I have a code that generates all of the possible combinations of 4 integers between 0 and 36.

This will be 37^4 numbers = 1874161.

My code is written in MATLAB:

```
i=0;
for a = 0:36
for b= 0:36
for c = 0:36
for d = 0:36
i=i+1;
combination(i,:) = [a,b,c,d];
end
end
end
end
```

I've tested this with using the number `3`

instead of the number `36`

and it worked fine.

If there are 1874161 combinations, and with An overly cautions guess of 100 clock cycles to do the additions and write the values, then if I have a 2.3GHz PC, this is:

1874161 * (1/2300000000) * 100 = 0.08148526086

A fraction of a second. But It has been running for about half an hour so far.

I did receive a warning that `combination changes size every loop iteration, consider predefining its size for speed`

, but this can't effect it that much can it?

Answer:

**As @horchler suggested you need to preallocate the target array**

- this is because your program is not
`O(N^4)`

without preallocation - each time you add new line to array it need to be resized
- so new bigger array is created (as matlab do not know how big array it will be it probably increase only by 1 item)
- old array is copied to it
- then deleted
- so when you have 10 items in array and adding 11th
- then a copying of 10 items is added to iteration ...
- if I am not mistaken that leads to something like
`O(N^12)`

which is massively more huge - estimated as
`(N^4)*(1+2+3+...+N^4)=((N^4)^3)/2`

- also the reallocation process is increasing in size breaching CACHE barriers
- slowing down even more with increasing
`i`

above each CACHE size barrier

**The only solution to this without preallocation is to store the result in linked list**

- not sure Matlab has this option
- but that will need one/two pointer per item (32/64 bit value)
- which renders your array
`2+`

times bigger

**If you need even more speed then there are ways (probably not for Matlab):**

- use multi-threading for array filling is fully parallelisable
- use memory block copy (
`rep movsd`

) or DMA the data is periodically repeating - You can also consider to compute the value from i on the run instead of remember the whole array, depending on the usage it can be faster in some cases...

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