Intel® oneAPI Math Kernel Library
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## best way to add a value to all elements in a vector

Beginner
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Hi,

Is there a betterway to add a value to all elements in a vector than this:

cblas_saxpy(arrSize, &m_anyvalue, m_pU, 1, m_pX, 1 );

where

m_pU = a vector of ones

m_pX = the vector I want to add m_anyvalue to

Thanks

13 Replies
Employee
1,015 Views

Hello,

cblas_saxpydoesn't lookright routine to use. My recommendation will be:

vsLinearFrac( n, a, b, scalea, shifta, scaleb, shiftb, y )

Description

The v?LinearFrac function performs linear fraction transformation of vectors a by vector b with scalar parameters: scaling multipliers scalea, scaleb and shifting addends shifta, shiftb:

y=(scaleaa+shifta)/(scalebb+shiftb), i=1,2 n

Just set scalea=1.0f, scaleb=0.0f, shiftb=1.0f

I hope that helps,
Sergey

Beginner
1,015 Views

Is the function smart enough to avoid performing the unnecessary computations?
Employee
1,015 Views
Hello again,

Yes, the function should be able to effectively handlesuch subtle cases. Let me know if it works fine.

With one more note. y=x+scale stresses the ADD unit only. It means in particular that, for example, MUL unit is iddle while performaing vector ADD. (Please remember all modern Intel CPUs can issue add and mul operations simultaneously). If you use Intel C++ compiler (which has quite good vectorizer) you might get better overall performance if you can mix this computation (y=x+scale) with other stuff so that all CPU compute units are busy.

Regards,
Sergey
Beginner
1,015 Views

Oh no, it seems there is no overload for complex input...

Employee
1,015 Views
Correct. Only real single and double precision variant for v?LinearFrac
Black Belt
1,015 Views
So you may be better off using your compiler (such as Intel or recent gcc).
Beginner
1,015 Views
How? (I am not really a programmer so it is not obvious to me) Do you mean I should just write a for loop and trust the compiler to optimize it?
Black Belt
1,015 Views
How? (I am not really a programmer so it is not obvious to me)

In that case, you may want to use mathematical reasoning to see if you can avoid a redundant calculation altogether.

For example, if z is a complex-valued n-vector, and the desired result is, say, w = z = u + i v, and you wish to add the real number c to the real parts of the components of z, you can compute w1 = (u + n c) + i v as the modified result.
Beginner
1,015 Views
Sure, but how do I optimize it? The re/im parts of the complex numbers are interleaved in the array. I cant use the functions mentioned above and writing a for-loop with a step increment of 2 doesn't seem so tempting.

Wouldn't it be nice to add a function to VML that does addition with a constant?
Black Belt
1,015 Views
I don't think that you understood the thrust of my remarks. There is nothing to optimize in a calculation that is not even performed!

So far, you have stated that you want a routine to add a constant to a vector, and you did not like the ones that were suggested.

If you state details of how the added constant plays a role in subsequent calculations, further suggestions may become possible.
Employee
1,015 Views
Exactly.

for (i=0; i{
x += c;
}

Intel C++ compiler should nicely vectorize the loop

Thanks,
Sergey
Beginner
1,015 Views
mecej4:

Yup, nothing like a calculation that never needs to be done:) I don't have any particular problem, but I can imagine that in some cases one can get away with an initialization to a contant instead of an addition. Example:

a = zeros
a = a + b*c
a = a + d

can be changed to

a = d
a = a + b*c

Is there something like a "vector-memset" in MKL?
Beginner
1,015 Views
Ah - nice! Thanks Sergey!