using repmat instead of for loop

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Mason
Mason 2011-10-13
I have a 1x24 matrix A and a 2000x24 matrix D
I want to take column 1 in A and multiply it by all the rows in column 1 of D, then take the second column of A and multiply it by all the rows in column 2 of D and do that for all 24 columns without using a for loop.
A coworker suggested:
G = D .* repmat(A(1:channels), size(D,1),1);
Hoping G is a 2000x24 matrix. It is, but its not doing what i want it to. Any suggestions?

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回答（3 个）

the cyclist
the cyclist 2011-10-13
That should work. But even better is:
>> G = bsxfun(@times,A,D);
If that does not give the answer you expect, I think you need to be a little bit more detailed about what you want, possibly with a small explicit example.
Mason
Mason 2011-10-13
I guess im not understanding something. When i added the bxnfun it did the same thing. Here is what im trying to do. I did it correctly using for loops but im trying to do it now without the for loops.
here is with for loops:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% clear all clc d = ReadSegy('MMAC1.SGY'); % load segy data created in knudsen channels = size(d,2); % total number of channels samples = size(d,1); % total number of samples in time shift=max(max(d))/3 ; % offset shift used to plot seismic data highest_values = max(d); % Builds matrix for highest amplitude of each channel highest_value = max(highest_values); % Largest amplitude of data set
%%%%%%%%%%%%%%%%%%%%%%% Plotting the data as Traces %%%%%%%%%%%%%%%%%%%%%%
figure(1)
for i=1:channels
plot(d(:,i)+shift*(i-1)) % plots pre-normalized values
hold on
view(90,90)
end
figure(2)
normalized_signals = ones(samples(1),channels(1)); % initializes matrix for plotting normalized signals
normalized_signals2 = ones(samples(1),channels(1)); % initializes matrix for imagesc of normalized signals
normalized = highest_values / highest_value; % matrix of percentages each channel peak value is to the max amplitude value
for i=1:channels
normalized_signals(:,i) = d(:,i) ./ normalized(:,i);% normalized signals on all 24 channels to be plotted
normalized_signals2(:,i) = normalized_signals(:,i) / highest_value;% normalized signals on all 24 channels for imagesc
plot(normalized_signals(:,i)+shift*(i-1)); % plots normalized values shifted
hold on
view(90,90)
end
figure(3) % flips origin to top left corner
imagesc(normalized_signals2);
colormap(gray)
colorbar
%%%%%%%%%%%%%%%%%%%end of program %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Here is what i tried after your suggestion:
clear all
clc
d = ReadSegy('MMAC1.SGY'); % load segy data created in knudsen
channels = size(d,2); % total number of channels
samples = size(d,1); % total number of samples in time
shift = max(max(d))/3 ; % offset shift used to plot seismic data
highest_values = max(d); % Builds matrix for highest amplitude of each channel
highest_value = max(highest_values); % Largest amplitude of data set
%%%%%%%%%%%%%%%%%%%%%%% Plotting the data as Traces %%%%%%%%%%%%%%%%%%%%%%
figure(1)
Shifted = d + shift * repmat(1:channels, size(d,1),1 );
plot(Shifted, 'b');% plots pre-normalized values
view(90,90)
figure(2)
fractions_24 = highest_values / highest_value; % Makes a 1x24 matrix of fractions of max amplitudes
scaled = bsxfun(@times,d,fractions_24); % Attempts to take 1st column of fractions_24 and multiply
% it through all rows of the first column of d,
% and do this for all 24 channels.
normalized = scaled / highest_value; % Drops all amplitudes to a max of one rest scaled to thatone
shift = max(max(normalized))/3;
shifted = shift * bsxfun(@times,1:24,normalized);% Shifts all channels to even spacing
plot(normalized) % Plots the normalized waveforms
view(90,90)
figure(3)
imagesc(normalized);
colormap(gray)
colorbar
%%%%%%%%%%%%%%%%%%end of program %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  2 个评论
the cyclist
the cyclist 2011-10-13
You had errors in both figures 4 and 5. In both cases, you used "shift" instead of "shift-1". In figure 5, you also multiplied by the shift instead of adding. I think you may also have multiplied instead of divided in one place, but I started to get confused, so I basically went back to your figure 1 and 2 code and translated it directly, rather than starting from your figure 4 and 5 code.
I believe the following replicates what you want.
figure(4)
Shifted = bsxfun(@plus,d,shift*(0:channels-1));
plot(Shifted, 'b');% plots pre-normalized values
view(90,90)
figure(5)
normalized = highest_values / highest_value; % matrix of percentages each channel peak value is to the max amplitude value
normalized_signals = bsxfun(@rdivide,d,normalized);% normalized signals on all 24 channels to be plotted
normalized_signals2 = normalized_signals / highest_value;% normalized signals on all 24 channels for imagesc
shifted = bsxfun(@plus,normalized_signals,shift*(0:channels-1))
plot(shifted,'b'); % plots normalized values shifted
view(90,90)
Jan
Jan 2011-10-13
For the "clear all" see: http://www.mathworks.com/matlabcentral/answers/16484-good-programming-practice#answer_22301

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Mason
Mason 2011-10-13
Cyclist, thank you very much. This new function is awesome! I'm trying to do my very best staying away from for loops. You guys rock!
  1 个评论
the cyclist
the cyclist 2011-10-13
Glad to help. Next time, rather than adding your own "Answer" as a response, try using the "Comment on this Answer" feature. As this thread currently stands, it is a little bit tricky for you to accept my answer as being helpful, because that help is spread out.

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