Struct with variable size of array - c

Question

I want to save data in arrays called plist. These arrays can vary in size and are part of a structure called ParticleList. I know how to create one list of size n[0]. n[0] for example is of size 2. Thus, a list of size 2. But what do I have to do, if I want to create several lists with size n[0], n[1], n[2] of type ParticleList?
To cut a long story short: How should I modify my code in order to access lists of variable size somehow like pl[numberOfList].plist[PositionInArray] = -1 or `pl[numberOfList] -> plist[PositionInArray] = -1'
#include <stdlib.h>
#include <stdio.h>
typedef struct{
double *plist;
int plistSize;
} ParticleList;
void sendPar(int *n, int nl){
// Allocate memory for struct ParticleList
ParticleList *pl = malloc(sizeof(ParticleList));
// Allocate memory for list
pl->plist = malloc(sizeof(double)*n[0]);
// Fill list with data
for(int k=0; k<n[0]; k++){
pl->plist[k] = -1;
}
// Write size of list into file
pl->plistSize = n[0];
// Print data
printf("Content of list:\n");
for(int k=0; k<n[0]; k++){
printf("%lf\n", pl->plist[k]);
}
printf("Size of list: %d\n", pl->plistSize);
// Free memory
free(pl);
}
int main(){
// Number of lists
int nl = 3;
// Size of lists
int n[nl];
n[0] = 2;
n[1] = 3;
n[2] = 4;
sendPar(n, nl);
}

Answer 1

Do you mean something like this?
typedef struct{
int plistSize;
double* plist;
} ParticleList;
int main()
{
int i, z = 0;
/* Assuming you have three lists with three different sizes */
double list1[2] = {-1.0, -1.1};
double list2[3] = {-2.0, -2.1, -2.2};
double list3[4] = {-3.0, -3.1, -3.2, -3.3};
/* Create an array of three Particle Lists */
ParticleList pl[3] = {{list1, 2},{list2, 3},{list3, 4}};
/* Access the values in the Particle Lists */
for(i = 0; i < 3; i++)
{
printf("ParticleList pl[%i]:\n", i);
for(z = 0; z < pl[i].plistSize; z++)
{
printf("pl[%i].plist[%i] = %f\n", i, z, pl[i].plist[z]);
}
}
/* Change the first item of the second list */
pl[1].plist[0] = 2.3;
}
This way you can access each item in each list by
pl[<index of list>].plist[<index of list item>]
A bit more dynamic by using flexible array members (this way one of the lists can be replaced by another list of different size):
Note that I changed the struct!
typedef struct{
int plistSize;
double plist[];
} ParticleList;
int main()
{
int i, z = 0;
ParticleList *pl[3];
/* Allocate memory for the lists */
pl[0] = malloc( sizeof(ParticleList) + sizeof(double[2]) );
pl[0]->plistSize = 2;
pl[1] = malloc( sizeof(ParticleList) + sizeof(double[3]) );
pl[1]->plistSize = 3;
pl[2] = malloc( sizeof(ParticleList) + sizeof(double[4]) );
pl[2]->plistSize = 4;
/* Write the values in the Particle Lists */
for(i = 0; i < 3; i++)
{
printf("ParticleList pl[%i]:\n", i);
for(z = 0; z < pl[i]->plistSize; z++)
{
pl[i]->plist[z] = -i;
}
}
/* Print the values */
for(i = 0; i < 3; i++)
{
printf("ParticleList pl[%i]:\n", i);
for(z = 0; z < pl[i]->plistSize; z++)
{
printf("pl[%i]->plist[%i] = %f\n", i, z, pl[i]->plist[z]);
}
}
/* Change the first value of the second list */
pl[1]->plist[0] = -1.1;
/* Replace the first list by a new one */
free(pl[0]);
pl[0] = malloc( sizeof(ParticleList) + sizeof(double[5]) );
pl[0]->plistSize = 5;
/* Assign some new values to the new list 1 */
pl[0]->plist[0] = -4.1;
pl[0]->plist[1] = -4.2;
pl[0]->plist[2] = -4.3;
pl[0]->plist[3] = -4.4;
pl[0]->plist[4] = -4.5;
/* Print the values */
for(i = 0; i < 3; i++)
{
printf("ParticleList pl[%i]:\n", i);
for(z = 0; z < pl[i]->plistSize; z++)
{
printf("pl[%i]->plist[%i] = %f\n", i, z, pl[i]->plist[z]);
}
}
/* free all lists before exiting the program */
for(i = 0; i < 3; i++)
{
free(pl[i]);
}
return 0;
}

Answer 2

It would seem you are looking for the language feature called flexible array member. It works like this:
typedef struct{
int plistSize;
double plist[];
} ParticleList;
ParticleList *pl = malloc( sizeof(ParticleList) + sizeof(double[n]) );
pl->plistSize = n;
...
free(pl);
Where n is the size you want plist to have.