Help us understand the problem. What is going on with this article?

fortranで配列の平均値計算を高速化

More than 3 years have passed since last update.

普通の書き方(昔ながらの書き方)

real(4) :: data(N)
のように,N個のデータが入った配列の平均値を求めるには,昔ながらの書き方だと,

function mean(data) result(out)
    real(4), intent(in) :: data(:)
    real(4) :: out
    integer(4) :: i

    out = 0
    do i = 1, size(data)
        out = out + data(i)
    end do
    out = out / size(data)
end function mean

こんな物が真っ先に浮かぶだろう.
もしもdataに欠損値があるのなら,

function mean(data, undef) result(out)
    real(4), intent(in) :: data(:), undef
    real(4) :: out
    integer(4) :: i, N

    out = 0
    N = 0
    do i = 1, size(data)
        if(data(i) /= undef) then
            out = out + data(i)
            N = N + 1
        end if
    end do
    out = out / N
end function mean

こんな風に書ける.
しかし,これだと,いちいちdataのメモリにアクセスするためあまり高速ではない.
fortranプログラムの高速化は,いかに組み込み関数や配列演算を駆使するかというのが基本らしいので,ちょこっとメモ.

欠損値のなしのデータの場合

function mean(data) result(out)
    real(4), intent(in) :: data(:)
    real(4) :: out

    out = sum(data) / size(data)
end program mean

これは,簡単ですね.

欠損値ありのデータの場合

function mean(data, undef) result(out)
    real(4), intent(in) :: data(:), undef
    real(4) :: out

    out = sum( data, mask = data /= undef ) / count( mask = data /= undef )
end function mean

組み込み関数のマスクを利用すれば,コードも短く,速度も速いらしい

Why not register and get more from Qiita?
  1. We will deliver articles that match you
    By following users and tags, you can catch up information on technical fields that you are interested in as a whole
  2. you can read useful information later efficiently
    By "stocking" the articles you like, you can search right away
Comments
No comments
Sign up for free and join this conversation.
If you already have a Qiita account
Why do not you register as a user and use Qiita more conveniently?
You need to log in to use this function. Qiita can be used more conveniently after logging in.
You seem to be reading articles frequently this month. Qiita can be used more conveniently after logging in.
  1. We will deliver articles that match you
    By following users and tags, you can catch up information on technical fields that you are interested in as a whole
  2. you can read useful information later efficiently
    By "stocking" the articles you like, you can search right away
ユーザーは見つかりませんでした