Sum specific values from a matrix

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Sören Gevaert 2020-2-20

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评论： Sören Gevaert 2020-2-25

hello

I have a matrix existing of 2 colums and a variable number of rows. the first colum is "amplitude" the second colum is "frequency". for some frequencys in a specific range i want to change the amplitude. so for example: for frequencys from 5 to 8 the amplitude has to be multiplied by 2. for frequencys from 9 to 14 the amplitude needs to be multiplied by 4 and so on. for that i used if loops in a for loop as seen in the code below. now for each range of frequency i want the amplitudes to be added. so for frequencys 4 to 8, all the amplitudes needs to be multiplied by 2 and then al these amplitudes need to be added to each other and saved in a new variable. can someone help me to solve my problem.

thanks in advance

treshold1 = 0.0001;    % treshold amplitude
treshold2 = 0;         %treshold freq
%K(K(:,1)<treshold1,:)=[0];
K(K(:,2)<treshold2,:)=[0];
for i = 1:size(K,1)
if K(i,2) >=5 && K(i,2)<9  
    K(i,1)*2
    %K1 = sum of the amplitude for freq 5 to 8
if K(i,2) >=9 && K(i,2)<15
    K(i,1)*4
    %K2 = sum of the amplitude for freq 9 to 14
end
end

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Answer 1

Guillaume 2020-2-24

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"What im trying to do is to add all the amplitudes off column 1 for a specific frequency range"

This is a different question from what you originally asked, so would have been better started as a new question.

Anyway, for this you'd use discretize to assign unique numbers to each frequency band and splitapply to sum all rows that belong to a given band:

frequencybands = [0, 24, 51.5, 80, Inf];  %Creates 4 frequency bands: [0, 24), [24, 51.5), [51.5, 80), [80, Inf]
bandnumber = discretize(K(:, 2), frequencybands);  %assign unique band number to members of a band
bandamplitude = splitapply(@sum, K(:, 1), bandnumber);  %sum amplitudes that belong to the same band

Another option would be to convert your matrix to a table and then use groupsummary. A table also has the advantage of making it clearer what each column represent.

ampfreq = array2table(K, 'VariableNames', {'Amplitude', 'Frequency'});
frequencybands = [0, 24, 51.5, 80, Inf];  %Creates 4 frequency bands: [0, 24), [24, 51.5), [51.5, 80), [80, Inf]
bandamplitude = groupsummary(ampfreq, 'Frequency', freqbands, 'sum', 'Amplitude');  %sum amplitude according to frequency bands

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Sören Gevaert 2020-2-24

Hi Guillaume

Thanks for the help, this seems to work verry well. I expressed my first question a bit wrong. The first thing was to add the amplitudes and after that, these values needs to be multiplied with different factors. Now i can continue to work from this.

Kind regards

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Answer 2

Athul Prakash 2020-2-23

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编辑：Athul Prakash 2020-2-23

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You may use logical indexing as follows:

cond1 = K(:,2)>=5 & K(:,2)<9; %these are logical indices to select out your given ranges.
cond2 = K(:,2)>=5 & K(:,2)<9;
K(cond1,:) = K(cond1,:)*2;
K(cond2,:) = K(cond2,:)*4;

I'm not sure what you wish to do after this.

Hope it Helps!

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Athul Prakash 2020-2-25

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For a closed interval [fMin fMax],

% suppose the matrix above is called 'M'
logical_indices = ( (M(:,2) >= fMin) & (M(:,2) <= fMax) );
amps_in_range = M(logical_indices, 1);
result = sum(amps_in_range);

You may compute a similar sum for all frequency bands.

Hope it Helps!

Sören Gevaert 2020-2-25