Some way for allocating and initializing array faster than plain loop-and-set

Question

This is how I allocate and initialize an array in C++.

char *dataPtr;

dataPtr= new char [40000]; 

int i; 

for(i=0;i<40000;i++)

{
    dataPtr[i]=200; 
}

Is there any other faster and cleaner way of achieving this as the for loop is taking considerable time for my application?

I legitimately doubt this loop is taking any time on your application. — Rapptz, Jul 02 '14 at 07:50
I completely agree with Rapptz. Maybe if you have optimizations off. Anyways, fill_n or memset might buy you some extra time if you have some luck, but most probably you are trying to fix something that's not broken, or at least, you're looking at wrong thing or you're not measuring the time correctly — quetzalcoatl, Jul 02 '14 at 07:52

score 4 · Answer 1 · answered Jul 02 '14 at 07:49

4

Use an std::vector:

std::vector<char> data(40000, 200);

answered Jul 02 '14 at 07:49

vz0

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may be I am doing something wrong , but I am getting this error: Error 2 error C2040: 'dataPtr' : 'std::vector<_Ty>' differs in levels of indirection from 'char *' – user2799508 Jul 02 '14 at 07:55
I just added these two lines to my code: char *dataPtr; std::vector dataPtr(40000, 200); – user2799508 Jul 02 '14 at 07:56
@user2799508 Just `#include ` and `std::vector dataPtr(40000, 200);` – P0W Jul 02 '14 at 07:57
3

@user: You don't need to add the first line you quoted, `char*dataPtr`. The vector-line can handle it by itself. Use **either** an array, **or** a vector. Consider the vector to be a super array on steroids. – quetzalcoatl Jul 02 '14 at 07:58

Dietrich Epp · Answer 2 · 2014-07-02T07:54:31.693

4

You can use memset(), which has a core of optimized assembly with SSE/SSE2/etc instructions as appropriate at runtime. The memset() function is designed to do exactly this task: set each char in an array of char to a specific value, as quickly as possible.

#include <cstring>

char *dataPtr = new char[LENGTH];
std::memset(dataPtr, 200, LENGTH);

However, modern compilers will do this for you, you can check the assembly, and you might find a call to memset() or something similar in the original code that used a for loop. It's only 40 kB anyway, so you're not going to save much time, unless you have to initialize the array very often.

edited Jul 02 '14 at 07:54

answered Jul 02 '14 at 07:49

Dietrich Epp

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@haccks: I fail to see the relevance of that link... could you explain? – Dietrich Epp Jul 02 '14 at 07:52
@quetzalcoatl: The C++ standard defines a `char` to be one byte... no matter how wide it is. So the guarantee is there, but the definition of "byte" is not the one you are used to. – Dietrich Epp Jul 02 '14 at 07:59
ouch, you're right. That's byte that's of undefined width, not char, I often mistake them when writing in haste. – quetzalcoatl Jul 02 '14 at 08:00

score 3 · Answer 3 · answered Jul 02 '14 at 07:49

3

To initialize each array elements by 200 you can use

std::fill_n(dataPtr, 40000, 200);

answered Jul 02 '14 at 07:49

haccks

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FWIW, `std::fill_n` will be not any _fast_, it uses loop too – P0W Jul 02 '14 at 07:52
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@P0W: That depends on the compiler. I know MSVC++ will use `memset()` if the parameters to `std::fill` or `std::fill_n` are pointers to `char`. – Blastfurnace Jul 02 '14 at 17:11

Some way for allocating and initializing array faster than plain loop-and-set

3 Answers3