Why does my <= statement not work?

I'm running a simple loop that applies a less than or equal to inequality to a value. Unfortunately, it is overlooking this value and not working. Here is my code

% Initial Conditions 
clearvars
close all
Ms=1.989E30;
m = logspace(-2, 2, 400);
% m = 1E2:100:1E4
% (m<0.08, m**-0.3, numpy.where(m < 0.5, 0.08**-0.3 * (m/0.08)**-1.3, 0.08**-0.3 * (0.5/0.08)**-1.3 * (m/0.5)**-2.3))
%% def chabrier03individual(m):
	k = 0.158*exp(-(-log(0.08))^2/(2*0.69^2))
if m<=1
    u = 0.158*(1./m)*exp(-(log(m)-log(0.08))^2/(2 * 0.69^2));
else
    v = k*m;
end 

[Stephen23]

The comprison works as documented. Note the if documentation states " An expression is true when its result is nonempty and contains only nonzero elements (logical or real numeric). Otherwise, the expression is false."

You provided if with a 1x400 logical vector as its condition, it contains no non-zero elements:

>> nnz(m<=1)
ans = 0

Based on this I would expect the else part to be executed, and this is indeed what occurs when i run your code.

Note that your vector m contains just one value repeated 400 times over:

>> unique(m)
ans =  100

It is not clear why you wanted to create that 1x400 vector in such an obfuscated way.

EDIT: the OP originally defined

m = logspace(2, 2, 400);

but has since edited the question and changed it to:

m = logspace(-2, 2, 400);

The relevance of reading the if documentation is unchanged.

Answer 1

[Shekhar Vats]

Can you please say more about your issue ? It's working as expected for me.

[HC98]

Hi yes, I couldn't get the u value to print, It does not recognise that instruction.

[Stephen23]

"I couldn't get the u value to print,"

Because your if condition is not true, I see no reason to expect u to be defined.

[Stephen23]

HC98' "Answer" moved here:

Apologies, I made an error in my code, I've added the correct version here

[HC98]

U shoiuld be defined... the entire point of this function is to produce values for m less than 1.

[Stephen23]

"U shoiuld be defined."

No. Your non-scalar logical vector does not contain only non-zeros:

>> nnz(m<=1)
ans =  200
>> numel(m)
ans =  400

Read the if documenttion to understand why this is relevant.

Most likely using if is entirely the wrong approach anyway, and you should be using indexing.

[HC98]

All I need is a clue why it won't work. I'm trying to transcribe it in MATLAB having found a copy of the code in Python which reads:

def chabrier03individual(m):
	k = 0.158 * exp(-(-log(0.08))**2/(2 * 0.69**2))
	return numpy.where(m <= 1,\
	        0.158*(1./m) * exp(-(log(m)-log(0.08))**2/(2 * 0.69**2)),\
	        k*m**-2.3)

[Stephen23]

The equivalent to numpy's where is to use indexing:

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

x = ...;
y = ...;
idx = m<=1;
z = y;
z(idx) = x(idx)