Removing if statements when obtaining data from a table - matlab

Question

I have the following table
nc a b
0.9 <= nc 0.33 -0.45
0.5 <= nc < 0.9 0.95 -0.75
0.1 <= nc < 0.5 2.2 -0.97
nc < 0.1 1.18 -0.77
which says that if the observed cloud cover if between the ranges shown then a and b are given as the values expressed in the table. I have written some code in matlab that should return these values given a vector of nc values:
nc = [0.1, 0.6, 0.5 ,0.2, 0.9];
a = nan(length(nc),1); % pre-allocate arrays
b = nan(length(nc),1);
for i = 1:length(nc)
if nc(i) >= 0.9;
ai = 0.33;
bi = -0.45;
elseif nc(i) >= 0.5 & nc(i) < 0.9
ai = 0.95;
bi = -0.75;
elseif nc(i) >= 0.1 & nc(i) < 0.5
ai = 2.2;
bi = -0.97;
elseif nc(i) < 0.1
ai = 1.18;
bi = -0.77;
end
a(i) = ai;
b(i) = bi;
end
However, this seems like a very long way of doing this. In addition, this code will eventually be fed into a number of other functions and I'm a bit worried that having so many if statements and a for loop in general will slow things down. Could anyone comment on this or suggest a faster way (if useful) of doing this?

Answer 1

loops in matlab are slow. You can improve your solution vectorizing the operation:
nc = [0.1, 0.6, 0.5 ,0.2, 0.9];
a = 2.2 * ones(length(nc),1); % by default in the 0.1-0.5 range
b = -0.97 * ones(length(nc),1);
f = find(nc >= 0.9);
a(f) = 0.33; b(f) = -0.45;
f = find(nc >= 0.5 & nc < 0.9);
a(f) = 0.95; b(f) = -0.75;
f = find(nc < 0.1);
a(f) = 1.18; b(f) = -0.77;
find will list the indices of the element that match the criteria, so you can modify the elements in the other vectors.

Answer 2

Or you can use logical indexing, by-passing the need to use the find function:
nc = [0.1, 0.6, 0.5 ,0.2, 0.9];
a = 2.2 * ones(length(nc),1); % by default in the 0.1-0.5 range
b = -0.97 * ones(length(nc),1);
a(nc >= 0.9) = 0.33; b(nc >= 0.9) = -0.45;
a(nc >= 0.5 & nc < 0.9) = 0.95; b(nc >= 0.5 & nc < 0.9) = -0.75;
a(nc < 0.1) = 1.18; b(nc < 0.1) = -0.77;