calculate histogram of 2 variables

Question

I have a vector containining the speed of 200 walks:

a = 50;
b = 100;
speed = (b-a).*rand(200,1) + a;

and another vector contaning the number of steps for each walk:

a = 8;
b = 100;
steps = (b-a).*rand(200,1) + a;

I would like to create a histogram plot with on the x axis the speeds and on the y axes the sum of the steps of each speed.

What I do is the following but I guess there is a more elegant way to do this:

unique_speed = unique(speed);
y_unique_speed  = zeros(size(unique_speed));
for i = 1 : numel(unique_speed)

    speed_idx = unique_speed(i);
    idx = speed==speed_idx ;
    y_unique_speed  (i) = sum(steps (idx));

end

With the code you give here, it is very unlikely that 2 `speed` values will be equal. Are they generated the same way in your actual code? — BillBokeey, May 20 '16 at 11:56
Something like [this](http://stackoverflow.com/a/18640944/2586922)? — Luis Mendo, May 20 '16 at 12:09

nirvana-msu · Accepted Answer · 2016-05-20T21:37:37.717

First you need to discretize your speed variable. Unlike in the other answer, the following allows you to choose arbitrary step size, e.g. I've chosen 1.5. Note that the last bin edge should be strictly more than the maximum data point, hence I used max(speed)+binstep. You can then use histc to determine which bin each pairs falls into, and accumarray to determine total number of steps in each bin. Finally, use bar to plot.

binstep = 1.5;
binranges = (min(speed):binstep:max(speed)+binstep)';
[~, ind] = histc(speed, binranges);
bincounts = accumarray(ind, steps, size(binranges));
hFig = figure(); axh = axes('Parent', hFig); hold(axh, 'all'); grid(axh, 'on');
bar(axh, binranges, bincounts); axis(axh, 'tight');

score 0 · Answer 2 · answered May 20 '16 at 11:51

0

First, bin the speed data to discrete values:

sspeed = ceil(speed);

and then accumulate the various step sizes to each bin:

numsteps = accumarray(sspeed, steps);
plot(numsteps)

answered May 20 '16 at 11:51

Airidas Korolkovas

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calculate histogram of 2 variables

2 Answers2