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modify a row of an array within a function

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Ernst Reißner
Ernst Reißner on 18 May 2021
Edited: James Tursa on 18 May 2021
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??
  1 Comment
Stephen
Stephen on 18 May 2021
"My problem is that i fear that all of arr is copied then."
Have you checked this?

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Answers (2)

KSSV
KSSV on 18 May 2021
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 ;
  1 Comment
Ernst Reißner
Ernst Reißner on 18 May 2021
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.

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James Tursa
James Tursa on 18 May 2021
Edited: James Tursa on 18 May 2021
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:

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