c loop through struct array in reverse

Question

I'm trying to loop through an array of structs in reverse, not quite sure how to go about it. This is how I'm looping through normally:

    struct Thing* ptr = things;
    struct Thing* endPtr = things + sizeof(things)/sizeof(things[0]);

    for(ptr < endPtr)
    {
        // do stuff
    }

What is `things`? And why not use a loop using the *indexes* instead? — Some programmer dude, Nov 11 '16 at 09:38
`ptr[(sizeof(things)/sizeof(things[0]))-1]` is the last element, so starting from that one down to 0 index... — LPs, Nov 11 '16 at 09:39
@WhozCraig `#define for while`. Works like a charm. And it also allows me to do `do {...} for (ptr < endPtr);` :) — Lundin, Nov 11 '16 at 14:56
obscuring the code by eliminating the normal usage of the `for` statement is a very bad idea. As it is, the posted code does not compile and this line: `struct Thing* endPtr = things + sizeof(things)/sizeof(things[0]);` yields a pointer to just past the end of the array rather than a pointer to the last element — user3629249, Nov 12 '16 at 18:42

score 2 · Answer 1 · answered Nov 11 '16 at 09:53

2

Assuming N >= 0, the item-magnitude of your things sequence , you can use indexes, of course, but really you simply need a single pointer:

struct Thing *ptr = things + N;
while (ptr != things)
{
    --ptr;

    // do something with *ptr;
}

answered Nov 11 '16 at 09:53

WhozCraig

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@IanAbbott technically, you can't shorten this using a post-decrement in the while-expression. I've asked only two questions on SO, [one of them directly relating to that very subject.](https://stackoverflow.com/questions/30512669/does-applying-post-decrement-on-a-pointer-already-addressing-the-base-of-an-arra) Hard as it is to fathom, the final decrement actually invokes UB. I chose the code in this post for that very reason. – WhozCraig Nov 11 '16 at 10:36
Yes, I realized my mistake just after posting my comment and deleted it. (For reference, I suggested post-decrementing `ptr` in the test: `while (ptr-- != things)`. The problem with that is the final decrement leaves `ptr` less than `things`, which is UB, as mentioned by WhozCraig.) – Ian Abbott Nov 11 '16 at 10:39
@IanAbbott I'm pretty sure `while (ptr != things && --ptr)` *would* work, however, since the decrement would be skipped on the short circuit eval. It would, however, be oddly cryptic to anyone reviewing the code. – WhozCraig Nov 11 '16 at 10:44
@WhozCraig Interesting, I just recently asked a similar `++` [question](http://stackoverflow.com/q/40537682/2410359) to your `--` [post](https://stackoverflow.com/questions/30512669/does-applying-post-decrement-on-a-pointer-already-addressing-the-base-of-an-arra). – chux - Reinstate Monica Nov 11 '16 at 14:46

Muhammad Mohsin Khan · Answer 2 · 2016-11-11T13:56:41.953

0

This can be a possible solution

 // assuming 'Thing' is the structure in question and 'size' is the length of array
struct Thing array[size];   // created an array of structure Thing having length equals size

struct Thing *ptr;  // pointer of type struct Thing 
ptr = array;
ptr = ptr + (size-1); // point the pointer to last element of the array
int i; //counter

for( i=0; i < size; i++ )
{
    // do something with the pointer ptr
    ptr--; 
}

edited Nov 11 '16 at 13:56

answered Nov 11 '16 at 10:11

Muhammad Mohsin Khan

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Your `ptr = ptr + ...` invokes undefined behavior. `ptr` is indeterminate when its value is taken with the right side expression of that assignment. And the size computation is wrong anyway. Pointer arithmetic strides to the size of the pointed-to object type already. – WhozCraig Nov 11 '16 at 10:39
`ptr` should initialized before using. You program results in UB. – babon Nov 11 '16 at 12:32

score 0 · Answer 3 · answered Nov 11 '16 at 14:49

0

It is possible to use pointers in a down-counting loop, but I would instead prioritize readability:

size_t size = sizeof(things) / sizeof(things[0]);

for(size_t i=0; i<size; i++)
{
  size_t index = size - i - 1;
  things[index] = something;
}

answered Nov 11 '16 at 14:49

Lundin

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c loop through struct array in reverse

3 Answers3