modify a row of an array within a function

I have a 2 dimensional array arr and at various places i want to replace the ii-th row.

So I would like to write a function

function arr = replace(arr, ii)

and call that, e.g. in a loop but also with fixed ii.

My problem is that i fear that all of arr is copied then.

I just want a passed by reference and replace the row.

Is this feasable??

[Stephen23]

"My problem is that i fear that all of arr is copied then."

Have you checked this?

Answer 1

[KSSV]

If a is an array of size m*n, you can repalce i'th row with a row matrix b of size 1*n using:

a(i,:) = b ; 

[Ernst Reißner]

I know.

that is exactly what I would do within the function replace(...).

But my question is how i can avoid that when invoking replace all of a is copied

although within replace i just replace a single row.

I know, if I just read from a, it is not copied, but it is if i modify.

RIght now i wonder whether if i could define a to be an array of arrays,

accessing as a(ii)(jj).. this would spare someting. but far from ideal.

Answer 2

[James Tursa]

This can sometimes be done inplace depending on how you write the function and how you call the function. One requirement is your function needs to be called from within another function (easy to do). Another requirement is that your variable can't be shared with another variable (not easy to guarantee unfortunately). See this blog:

https://blogs.mathworks.com/loren/2007/03/22/in-place-operations-on-data/?s_tid=srchtitle