How to sum all instances of a value across a variable?

My variable follows this format

a =  [1 2 2 1]

etc.

So all of the values are either 1, or 2.

How do I sum all of the instances of the number 1 across this variable?

[Jan]

Is this a homework question? If so, what have you tried so far? Do you want to solve it with a loop or with the efficient builtin method unique and histcounts?

As usual I could offer a method based on RunLength. After I have suggested it a hundred of times in the forum already, I think about changing my nick name to "JanLength".

Answer 1

[Jan]

a      = [1 2 2 1]
k      = 1;
sumOf1 = sum(a(a == k))

or

sumOf1 = k * sum(a == k)

But maybe you want to get the result for all elements of a? Then please explain this. unique and histcounts will be useful.

[Louisa Thomas]

The first part was great thank you. It worked perfectly! It's part of a data extraction script, so I just worked with that format, and it worked :)