arrays with different sizes..

This is a discussion on arrays with different sizes.. within the C Programming forums, part of the General Programming Boards category; i want to define arrays with different sizes.. something like this.. Code: struct block{ int id; int points[3]; int cells[3]; ...

  1. #1
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    arrays with different sizes..

    i want to define arrays with different sizes..

    something like this..
    Code:
    struct block{
    	int id;
    	int points[3];
    	int cells[3];
    	struct covar covariant[cells[0]][cells[1]][cells[2]];
    };
    however since cells is defined inside the same structure i cannot define it like this..
    what is the way out??

  2. #2
    Gawking at stupidity
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    Dynamic memory allocation would be one option. Look at the malloc() function.
    If you understand what you're doing, you're not learning anything.

  3. #3
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    can you elaborate..
    afaik malloc is used for pointers..
    i couldnt find its use to initialize arrays...

  4. #4
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    You can simulate an array using malloc():
    Code:
    {
    int size;
    int *array;
    
    size = 5;
    
    array = malloc(sizeof(int) * size);
    // Now you can access array[0] through array[4].
    free(array);
    }
    If you understand what you're doing, you're not learning anything.

  5. #5
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    hey thanks that worked.. but i am unable to rewind the pointer.. how do i do that?

  6. #6
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    Rewind the pointer? I'm assuming you're doing something like:
    Code:
    {
      char *ptr = malloc(10);
    
      strcpy(ptr, "foo");
      while(*ptr)
      {
        putchar(*ptr);
        ptr++;
      }
      // Uh oh! I lost the original start of the memory allocation
    }
    If that's what you mean then there's no way to get it back. The way to handle that situation correctly is to leave a pointer pointing to the beginning and don't change it. Use a second pointer to walk around instead.
    Last edited by itsme86; 06-30-2006 at 03:36 PM. Reason: Oops...forgot to increment pointer
    If you understand what you're doing, you're not learning anything.

  7. #7
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    hi.. i ve been rtying to do what u said..
    but it gives me segmentation fault as soon as i try to declare another pointer..
    am i missing something here..

  8. #8
    Gawking at stupidity
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    I mean something like this:
    Code:
    {
      char *ptr, *ptr2;
    
      ptr = malloc(10);
    
      strcpy(ptr, "foo");
      ptr2 = ptr;
    
      while(*ptr2)
      {
        putchar(*ptr2);
        ptr2++;
      }
    
      free(ptr);
    }
    Maybe you could post the code you're actually trying and I or someone else can find the problem.
    If you understand what you're doing, you're not learning anything.

  9. #9
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    yes i got what you were trying to say..
    but i just found that declaring any more pointers in my program causes seg. fault...
    here's the code...
    if i delelte any one of the colored part the seg. fault goes away ..
    otherwise however.. any pointer declaration gives an error..
    i have a 768 mb ram on my comp.. could it be a memory problem??

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include "structures.c"
    
    int main()
    {
    FILE *stream;
    
    if((stream=fopen("block2.dat","r"))!=NULL){
    	printf("Succesfully opened file\n");
    }
    else{
    	printf("Failed to open file\n");
    	exit(1);
    }
    
    char number[5];char temp[10];
    
    char num_i[5],num_j[5],num_k[5];
    int j=0;int k=0;int i=0;int l,m,z,loop_var;
    
    fscanf(stream,"%s", number);
    num_blocks=atoi(number);
    printf("%i\n",num_blocks);
    struct block arr_blocks[num_blocks];
    int no_i,no_j,no_k,total_points;
    
    for(i=0;i<num_blocks;i++){
    	fscanf(stream,"%s	%s	%s", num_i,num_j,num_k);
    	arr_blocks[i].id=i;
    	arr_blocks[i].points[0]=atoi(num_i);
    	arr_blocks[i].points[1]=atoi(num_j);
    	arr_blocks[i].points[2]=atof(num_k);
    	arr_blocks[i].cells[0]=(atoi(num_i))-1;
    	arr_blocks[i].cells[2]=(atoi(num_j))-1;
    	arr_blocks[i].cells[1]=(atoi(num_k))-1;
    }
    
    struct point *p_map,*p_map_2;
    struct covar ***p_covariant[num_blocks];
    struct contra ***p_contravariant[num_blocks];
    
    for(loop_var=0;loop_var<num_blocks;loop_var++){
    	total_points=(arr_blocks[loop_var].points[0])*
    				(arr_blocks[loop_var].points[1])*(arr_blocks[loop_var].points[2]);
    
    	//allocating memory for covar
    	p_covariant[loop_var]= 
    		(struct covar***)malloc(sizeof(struct covar**)*((arr_blocks[i].cells[0])));
    	for (i=0; i<(arr_blocks[i].cells[0]); i++)
    	{
    		p_covariant[loop_var][i] = 
    		(struct covar**)malloc(sizeof(struct covar*)*(arr_blocks[loop_var].cells[1]));
    		for (j=0; j<(arr_blocks[loop_var].cells[1]); j++)
    		{
    			p_covariant[loop_var][i][j] =
    				 (struct covar*)malloc(sizeof(struct covar)*
    					(arr_blocks[loop_var].cells[2]));
    		}
    	}
    
    	**p_covariant[loop_var]=&p_covariant[loop_var][0][0][0];
    
    	p_contravariant[loop_var] = (struct contr..........*)malloc(sizeof(struct contr..........)*
    					(arr_blocks[i].cells[0]));
    	for (i=0; i<(arr_blocks[i].cells[0]); i++)
    	{
    		p_contravariant[loop_var][i] = 
    		(struct contr..........)malloc(sizeof(struct contra*)*(arr_blocks[loop_var].cells[1]));
    		for (j=0; j<(arr_blocks[loop_var].cells[1]); j++)
    		{
    			p_contravariant[loop_var][i][j] =
    			 (struct contra*)malloc(sizeof(struct contra)
    						*(arr_blocks[loop_var].cells[2]));
    		}
    	}
    	**p_contravariant[loop_var]=&p_contravariant[loop_var][0][0][0];
    
    	//Read all the co-ordinates from the file
    	p_map=(struct point*)malloc(sizeof(struct point)*total_points);
    	printf("%u\n",p_map);
    	p_map_2=p_map;
    	printf("%u\n",p_map_2);
    	j=0;k=0;
    	for(k=0;k<3;k++){
    		for(i=0;i<total_points; i++){
    			fscanf(stream,"%s",temp);
    			(*p_map).coords[k]=atof(temp);
    			p_map++;
    		}
    	}
    }
    return 0;
    }

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