Adding elements to Struct Array

Question

struct GENERATIONS
{
char generation[MAX_ROWS][MAX_COLS];
int hasCycle;
};

typedef struct GENERATIONS Generation;

I have an array of type struct:

Generation generations[MAX_GENERATIONS];

I declare a Generationvariable like this:

Generation *currentGeneration = NULL;
currentGeneration = (Generation *) malloc(sizeof(Generation));

and attempt to add a generation to an array of generations: numGenerations is set to 0 then incremented via a loop.

copyGeneration(currentGeneration);
generations[numGenerations] = currentGeneration;

Yet each time, I get the error incompatible types when assigning to type 'Generation' from type 'struct Generation *. I understand this has to do with pointers which I do not understand but need.

Why is it that when I declare the array as:

Generation *generations[MAX_GENERATIONS];

Everything suddenly works?

Patrick Roberts · Accepted Answer · 2015-06-17 02:22:51Z

2

Each currentGeneration is a pointer to a Generation. Yet when you declare an array Generation generations[MAX_GENERATIONS] it expects each index to be a Generation, not a pointer to one. But when you declare the array as Generation *generations[MAX_GENERATIONS] it expects each index to be a pointer to a Generation, which is what you are assigning to each index.

answered Jun 17, 2015 at 2:22

Patrick Roberts

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3 Comments

Michael Miner Over a year ago

that makes sense, when I was testing getting elements from the generations array, I would create a new generation (as shown above) and do g = generations[0]. I was then able to use g to access all the "objects" data with out using the & to dereference it, why is that?

Patrick Roberts Over a year ago

Did you access with ->? That's how you access data on a struct pointer. e.g. g->hasCycle.

Michael Miner Over a year ago

yeah that is exactly how I did it

jangler · Accepted Answer · 2015-06-17 02:22:45Z

1

The error is telling you exactly what's wrong. Your variable currentGeneration is of type "pointer to Generation", and your variable generations is of type "array of Generation". You can't assign a pointer to Generation to an index of an array of Generation--you can only assign a Generation.

When you declare the array as Generation *generations[MAX_GENERATIONS], everything works because you're assigning a pointer to Generation to an index of an array of pointers to Generation.

answered Jun 17, 2015 at 2:22

jangler

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Comments

Mayukh Sarkar · Accepted Answer · 2015-06-17 11:09:11Z

1

To solve this problem, you can proceed in a different way. What you can do is this

#define MAX_GENERATIONS 1024 // you can take some other value too
#include <stdio.h>
#include <stdlib.h>
static int count = 0

Generation** push(Generation** generations, Generation obj){
 count++;
 if (count == MAX_GENERATIONS){
   printf("Maximum limit reached\n");
   return generations;

 if ( count == 1 )
   generations = (Generation**)malloc(sizeof(Generation*) * count);
 else
   generations = (Generation**)realloc(generations, sizeof(Generation*) * count);

 generations[count - 1] = (Generation*)malloc(sizeof(Generation));
 generations[count - 1] = obj;

 return generations;
}

int main(){
  Generation** generations = NULL;
  Generation currentGeneration;
  // Scan the the elements into currentGeneration
  generations = push(generations, currentGeneration); // You can use it in a loop
}

answered Jun 17, 2015 at 11:09

Mayukh Sarkar

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2 Comments

Michael Miner Over a year ago

as good as this looks, it seems a bit advanced and my prof may know that it came from a source other than me. I will say thank you an I will try this in the future!

Mayukh Sarkar Over a year ago

Okay no prob..but the objective is that you understand the logic here..and if possible help me in my reputation by up voting it..

Paul · Accepted Answer · 2015-06-17 02:23:42Z

0

currentGeneration is a Generation *, not a Generation.

You need an array of Generation * to hold it, not an array of Generation.

answered Jun 17, 2015 at 2:23

Paul

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