NL = (x/x0).^(1./(Omega1-1)).*((1-x0)./(1-x)).^(Omega1./(Omega1-1))
Because x is a vector, and you are doing element-by-element operations, unless one of the other variables is not a scalar or row vector, then NL would be a row vector. NL is not going to be a scalar, because x is not a scalar and you are not doing any reduction operations such as sum()
In MATLAB, when you test a condition using if or while, the condition is considered true if all of the values being tested are non-zero. NL >= 1 with NL non-scalar is going to evaluate to a logical matrix the same sizes as NL. The test would then be considered to be true if all of the NL values are greater than or equal to 1. It is as-if you had written
if you want the condition to be true as soon as at least one of the tests is satisfied, then write in any(), as in
But you should be considering whether you need to loop over the x values, like
NL(xidx) = (x(xidx)/x0).^(1./(Omega1-1)).*((1-x0)./(1-x(xidx))).^(Omega1./(Omega1-1));
