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Scatter use different colors for data-points

Asked by Cliff Karlsson on 16 Sep 2018 at 16:01
Latest activity Edited by dpb
on 17 Sep 2018 at 14:18
Accepted Answer by dpb

I experimenting with the carbig dataset and want to use scatterplot3 to display some categories with different colors depending on how many cylinders the car got. How could I modify my code to work as expected? This is what I have tried:

load carbig
colors = [[1 0 0] [0 1 0] [0 0 1] [1 1 0] [1 0 1]];
c = [0 0 0]*length(Cylinders);
for i=1:length(Cylinders)
    if Cylinders(i) == 8
       c(i) = colors(5); 
    else
        c(i) = colors(Cylinders(i)-2);
    end   
end
scatter3(Horsepower,Weight, MPG,[],c, 'Marker','.')
xlabel('Horsepower')
ylabel('Weight')
zlabel('Miles per Gallon')
title('Car Database')

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1 Answer

Answer by dpb
on 16 Sep 2018 at 17:55
Edited by dpb
on 16 Sep 2018 at 21:05
 Accepted Answer

Close, but... :) (Always a "but", isn't there or wouldn't be asking a Q?) VBG

colors = [[1 0 0] [0 1 0] [0 0 1] [1 1 0] [1 0 1]];

makes a 1x15 row vector instead of an array of 5 color triplets; you're missing the semicolon between the entries to keep them from just being strung together in one long row--

colors = [[1 0 0]; [0 1 0]; [0 0 1]; [1 1 0]; [1 0 1]];

Then, don't need the loop to assign them, use optional output from unique to locate the cylinder size in the array and use as lookup into the colors table:

[~,~,ib]=unique(Cylinders);   % vector of each unique cylinder size index in Cylinders array
scatter3(Horsepower,Weight, MPG,[],colors(ib,:), 'Marker','.')  % scatter using lookup into colors

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Right, I shoulda' cut 'n paste instead of just typing air code, Walter. Good catch. Fixed...

Thanks but I still have a hard time understanding [~,~,ib]=unique(Cylinders); what does the ~ do?

They just throw away the first two return values that we're not interested in for this application....but lets us get the third by being there for positional placeholders.

>> help punct
 Punctuation.
  .   Decimal point. 325/100, 3.25 and .325e1 are all the same.
  .   Array operations.  Element-by-element multiplicative operations
    .
    .
    .
    .
   ~   The tilde character can be used in function definitions to
       represent an input argument that is unused within the function.
       It can also be used to indicate that an output argument of a
       function call is to be ignored.  In this case, it must appear
       within [ ] and be separated by commas from any other arguments.

Look at the documentation for unique to understand what the outputs are and why it's the third (and only the third) that we're interested in here.

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