How do I change the colour of certain indices in scatterplot?

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Hello, I am very new to this.
I am looking to make a scatter plot with some points plot as black point and some as red according to the indices.
I have a 19 elements array and for the values at indices 2, 4, 6, 9, 13, 15, 16 I want the point to be red and the other ones to be black.
Let A be my 19 elemets array
Let X be my X array
Let indice = [2 4 6 9 13 15 16]
I first wrote this:
scatter(X,A,'*k') and now how can I modify this to get the red points at the proper values?
I guess I need to do a if or for but I am not quite sure how do do this. Can anyone help me out?
Thanks

Accepted Answer

Adam Danz
Adam Danz on 24 Sep 2020
Edited: Adam Danz on 24 Sep 2020
Here's are 4 demo that all achieve the same result.
Xvals = [1,2,3,4,5,6,7,8,9];
Yvals = [2,5,5,5,2,2,5,5,5];
% Show scatter plot were all y-val greater than 3 are red
% and all y-vals less than or equal to 3 are black.
% METHOD 1
cla()
col = [0 0 0; 1 0 0]; % [black; red]
isGreater = Yvals > 3;
scatter(Xvals, Yvals, 50, col(isGreater+1,:), 'filled')
% METHOD 2 (variation of method 1)
cla()
baseColors = [0 0 0; 1 0 0]; % [black; red]
isGreater = Yvals > 3;
cmap = baseColors(isGreater+1,:);
scatter(Xvals, Yvals, 50, cmap, 'filled')
% METHOD 3
ax = cla();
isGreater = Yvals > 3;
scatter(Xvals, Yvals, 50, isGreater+1, 'filled')
ax.Colormap = [0 0 0; 1 0 0]; % [black; red]
% METHOD 4
cla()
hold on
isGreater = Yvals > 3;
scatter(Xvals(~isGreater), Yvals(~isGreater), 50, 'k', 'filled')
scatter(Xvals(isGreater), Yvals(isGreater), 50, 'r', 'filled')
  5 Comments
Adam Danz
Adam Danz on 25 Sep 2020
"I didn't realised it was suboptimal"
Yes, logial indexing is faster and maintains the size of the array which is usually helpful.
"I tried your first option and I don't know why is only shows the red markers."
DId you change the threshold? My threshold is at 3 and all of your data are well above 3 so they would all be red if you didn't change the threshold.
"But the second option works so I just changed my code for this one!"
Glad to hear it!
A LL
A LL on 25 Sep 2020
Maybe my last answer wasn't clear but I was refering to the two alternative solutions you proposed to me.
Alternative 1:
plot(A(~indice),Value(~indice),'*k');
plot(A(indice),Value(indice),'*r');
==>This one did not work for me and only plot the red markers... not sure why...
Alternative 2:
otherIndices = setdiff(1:numel(A), indices);
plot(A(otherIndices),Value(otherIndices),'*k');
plot(A(indice),Value(indice),'*r');
==>This one worked and I change my code for this!
Anyway, thanks again!

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