Standard C library numeric array functions?

bulliver · 04-12-2006, 06:09 PM

Hello all,

I am just trying to play around with some C here, integer arrays in particular. I am running into difficulty because I cannot find any functions that deal with integer arrays in the glibc docs.

Specifically, I want to step through an array from beginning to end. The problem is I don't know how long my array is at compile time. I have two books on C here, and both of them use a fixed length arrays in their example code which makes it trivial to iterate over them using a for loop. What do you do if you don't know the length? I have found that:

Code:

int arraylength = ((int)sizeof array / 4);

sort of works, because an int is four bytes, but this seems really tacky, and non-portable. All the array functions in the glibc docs assume you have a char array. Is there a numeric array equivalent to '\0'?

Any pointers to example code, docs, or anything would be appreciated.

Thanks.

chrism01 · 04-12-2006, 06:33 PM

I know I haven't used C for a few yrs, but when I did (for a few yrs) I seem to remember that one slight inconvenience was that (in std C anyway) array lengths have to be declared at compile time.
I'm open to correction if anyone knows better...

bulliver · 04-12-2006, 07:02 PM

Quote:

array lengths have to be declared at compile time.

Right, sure. I guess I need to explain more. The length of the array is decided by the user by command line argument, so:

Code:

int x = atoi(argv[1]);
unsigned long set[x];

where 'set' is my array.

Here is some psuedocode for what I am trying to do:

Code:

while (test set[0]) {
    for (each set member) {
        if (a different test on set[member]) {
            remove set[member]
            reset 'set' indices
        }
    }
    shift set[1] >> set[0]
}

Initializing and populating the array isn't a problem, and even the first iteration of the while loop is no prob because I know the size of the initial array, but after removing items from it I no longer know the size.

This begs another question: are there functions to remove arbitrary array members and reset the indices, or do I have to use a temporary array to keep the numbers that pass the test, then overwrite set with the temp array?

graemef · 04-12-2006, 07:14 PM

As I understand your problem it is just a case of keeping track of the size of the array. You have an initial size ArySize, you remove an element from the array, so the size is --ArySize

If you wish to remove an element there are a couple of ways of doing it, but you will have to do it. One could be to use a temp as you suggested another could be to just overwrite the element to be removed and shuffle everything to the left, hence:

myArray [1,4,6,5,8,9,3,7]
ArySize = 8
now remove the 5
myArray [1,4,6,8,9,3,7,7]
ArySize = 7
(and so the final element is ignored.)

graeme

bulliver · 04-12-2006, 07:36 PM

Quote:

As I understand your problem it is just a case of keeping track of the size of the array. You have an initial size ArySize, you remove an element from the array, so the size is --ArySize

Of course! Thanks. I guess I just thought finding the length of an array would be a common enough task there would be a standard function for it.

Quote:

another could be to just overwrite the element to be removed and shuffle everything to the left,

Ok, I like this idea, but I don't understand your example. Where is the extra 7 coming from? How are you removing the 5? How do you 'shuffle' the elements left?

I have found some code:

Code:

memmove(array, array+1, sizeof(int) * 4);

which will do the 'shift set[1] >> set[0]' which I need, but I am unsure how to use it do remove an element in the middle of an array...

graemef · 04-12-2006, 07:41 PM

The pseudo code might be like this

Code:

remove (int ele)
for (int i = ele ; i < ArrSize; i++)
   myArray[i] = myArray[i+1]
--ArrSize;

Where ele is the element number to be removed, in the example I gave above it would be 3

graemef · 04-12-2006, 07:49 PM

Okay now for one concern I have. An array in C is declared with a constant size see. I suspect that what you will get with you above example (if it compiles) is an array that will stomp all over your memory and one day you will get a segmentation fault and be quite sad.

tamoneya · 04-12-2006, 08:18 PM

i dont see how it could mess up your memory if you are making it smaller. But if you are really worried abotu that you should jst have it make a new array with a new size and copy the elements you want to keep into and then change the pointer. Then repeat your loop or whatever.

graemef · 04-12-2006, 08:45 PM

Sorry I should of made it clearer, I was worried about the following statement:

Code:

int x = atoi(argv[1]);
unsigned long set[x];

That was to be used to create the array