# "if sum" (how operator "sum" is possible next to "if"...?)

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Jinho Lee on 18 Jun 2022
Answered: Jinho Lee on 18 Jun 2022
I'm trying to undedrstand matlab code for Ising model ("https://kr.mathworks.com/matlabcentral/fileexchange/75149-2d-ising-model")
In the program, there is code like:
if sum(ismember([0:100:iter],i))
I understand ismember but I couldn't understand "if sum"
Q1. Next to "if", there should be logic operator but how can "sum" can be placed
Q2. what is the meaning of sum of logic array, it just number ..?
per isakson on 18 Jun 2022
Edited: per isakson on 18 Jun 2022
Read the documation of if, elseif, else carefully and you will find that if some_thing_numerical is allowed. That's because in Matlab if appeared decades before logical.

Image Analyst on 18 Jun 2022
The sum can either be 0 or some number. ismember gives you a 1 if the item, i, occurs and 0 if not. It can give you a whole vector os 1's and 0's. The sum counts the number of times there is a 1, and so the if block executes if i is found in the vector [0, 100, 200, ......iter] at least once.

Stephen23 on 18 Jun 2022
Edited: Stephen23 on 18 Jun 2022
"Next to "if", there should be logic operator"
The IF documentation actually states that it must be an expression. It also states that "An expression is true when its result is nonempty and contains only nonzero elements (logical or real numeric). Otherwise, the expression is false."
"but how can "sum" can be placed"
Just as the IF documentation states, if the SUM() returns only non-zero elements then the expression is considered true. Otherwise it will be false.
"what is the meaning of sum of logic array, it just number ..?"
Yes, SUM() just adds up all of the trues (as ones). In this usage, these are equivalent:
IF SUM(logicalarray)
IF ANY(logicalarray)

Jinho Lee on 18 Jun 2022
I got it!
Thanks