Circular shifting an array with `memmove`

Question

Say I have an integer array like such

#define MAX 5
int bar [MAX] = {0};
int foo [MAX] = {3,1,0,0,0};

Now I'd like to shift this array so that all null entries are to the left, i.e. bar = {0,0,0,3,1}.

I thought I could do this by

Finding the number of shifts I must perform
Using memmove() to do the shift.

I solved 1. with the following loop

for (shift = MAX - 1; shift >= 0; --shift) {
    if (foo[shift]) break;
}

But I do not know how to use memmove() now to perform the shift, I attempted to do memmove(bar + shift, foo, MAX * sizeof(*foo)-1); but with no success.

Is memmove() the correct tool for the job? How can I circulate an integer array as described? What if this is happening inside a function where bar is a passed as a pointer?

@EugeneSh. Do I need an intermediate buffer? I'm moving `foo` into `bar`, so `foo` will be intact, no? Couldn't I just move `foo` again? — Bernardo Meurer, Mar 27 '17 at 20:41
Right. Though you want it in-place. In this case you can use `memcpy` instead.. — Eugene Sh., Mar 27 '17 at 20:43
@EugeneSh.: I thought the *shift* was meant to happen in place, `memcpy` is fine to move the appropriate elements into the right places into `bar`. — chqrlie, Mar 27 '17 at 20:52

ikegami · Accepted Answer · 2017-03-28T12:24:19.160

4

The formula you used to calculate shift does not give you the number of positions by which to shift.
You're not moving the correct number of bytes (MAX * sizeof(*foo)-1 is in no way based on the number of trailing zeroes).

#include <string.h>

#define MAX 5
int bar[MAX] = {0};
int foo[MAX] = {3,1,0,0,0};

int i;
int shift = 0;
for (i=MAX; i--; ) {
   if (foo[i]) {
      shift = MAX - (i + 1);
      break;
   }
}

memcpy(bar+shift, foo, (MAX-shift) * sizeof(*foo));
memcpy(bar, foo+(MAX-shift), shift * sizeof(*foo));

You can do it in-place since you know what values to "rotate in".

memmove(foo+shift, foo, (MAX-shift) * sizeof(*foo));
memset(foo, 0, shift * sizeof(*foo));

memmove must be used instead of memcpy since the destination and source buffers may overlap.

edited Mar 28 '17 at 12:24

answered Mar 27 '17 at 20:44

ikegami

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Hmmm, after the`for()`, `shift` can have the values of `0, 2, 3,4,5,6`. Not all those values work well in the following `mem...()` code. – chux - Reinstate Monica Mar 27 '17 at 21:51
@chux How would you fix that? – Bernardo Meurer Mar 27 '17 at 22:20
@ikegami, Implementing the above yields me a "exit code 139" SIGSEGV. If `bar` is a pointer must I do something special when passing it to `memcpy()`? – Bernardo Meurer Mar 27 '17 at 22:28
The shift-estimation part is what breaks this. – Bernardo Meurer Mar 27 '17 at 23:08
@BernardoMeurer I find your post unclear. "all null entries are to the left". What about `0,1,0,2,0`? This answer is likely on the right track, just needs a minor fix. – chux - Reinstate Monica Mar 28 '17 at 02:06
@chux In my use case that will never happen, although I guess finding a more general solution is never bad. – Bernardo Meurer Mar 28 '17 at 02:13
Fixed. . . . . . . – ikegami Mar 28 '17 at 12:24

Circular shifting an array with `memmove`

1 Answers1