Avoiding unpacking an array when altering its dimension

Question

I am trying to flatten a matrix from 3-dimensional to 2-dimensional one.

The original one has dimension for example {10,120,120}:

list1 = RandomReal[1, {10, 120, 120}];

I want to convert it to a new matrix with {10,120*120} dimension. I am using:

On["Packing"]  (*For checking if exist unpacked arrays *)  
Flatten[Map[Flatten, {list1}, {-3}], 1];

But this will generate unpacked array. How should I avoid this issue?

Answer 1

An alternative to using Flatten as in the comments is to use ArrayReshape. For large arrays, ArrayReshape should be significantly faster. For example:

list1 = RandomReal[1, {10, 1200, 1200}];

Comparison:

r1 = Flatten[list1, {{1}, {2, 3}}]; //RepeatedTiming
r2 = ArrayReshape[list1, {10, 1200 1200}]; //RepeatedTiming

r1 === r2

{0.120, Null}

{0.025, Null}

True