How to flatten only some dimensions of a numpy array

Question

Is there a quick way to  sub-flatten  or flatten only some of the first dimensions in a numpy array   For example  given a numpy array of dimensions  50 100 25   the resultant dimensions would be  5000 25

User · Answer

An alternative approach is to use numpy resize   as in   In  37   shp    50 100 25  In  38   arr   np random random sample shp  In  45   resized arr   np resize arr   np prod shp  2    shp -1    In  46   resized arr shape Out 46    5000  25     sanity check with other solutions In  47   resized   np reshape arr   -1  shp -1    In  48   np allclose resized arr  resized  Out 48   True

User · Answer

Take a look at numpy reshape     gt  gt  gt  arr   numpy zeros  50 100 25    gt  gt  gt  arr shape    50  100  25    gt  gt  gt  new arr   arr reshape 5000 25   gt  gt  gt  new arr shape       5000  25     One shape dimension can be -1     In this case  the value is inferred from    the length of the array and remaining dimensions   gt  gt  gt  another arr   arr reshape -1  arr shape -1    gt  gt  gt  another arr shape    5000  25

User · Answer

A slight generalization to Peter s answer -- you can specify a range over the original array s shape if you want to go beyond three dimensional arrays   e g  to flatten all but the last two dimensions   arr   numpy zeros  3  4  5  6   new arr   arr reshape -1   arr shape -2    new arr shape    12  5  6    EDIT  A slight generalization to my earlier answer -- you can  of course  also specify a range at the beginning of the of the reshape too   arr   numpy zeros  3  4  5  6  7  8   new arr   arr reshape  arr shape  2   -1   arr shape -2    new arr shape    3  4  30  7  8

User · Answer

A slight generalization to Alexander s answer - np reshape can take -1 as an argument  meaning  total array size divided by product of all other listed dimensions    e g  to flatten all but the last dimension    gt  gt  gt  arr   numpy zeros  50 100 25    gt  gt  gt  new arr   arr reshape -1  arr shape -1    gt  gt  gt  new arr shape    5000  25

[python] How to flatten only some dimensions of a numpy array

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