Matlab:如果索引在范围内，则求和相应的值 [英] Matlab: Sum corresponding values if index is within a range

问题描述

我一直在疯狂地试图找到一种加快速度的方法.现在，我当前的代码讨论了大约200秒，遍历了77000个事件.我希望有人可以帮助我加快速度，因为我必须做500左右.

I have been going crazy trying to figure a way to speed this up. Right now my current code talks ~200 sec looping over 77000 events. I was hoping someone might be able to help me speed this up because I have to do about 500 of these.

问题: 我有两个数组(均为200000x1)，它们对应于77000个事件中命中的能量和位置.我将每个事件的范围分为两个数组，event_start和event_end.我要做的第一件事是在特定范围内寻找位置，然后将对应的能量放入其自己的数组中.为了从这些信息中获取所需信息，我循环浏览每个事件及其对应的开始/结束，以总结每个事件所产生的所有能量.我的代码如下:

Problem: I have arrays (both 200000x1) that correspond to Energy and Position of a hit over 77000 events. I have the range of each event separated into two arrays, event_start and event_end. First thing I do is look for the position in a specific range, then I put the correspond energy in its own array. To get what I need out of this information, I loop through each event and its corresponding start/end to sum up all the energies from each it hit. My code is below:

    indx_pos = find(pos>0.7 & pos<2.0);
    energy = HitEnergy(indx_pos);

    for i=1:n_events
        Etotal(i) = sum(energy(find(indx_pos>=event_start(i) …
        & indx_pos<=event_end(i))));    
    end

样本输入和输出:

% Sample input 
% pos and energy same length

n_events = 3;
event_start = [1 3 7]';
event_end   = [2 6 8]';

pos = [0.75 0.8 2.1 3.6 1.9 0.5 21.0 3.1]';
HitEnergy = [0.002 0.004 0.01 0.0005 0.08  0.1 1.7 0.007]';


%  Sample Output
Etotal = 0.0060
         0.0800
              0

推荐答案

方法1:一般情况

使用 bsxfun 和matrix-multiplication的一种方法-

mask = bsxfun(@ge,indx_pos,event_start.') & bsxfun(@le,indx_pos,event_end.')
Etotal = energy.'*mask

如果indx_pos中包含很多元素，则可能有点memory-hungry.

This could be a bit memory-hungry if indx_pos has lots of elements in it.

方法2:开始/结束范围不重叠的情况

对于这种特殊情况，可以使用 accumarray -

One can use accumarray for this special case like so -

%// Setup ID array for use in accumarray later on
loc(numel(pos))=0; %// Fast pre-allocation scheme
valids = event_end+1<=numel(pos);
loc(event_end(valids)+1) = -1*(1:sum(valids));
loc(event_start) = loc(event_start)+(1:numel(event_end));
id = cumsum(loc);

%// Set elements as zeros in HitEnergy that do not satisfy the criteria:
%// pos>0.7 & pos<2.0
HitEnergy_select = (pos>0.7 & pos<2.0).*HitEnergy(:);

%// Discard elments in HitEnergy_select & id that have IDs as zeros  
HitEnergy_select = HitEnergy_select(id~=0);
id = id(id~=0);

%// Accumulate summations as done inside the loop in the original code 
Etotal = accumarray(id(:),HitEnergy_select);

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