Dynamic Memory Allocation

• In the previous example, we had to set the array size for the population in the source code. If a user inputs a bigger population size than specified in the source code, it causes a problem. Also it is wasteful to make a big array when you are simulating with small population size.

• It will be convenient to adjust the size of array while the program is executing (so you don't have to recompile the source for differnt population sizes). This is called dynamic allocation of memory.

• Example:

  #include <stdlib.h>  /* for malloc() */
  #include <stdio.h>
  
  int main(void) {
    int *dynArrPtr;
    dynArrPtr = (int *) malloc(50 * sizeof(int));
    if (dynArryPtr == NULL) {
      fprintf(stderr, "No memory!!\n");
      exit(1)
    }
    /* do something with the array */
    free (dynArrPtr);
    return 0;
  }
  

  • sizeof(varType) returns the amount of memory space required for one variable of type varType. The amount of memory space is measured by a unit called bytes.

  • malloc(size) will allocate the memory of size, and return the address of this memory.

  • You need to tell that the returned address is interpreted as a pointer to a certain type (int * in this case). This is called casting. You have encountered casting when you were converting integer to double with (double).

  • It is important to check whether malloc() successfully find memory. If unsuccessful, the pointer become something called NULL pointer. This means that the pointer is NOT pointing to anywhere.

  • After you are done using the memory, you need to tell that you don't need the memory any more by free(ptr). If you keep making new arrays without freeing the ones you don't need any more, you might use up all memory. The memory will be released when the program ends.

• calloc()
  calloc(unsigned int numMember, unsigned int size) allocates memory for an array of nmemb elements of size bytes each and returns a pointer to the allocated memory. So instead of malloc() above, you can use:

  dynArrPtr = (int *) calloc(50, sizeof(int));
  

• Modify the genetic drift program to use dynamic memory allocation.