how to plot a color-coded matrix

Hello

I have a matrix of data including 3 column,for example:

a=[1 0 3;1 1 4;1 2 4;1 3 5] 

x=a(:,1);y=a(:,2);z=a(:,3);

1) in my real data, z is the value associated with (x,y). so I would like to have a 2D chart with x and y value being color-coded based on z value. like all the points in the chart with 1<z<2 are red and 2<z>3 are blue and etc. is there a way to do that?

2) to get what i want, I used the below command for a sample matrix with different z value but the chart is just giving me all points in blue color. what am i missing? what is the best syntax for what I want?

figure
scatter3(x(0<z<=2),y(0<z<=2),z(0<z<=2),'g.')
hold on

scatter3(x(2<z<=4),y(2<z<=4),z(2<z<=4),'k.')

hold on
scatter3(x(4<z<=6),y(4<z<=6),z(4<z<=6),'r.')

Thank you !

Answer 1

[Star Strider]

Try this:

a=[1 0 3;1 1 4;1 2 4;1 3 5]

x=a(:,1);
y=a(:,2);
z=a(:,3);

L1 = (z > 1) & (z <= 3);                                     % Logical Subscripts For Condittion #1
L2 = (z > 3) & (z <= 5);                                     % Logical Subscripts For Condittion #2

figure(1)
scatter3(x(L1), y(L1), z(L1), 'rp')
hold on
scatter3(x(L2), y(L2), z(L2), 'bp')
hold off
grid on

Change the conditions as necessary for your data.

[Walter Roberson]

Right. MATLAB inteprets A<B<C as ((A<B)<C) and gives a logical value (0 or 1) for the A<B and compares the logical value to C.

Exception: inside piecewise() of the Symbolic toolbox, but not generally in the Symbolic Toolbox, A<B<C is interpreted as B being in the range A to C exclusive.

[farfar]

Thank you very much. one more thing !how can I show the color bar with the plot so the user know I used blue for 0 to 100 and red for 100 to 200?

[Star Strider]

As always, my pleasure!

Using legend may be more practical:

    legend('0 - 100', '100 - 200')

If you want to use colorbar, try this (after your scatter3 call):

colormap([1 0 0; 0 0 1])
colorbar('Ticks', [0 0.5 1], 'TickLabels',[0 100 200])

This worked when I tested it (in R2018a). You may have to experiment with it.