generating a new array within a loop

Question

I'm doing a little programming experiment in C on ways to represent multiple lines of text.

I've come up with four ways: arrays of arrays, arrays of pointers, pointers of arrays, and pointers of pointers.

I've run into a little problem with pointers of arrays. Every time I create an array, it actually just reuses the array I already created.

Check out the output of the "pointers of arrays" (pa) part of my program to see what I mean:

variable    address         value
==========  ==============  =====
pa          0x10f500970

*(pa+0)     0x7fff6f0334bd

*(pa+0)[0]  0x7fff6f0334bd  t
*(pa+0)[1]  0x7fff6f0334be  e
*(pa+0)[2]  0x7fff6f0334bf  s
*(pa+0)[3]  0x7fff6f0334c0  t
*(pa+0)[4]  0x7fff6f0334c1
*(pa+0)[5]  0x7fff6f0334c2  0

*(pa+1)     0x7fff6f0334bd

*(pa+1)[0]  0x7fff6f0334bd  t
*(pa+1)[1]  0x7fff6f0334be  e
*(pa+1)[2]  0x7fff6f0334bf  s
*(pa+1)[3]  0x7fff6f0334c0  t
*(pa+1)[4]  0x7fff6f0334c1
*(pa+1)[5]  0x7fff6f0334c2  1

See how the address for each array are the same? Not very cool, is it?

So here's my question: Is there a way to actually declare a new array in a n > 1 loop, rather than rewrite the contents of the already existing array?

Here's the snipplet: (LIST_SIZE = 2 and TEXT_SIZE = 6)

void** pa = malloc(LIST_SIZE * sizeof(void*));

printh();

printf("pa\t\t%p\n\n", pa);

for(int i = 0; i < LIST_SIZE; i++)
{
  char txt[TEXT_SIZE + 1];

  sprintf(txt, "test %d", i);

  *(pa + i) = txt;

  printf("*(pa+%d)\t\t%p\n\n", i, *(pa + i));

  for(int j = 0; j < TEXT_SIZE; j++){
    printf("*(pa+%d)[%d]\t%p\t%c\n", i, j, &(((char*)*(pa+i))[j]), ((char*)*(pa+i))[j]);
  }

  printf("\n");
}

Thanks. If you have any other comments or suggestions, I'd be interested in hearing them, too.

The full source code is available at: https://gist.github.com/80m/7143558

jxh · Accepted Answer · 2013-10-24 20:31:25Z

1

The variable txt is local to the for loop. At each iteration, the old txt variable becomes invalid, and a different txt instance is created. This means the pointer that was assigned txt in the previous iteration is not pointing to a valid object when the next iteration starts.

When the loop is finished, none of your pointers point to anything valid.

One way to fix this is to dynamically allocate memory for each pointer you want to assign to.

    pa[i] = malloc(TEXT_SIZE+1);
    snprintf(pa[i], TEXT_SIZE+1, "test %d", i);

You say you want "pointers to arrays", yet you declared of void **pa, which makes pa a pointer to a pointer. If you want pa to a pointer to an array, you would declare it this way:

char (*pa)[TEXT_SIZE+1] = malloc(LIST_SIZE * sizeof(*pa));

So now, pa[0] would be one 7 byte array, and pa[1] would be a second 7 byte array. You do away with the need to do any dynamic allocation within the loop.

edited Oct 24, 2013 at 20:31

answered Oct 24, 2013 at 19:48

jxh

70.8k8 gold badges116 silver badges204 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

80m Over a year ago

Yes but wouldn't that turn "pointers of arrays" into "pointers of pointers"?

jxh Over a year ago

If a pointer variable is assigned the address of an object, it points to the object. It doesn't matter if the object is dynamically allocated or not. Dereferencing the pointer obtains the pointed to object.

80m Over a year ago

Well, first of all the code you provided doesn't compile (pa + i needs to be dereferenced). Secondly, where is the array that's being pointed to? You might be onto something but I'll have to experiment some more.

jxh Over a year ago

@80m: Sorry about that. My brain is not a high quality C compiler.

80m Over a year ago

Your edit appears to be correct. I'm impressed. How did you come up with that? I thought it would be an array of char*.

|

Shubham · Accepted Answer · 2013-10-24 20:26:53Z

1

Each time the loops runs pa points to the same txt. So, the address that pointer pa is pointing to remains the same. If you want to point this to another location then try creating a different char[], say txt1, and assign it to (pa+i) during second iteration.

I tried this:

int i=0,j=0;

void** pa = malloc(LIST_SIZE * sizeof(void*));

printf("pa\t\t%p\n\n", pa);

for(i = 0; i < LIST_SIZE; i++)
{

char txt[TEXT_SIZE + 1];
char txt1[TEXT_SIZE + 1];


  sprintf(txt1, "test %d", i);
  sprintf(txt, "test %d", i);
  if (i==0)
  {
    *(pa + i) = txt;
  }
  else
  {
    *(pa + i) = txt1;
  }

  printf("*(pa+%d)\t\t%p\n\n", i, *(pa + i));

  for(j = 0; j < TEXT_SIZE; j++){
    printf("*(pa+%d)[%d]\t%p\t%c\n", i, j, &(((char*)*(pa+i))[j]), ((char*)*(pa+i))[j]);
  }

  printf("\n");
}

and got different addresses for different iterations.

I think the problem is not with the pointer but with the variable it is trying to refer to and it remains the same variable every time. Hope this helps!

answered Oct 24, 2013 at 20:26

Shubham

111 bronze badge

1 Comment

80m Over a year ago

Yes but what if I changed the LIST_SIZE to 3? The same problem would occur except this time with the txt1 variable.

Collectives™ on Stack Overflow

generating a new array within a loop

2 Answers 2

7 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

7 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related