,sizeof(vetor*));
for (j=0;j<lx;j++)
{
p[i][j]=calloc(lz,sizeof(vetor));
}
}
}
I was using sizeof(double*) but the member p is a vetor struct type.
So with this i can make a member p[a][b][c] dynamically using pointers and let it a global member ( in my code i have to) and put the values a,b and c on terminal.
I would like to thank you for the answers which makes me think a lot about this problem. Thank you for attention too.

Answer 3

Note that the struct definition you provided ( struct Vector() ) as an example is not a struct definition.
But yes, you can use int argc, char *argv[] to read the command line for user input.
Then you can use something as simple as
If(argc != 2) {
printf("usage error, exiting");
return 0;
}
int array_size = atoi(argc[1]);
Then for example you can easily create an array of struct by declaring a typedef:
typedef struct {
double x;
double y;
double z
} MOLECULE;
MOLECULE molArray[array_size];//from user input - each of array_size instances contains x,y,z members.
There are several ways to instantiate a pointer reference to this example struct, i.e.
MOLECULE *pMolecule = malloc(array_size)*sizeof(*molecule));
Or
MOLECULE *pMolecule = {0};
MOLECULE molecule = {1.0,2.0,3.0};//simple sample of data
pMolecule = &modlecule;//pointer pointing to area in memory containing instance of MOLECULE
Note that with an array of struct, it becomes impracticable to use user input to populate the many members, reading that information from a file, or a database is probably best.
If you provide more details, this answer can do the same. Just flag me in a comment
EDIT To address question in comments "...but how it would if I needed a MOLECULE molArray[x][y][z] ? and x,y,z is a dynamic value which I enter in the beginning of the program?":
Using VLA allows a way to dynamically defined sizes of objects when the sizes needed are not known until run-time. example...
defined MOLECULE struct differently since you want 3D array of struct, no need to define members x, y, z. You need to decide what members will define your actual struct. Because I do not know, I can only guess at what characteristics you want to include in this struct illustration:
typedef struct {
char R;
char G;
char B;
}COLOR;
typedef struct {
double mass;
COLOR color;
double size;
char molec_formula[80];
}CHARAC;
typedef struct {
char name[80];
CHARAC charac;
}MOLECULE;
int main(int argc, char *argv[])
{
if(argc != 4) // prog.exe <x> <y> <z>
{
printf("usage error, exiting");
return 0;
}
int x = atoi(argv[1]);//could also use strtol() here as well
int y = atoi(argv[2]);
int z = atoi(argv[3]);
// from MOLECULE definition above:
MOLECULE molArray[x][y][z];//now you have an 3D array of a struct using VLA
...(more code)
So, if your program name was prog.exe, and you wanted a 3D array with dimensions: 10, 12, 13. then typing this on command line:
prog 10 12 13
Will result in creating molArray[10][12][13]