MATLAB: How to vectorize this code

for loopMATLABperformancesumvectorization

I have the following loop.It is needed to improve the performance and hence need to vectorize the loop.Is it possible?

I am facing difficulty in vectorizing due to use of "sum" operator in line 3 and 5.

frequency=randi([0 578],110,1);
axis(:,1)=[-275:5:270];
for k=1:size(frequency,1)
    if axis(k,1)<=0
        cumFrequency(k,1)=sum(frequency(1:k,1));
    elseif axis(k,1)>0
        cumFrequency(k,1)=sum(frequency(k:end,1));
    end
end

Thanks

Best Answer

frequency=randi([0 578],110,1);
axis=[-275:5:270]';
% get indices where axis is negative
axis_negative = (axis<=0);
% calculate the cumulative sum of frequency
cumFrequency_neg = cumsum(frequency);
% calculate the reverse cumulative sum of frequency
cumFrequency_pos = cumsum(frequency,'reverse');
% insert the forward and reverse sums in the indices according to axis
cumFrequency(axis_negative,1) = cumFrequency_neg(axis_negative);
cumFrequency(~axis_negative,1) = cumFrequency_pos(~axis_negative);

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MATLAB: Inversely proportional secondary axis as a reference of primary axis

Hi Owen,

From your question I understand that the number of ticks shown on the first axis is different than the number of ticks in the new axis that you have added even though the limits are as you would like them.

This is happening because based on the limits of the second axis, the plot is automatically selecting the tick labels. You can work around this issue by modifying the code a little bit. I have shown below how you can do that:

For the first axis and the second axis to have the same number of ticks, we need to specify the 'Xtick' parameter for one of the axis. We can do that by finding out how many ticks are there in the first axis. The steps are:

a) Find the Xticks for the first axis b) Calculate the length c) Create new array of Xticks for the second axis based on the range and number of ticks d) Set Xticks for the second axis

For simplicity, I am assuming the following data for x1, y1 and y2:

x1 = 1:0.01:2*pi;
y1 = cos(x1);
y2 = sin(x1);

The changes to the code are shown below (please look carefully at the portion marked with %new code):

[AX,H1,H2] = plotyy(x1,y1,x1,y2);
ax1 = gca;
delta = 0.1; 
set(ax1, 'Position', get(ax1, 'Position') + [0 delta 0 -delta]);
ax2=axes('Position', get(ax1, 'Position').* [1 1 1 0.001]-[0 delta 0 0],'Color','none');
domain=get(ax1, 'XLim')
domain=fliplr(domain).^-1
% new code begin
val=get(ax1, 'XTick')
l = length(val)
ticks = linspace(domain(1),domain(2),l)
set(ax2,'Xtick',ticks);
% new code end
set(ax2,'xdir','reverse','Xlim',domain);

I get the following output figure:

MATLAB: Obtaining a list of percentages/cumulative probabilities from a data vector given the percentile/quantile values

While MATLAB does not include an inbuilt function to accomplish this, it is possible to write some simple code to do so. The following page of the MATLAB documentation provides details of the algorithm used by the "prctile" and "quantile" functions.

http://www.mathworks.com/help/stats/quantiles-and-percentiles.html

The following code can be used to reverse these algorithms to arrive at the percentage and cumulative probability values given a data vector "X" and a list of percentile or quantile values "Y".

For "prctile",

>> sortedX = sort(X);
>> listPercentages = (100/length(X))*(0.5:1:length(X)-0.5);
>> percentages = interp1(sortedX,listPercentages,Y);

For "quantile",

>> sortedX = sort(X);
>> listCumProb = (1/length(X))*(0.5:1:length(X)-0.5);
>> cumProbs = interp1(sortedX,listCumProb,Y);

Best Answer

Related Solutions

MATLAB: Inversely proportional secondary axis as a reference of primary axis

MATLAB: Obtaining a list of percentages/cumulative probabilities from a data vector given the percentile/quantile values

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