How to create an error function comparing two matrices? - matlab

Question

I have two matrices in MATLAB. Each one is filled with 1 and 0 at different positions. I want to compare each element:
If there is a 1 match, I want it to record as True Positive.
If there is a 0 match, I want it to record as True Negative.
If one says 1 and the other says 0, I want to record as False Positive.
If one says 0 and the other says 1, I want to record as False Negative.
I tried just comparing the two matrices:
idx = A == B
But, that gives me a simple match, not telling me when there is a True Positive or Negative, etc.
Is there any specific function I could use, or any alternative?

Answer 1

You could just add the matrices in a prescribed way....
a = [1 0 1 0
1 1 0 0
0 0 1 1];
b = [1 0 0 0
0 0 0 1
0 0 1 0];
C = a + 2*b;
% For pairs [a,b] we expect
% [0,0]: C = 0, true negative
% [1,0]: C = 1, false positive
% [0,1]: C = 2, false negative
% [1,1]: C = 3, true positive
% C =
% [ 3 0 1 0
% 1 1 0 2
% 0 0 3 1 ]
If you have the Statistics and Machine Learning toolbox and you only want a summary, you might just need the function confusionmat.
From the docs:
C = confusionmat(group,grouphat) returns the confusion matrix C determined by the known and predicted groups in group and grouphat. [...]. C is a square matrix with size equal to the total number of distinct elements in group and grouphat. C(i,j) is a count of observations known to be in group i but predicted to be in group j.
For example:
a = [1 0 1 0
1 1 0 0
0 0 1 1];
b = [1 0 0 0
0 0 0 1
0 0 1 0];
C = confusionmat( a(:), b(:) );
% C =
% [ 5 1
% 4 2]
% So for each pair [a,b], we have 5*[0,0], 2*[1,1], 4*[1,0], 1*[0,1]
A similar function for those with the Neural Network Toolbox instead would be confusion.

Answer 2

You could just use bitwise operators to produce the four different values:
bitor(bitshift(uint8(b),1),uint8(a))
Produces an array with
0 : True Negative
1 : False Negative (a is true but b is false)
2 : False Positive (a is false but b is true)
3 : True Positive

Answer 3

One naive approach would be four comparisons, case by case:
% Set up some artificial data
ground_truth = randi(2, 5) - 1
compare = randi(2, 5) - 1
% Determine true positives, false positives, etc.
tp = ground_truth & compare
fp = ~ground_truth & compare
tn = ~ground_truth & ~compare
fn = ground_truth & ~compare
Output:
ground_truth =
1 0 1 0 0
0 1 1 0 1
1 1 0 1 0
0 1 0 1 1
0 0 0 1 0
compare =
0 1 1 0 1
0 1 1 1 0
1 1 0 0 1
1 1 1 0 0
1 1 1 1 1
tp =
0 0 1 0 0
0 1 1 0 0
1 1 0 0 0
0 1 0 0 0
0 0 0 1 0
fp =
0 1 0 0 1
0 0 0 1 0
0 0 0 0 1
1 0 1 0 0
1 1 1 0 1
tn =
0 0 0 1 0
1 0 0 0 0
0 0 1 0 0
0 0 0 0 0
0 0 0 0 0
fn =
1 0 0 0 0
0 0 0 0 1
0 0 0 1 0
0 0 0 1 1
0 0 0 0 0
That works, because 0 and 1 (or any positive value) are alternative representations for true and false.
To keep your main code clean, set up a separate function, say my_stats.m
function [tp, fp, tn, fn] = my_stats(ground_truth, compare)
% Determine true positives, false positives, etc.
tp = ground_truth & compare;
fp = ~ground_truth & compare;
tn = ~ground_truth & ~compare;
fn = ground_truth & ~compare;
end
and call it in your main code:
% Set up some artificial data
ground_truth = randi(2, 5) - 1
compare = randi(2, 5) - 1
[tp, fp, tn, fn] = my_stats(ground_truth, compare)
Hope that helps!

Answer 4

I found that I can use the find method and set two conditions, then just find the numbers of the element in each variable
TruePositive = length(find(A==B & A==1))
TrueNegative = length(find(A==B & A==0))
FalsePositive = length(find(A~=B & A==1))
FalseNegative = length(find(A~=B & A==0))
The confusionmatrix() method suggested by #Wolfie is also really neat, especially if you use the confusionchart() which provides a nice visualisation.