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c++arraysloops
mibrahimy
mibrahimy

Reputation: 732

Add Arrays without a loop C++

How do we add two arrays of the same size without writing an explicit loop in C++ For example:

int a[3]={1,2,3};
int b[3]={4,3,2};
int c[3]; //to contain sum of a and b

Upvotes: 5

Views: 2269

Answers (2)

Picaud Vincent
Picaud Vincent

Reputation: 10982

If you really want no loop and performance you can try that:

#include <emmintrin.h>
#include <iostream>

alignas(16) int32_t a[4] = {1, 2, 3, 0};
alignas(16) int32_t b[4] = {4, 3, 2, 0};
alignas(16) int32_t c[4];

main()
{
  __m128i simd_a = _mm_load_si128((__m128i*)a);
  __m128i simd_b = _mm_load_si128((__m128i*)b);


  // No loop for addition and performance is here.
  //
  __m128i simd_c = _mm_add_epi32(simd_a, simd_b);

  _mm_store_si128((__m128i*)c, simd_c);

  std::cout << "\n" << c[0] << " " << c[1] << " " << c[2] << " " << c[3];
}

More seriously if your vectors are smalls and if performance is a real concern you can use SIMD. Do not recode simd yourself, but use one of the excellent libraries that are available.

Examples of such C++ libraries:

Upvotes: 1

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170211

If an explicit loop is out of the question, then use a possibly implicit one. Let the standard library do it for you.

std::transform(a, a + 3, b, c, std::plus<int>{});

If you find yourself doing it on arrays of the same size often, you can even templatize it:

template<typename T, std::size_t N>
void add_arrays( T (const &a)[N], T (const &b)[N], T (&c)[N] ) {
  std::transform(a, a + N, b, c, std::plus<T>{});
}

The compiler will be nice and check the sizes for you. And you don't even have to stop there. There's loads of ways to make add_arrays usable in more contexts.

Having said all that. Try to shift away from using raw arrays. Use std::array as a default. It's a thin wrapper over a raw array, but it has value semantics. And if you need a fixed collection of things for some purpose (say you are doing graphics programming, so you store coordinates), try to use designated types for it instead.

Upvotes: 5

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