what does this line do

Question

Kamil Kacer 2020-11-20

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https://ww2.mathworks.cn/matlabcentral/answers/655133-what-does-this-line-do

回答： Walter Roberson 2020-11-21

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

d{i}(end+1:max(numOfTrainSamples)) = inf;

can anyone tell what does this do d has 3 iteration and numOfTrainSamples is equal to 1

It suppose to do something but when i ran it doeasnt do anything

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Answer 1

Walter Roberson 2020-11-21

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The code goes through all of the F cell entries, and finds the number of columns in each, recording the number of columns in numOfTrainSamples(i) at each point. It takes the maximum overall of the number of those. As it goes through and finds the distances, it extends the calculated distances out to the maximum number of samples that were encountered, extending with infinities.

The reason it does this is to be able to compare between the different F, by producing distance arrays that are the same size.

By the way, the way it initializes the arrays is incorrect, but in a way that does not matter. The initialization of d{i} with inf values is not needed as long as d is initialized to be a cell with the correct number of entries.

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Answer 2

John D'Errico 2020-11-20

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It does nothing on its own. A single line of code has little meaning, out of context.

What does it do? It sets some elements of a specific element of one cell in a cell array to inf. Actually, it APPENDS infs to the end of the vector. There will be max(numOfTrainSamples) infs appended.

Why it does that is what really matters.

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Kamil Kacer 2020-11-20

在 MATLAB Online 中打开

This is the code but i dont understand why it is there

function [Ps, winnerClass] = classifyKNN_D_Multi(F, testSample, k, NORMALIZE, useL1distance )
% function [Ps, winnerClass] = classifyKNN_D_Multi(F, testSample, k, NORMALIZE, useL1distance);
% 
% This function is used for classifying an unknown sample using the kNN
% algorithm, in its multi-class form.
%
% ARGUMENTS:
% - F: an CELL array that contains the feature values for each class. I.e.,
%      F{1} is a matrix of size numOfDimensions x numofSamples FOR THE FIRST
%      CLASS, etc.
%
% - testSample: the input sample to be classified
% - k: the kNN parameter
% - NORMALIZE: use class priors to weight results
% - useL1distance: use L1 instead of L2 distance
%
% RETURNS:
% - Ps: an array that contains the classification probabilities for each class
% - winnerClass: the label of the winner class
  %%error(nargchk(4,5,nargin))
  switch nargin
      case 4
           useL1distance = '1'; % euclidean distance if 4 variables included
      case 5
          useL1distance = '0'; % mahalanobis distance if 5 variables included
      otherwise
          disp('error')
  end
numOfClasses = length(F);
if (size(testSample, 2)==1)
    testSample = testSample';
end
% initilization of distance vectors:
numOfDims = zeros( 1, numOfClasses );
numOfTrainSamples = zeros( 1, numOfClasses );
d = cell(numOfClasses,1);
% d{i} is a vector, whose elements represent the distance of the testing
% sample from all the samples of i-th class
testSample(isnan(testSample)) = 0.0;
for i=1:numOfClasses
    [ numOfDims(i), numOfTrainSamples(i) ] = size( F{i} );
    d{i} = inf*ones(max(numOfTrainSamples), 1); % we fill it with inf values
    F{i}(isnan(F{i})) = 0.0;    
end
if (length(testSample)>1)
    for i=1:numOfClasses % for each class:
        if (numOfTrainSamples(i)>0)
            if ( useL1distance == 0)                
              %  d{i} = sum( abs(repmat(testSample, [numOfTrainSamples(i) 1]) - F{i}'),2); % L1
                d{i} =  pdist2(F{i}.', testSample, 'euclidean');
             else
                 %[size(repmat(testSample, [numOfTrainSamples(i) 1])) size(F{i}')]
                 %sum(sum(isnan(F{i}')))
                d{i} = sum( ((repmat(testSample, [numOfTrainSamples(i) 1]) - F{i}').^2 ),2); % L2 
                 d{i} =  pdist2(F{i}.', testSample, 'mahalanobis');
            end
            d{i} = sort(d{i});            
            d{i}(end+1:max(numOfTrainSamples)) = inf;                                    
        else
            d{i} = inf;
        end        
    end
else % single dimension (NO SUM required!!!)
    for i=1:numOfClasses
        if (numOfTrainSamples(i)>0)
            d{i}   = (abs(repmat(testSample, [numOfTrainSamples(i) 1]) - F{i}')'); 
            d{i} = sort(d{i});
            d{i}(end+1:max(numOfTrainSamples)) = inf;            
        else
            d{i} = inf;
        end
    end    
end
kAll = zeros(numOfClasses, 1);
for j=1:k
    curArray = zeros(numOfClasses, 1);
    for i=1:numOfClasses
        curArray(i) = d{i}(kAll(i)+1);
    end    
    [MIN, IMIN] = min(curArray);
    kAll(IMIN) = kAll(IMIN) + 1;
end
if ( NORMALIZE == 0 )
    Ps = (kAll ./ k);
else
    Ps = kAll ./ numOfTrainSamples';
    Ps = Ps / sum(Ps);
end
[MAX, IMAX] = max(Ps);
winnerClass = IMAX;