function [ alfan ] = an( vm )
alfan = 0.1 * ones(size(vm));
alfan(vm ~= 10) = (0.01*(10.0-vm(vm ~= 10))) ./ (exp((10.0-vm(vm ~= 10))/10.0)-1.0);
end
You were passing in a matrix of vm but testing the entire matrix with a single "if" statement. "if" applied to a matrix is only true if every element of the logical condition is true. In that case you were returning a scalar 0.1 instead of something the same size as vm.
In the case where some of the elements did not equal 1.0, the "else" was executed. In the else you had a matrix / a matrix. / is the matrix right division operator, like A / B meaning A * pinv(B) with that * meaning algebraic matrix multiplication and the pinv being the pseudo-inverse. The result you were getting from that was not the same size as vm so the overall result alfan was not the same size as you were expecting on output.
The replacement code I show here takes the time to copy the size of the input, sets the default value to the exceptional condition, and it applies the formula only to the places that are not exceptional, making sure to use the ./ element-by-element division instead of using / matrix division.
