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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

Adam Danz
on 24 Sep 2020 at 21:24

Edited: Adam Danz
on 24 Sep 2020 at 21:28

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')

Adam Danz
on 25 Sep 2020 at 2:30

"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!

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