c: pointers - how to increase every 2nd byte by X

I have a pointer that's开发者_StackOverflow holding 100 bytes of data. i would like to add 5 to every 2nd byte.

example:

1 2 3 4 5 6

will become:

1 7 3 9 5 11

Now i know i can do a for loop, is there a quicker way? something like memset that will increase the value of every 2nd byte ?

thanks

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A loop would be the best way. memset() is efficient for setting a contiguous block of memory. Wouldn't be much help for you here.

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In which format do you have your bytes? As concatenated chars? Or are the bytes a subpart of e.g. a uint32?

In general, a loop is the best way for doing this - even if you would be able to apply a pattern-like mask with memset, you would still need to create it and that would take the same amount of CPU cycles.

If you would have 4 bytes per element (e.g. uint32), you could cut the cpu cycles in half by creating a pre-defined mask for adding. But attention: such a solution would not check for overflows (pseudocode):

uint32* ptr = new uint32[16]; // creates 64 bytes of data
(...) fill data
for (int k=0; k < 16; ++k)
{
   // Hardcored Add-Mask for Little Endian systems
   ptr[k] += 0x05000500; // dereference and add mask to content
}

Edit: Please note that this assumes a little endian system and is C++ Pseudocode.

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If memset supported increasing the value of every nth byte, how do you think it would accomplish it?

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You can in fact make it faster by using loop unrolling, but you'll need to know that your array is a fixed size. Then you would skip the loop overhead by just repeatedly assigning the value:

array[ 1 ] += 5;
array[ 3 ] += 5;
array[ 5 ] += 5;
...

By doing this you don't have the overhead incurred by jump and test instructions that would be present in a loop, but you pay for it in code bloat.