change bar color based on value

Hi,

I am trying to debug a code that will take two identical functions, A = randi([-100,100]) and B = randi([-100,100]), which each have their own unique values and input them into a third function, C = A - B. The code runs on a loop so it determines a value for A, B and C, and then plots the value in real time on three separate plots before moving on to the next number. I have a code that is supposed to plot the value of C on bar graph with the color green or red based on the value of C (green if C >= 0, red if C < 0). However, the code that I have ends up taking whatever the first color that was plotted and plots all bars in that color. The code that I currently have is as follows:

for i = 1:100;
  A(i)=randi([-100,100]);
  B(i)=randi([-100,100]);
  C(i)=A(i)-B(i);

    subplot(2,2,1)
    plot(A,'b')
    hold on
    subplot(2,2,3)
    plot(B,'k')
    hold on
    subplot(2,2,[2,4])
    bar(C(C(:,1)>=0,:),'g');
    hold on
    bar(C(C(:,1)<0,:),'r');
    hold off
    drawnow
end

Any help to figure out where I'm going wrong in my code is greatly appreciated.

Answer 1

[Morgan Clendennin]

I was able to figure out the question myself with a little less than desired solution. By creating two new variables, topC and bottomC, I was able to add in an if statement that assigned the value of C or a value of 0 to the new variables at the current index based on the criteria I was previously using for the bar graph commands. Now I just plot topC and bottomC instead and it works fine.

Answer 2

[David Goodmanson]

Hi Morgan,

Unless you are trying to make a movie*, it's faster and more Matlablike to create A and B all at at once:

N = 100;
span = [-100 100];
A = randi(span,1,N);
B = randi(span,1,N);
C = A-B;

figure(1)
subplot(2,2,1)
plot(A,'b')
subplot(2,2,3)
plot(B,'k')
subplot(2,2,[2,4])
bar(C(C>=0),'g')
hold on
bar(C(C<0),'r')
hold off

I don't know exactly why the colors don't work for the for loop, but this way they do work all right.

*(there are more efficient ways to do that then replotting the entire array each time).

[Morgan Clendennin]

Hi David,

Thank you for your quick response. I am not however looking to make the A, B and C arrays all at once. I have it set up as a loop because I want only one value for each array created at one time and plotted. Hence, why I said plotted in real time. I am only looking for a way to debug why the bar functions are not plotting as they should.

[Morgan Clendennin]

I have modified the bar functions slightly and now I am getting a bar plot where the ever bar (including the ones already plotted) change color based on the current value.

bar(C(C(i)>=0,:),'g');
hold on
bar(C(C(i)<0,:),'r');
hold off

Answer 3

[Benjamin Kraus]

You should check out the latest version of MATLAB. Starting in MATLAB R2017b you can specify a color for each bar in a bar series independently.

In your case, I think this will work:

b = bar(C, 'FaceColor', 'g');
b.FaceColor = 'flat';
b.CData(C<0,:) = repmat([1 0 0],sum(C<0),1);