if else statement for elemental comparison

Hello,

I have 2 matrices, a & b, both 1024x1024. I am trying to create an "if/else" statement where each element in the same position in the two matrices are compared to each other. Depending on which value is greater, the subsequent analysis is performed. I have the following code, but it is currently running with "any" value in the matrix. Is there a way to do if/else on an elemental basis? I then want to find the average of all the resulting values (here, average of "ab" and "ba.") Is there a better way to do this than my current methods? This is what I have so far (and thanks for your help!):

a = load('0.txt');
b = load('9.txt');

if any(a > b)
    ab = (a-b)./(a+(2*b));
    abavg= mean(mean(ab,'omitnan'), 'omitnan')
    abSD= std2(ab);
else 
    ba = (b-a)./(b+(2*a));
    baavg= mean(mean(ba,'omitnan'), 'omitnan')
    baSD= std2(ba);
end

[Rik]

Use logical indexing.

a_part=a(a>b);b_part=b(a>b);

You are aware that a mean of means is not the same as a mean of all values?

[Emily Pendleton]

running mean() on my resulting matrix gave an array (1x1024). How do I get the average of the 1024x1024 matrix in one step?

[Benjamin Kraus]

You need to convert your matrix into a vector first. One way to do that is using :.

mean(ba(:))

[Emily Pendleton]

Got the mean part, thanks.

Still, I think I'm mis-using the logical indexing (I'm only getting values for ab and none for ba. I could check by hand if I should have values for ba, but it would take a while).

Did I appropriately use the logical indexing here? When I copied/pasted the answer from Rik Wisselink after "if", I got an error message.

if  a(a>b)
    ab = (a-b)./(a+(2*b));
    abavg = mean(ab(:), 'omitnan')
    abSD= std2(ab);
else 
    ba = (b-a)./(b+(2*a));
    baavg = mean(ba(:), 'omitnan')
    baSD= std2(ba);
end

Answer 1

[Rik]

The logical indexing shouldn't be used in combination with an if-statement. Let me give you a small example:

a=[3 5 6];
b=[5 4 8];
%then a>b results in [false true false] (or [0 1 0])
%a([0 1 0]) is 5, so a(a>b) is 5
%a(a<b) is [3 6]

That is called logical indexing, because you use a logical array to index another array.

The code below should solve your problem.

%dummy data
a=rand(1024);
b=rand(1024);

%create shorthand arrays with logical indexing
a1=a(a>b);
a2=a(a<=b);
b1=a(a>b);
b2=a(a<=b);

ab = (a1-b1)./(a1+(2*b1));
abavg = mean(ab(:), 'omitnan');
abSD= std2(ab);
ba = (b2-a2)./(b2+(2*a2));
baavg = mean(ba(:), 'omitnan');
baSD= std2(ba);

[Emily Pendleton]

Thank you!

Answer 2

[Benjamin Kraus]

You need to calculate both answers, then recombine the matrix using logical indexing. Something like this:

ab = (a-b)./(a+(2*b));
abavg = mean(ab(:), 'omitnan')
abSD= std2(ab);

ba = (b-a)./(b+(2*a));
baavg = mean(ba(:), 'omitnan')
baSD= std2(ba);

out1 = ab;
out1(b>a) = ba(b>a); % logical indexing to replace only values where |b>a|
avg = abavg;
avg(b>a) = baavg(b>a); % logical indexing to replace only values where |b>a|
SD = abSD;
SD(b>a) = abavg(b>a); % logical indexing to replace only values where |b>a|

[Benjamin Kraus]

Actually, looking back at your code a little closer, this may be what you are trying to do:

ab = (a-b)./(a+(2*b));
abavg = mean(ab(a>b), 'omitnan')
abSD= std2(ab(a>b));

ba = (b-a)./(b+(2*a));
baavg = mean(ba(b>a), 'omitnan')
baSD= std2(ba(b>a));

Also, don't forget about places where a == b.

[Emily Pendleton]

thank you!