Add single element to array in numpy

Question

I have a numpy array containing    1  2  3    I want to create an array containing    1  2  3  1    That is  I want to add the first element on to the end of the array   I have tried the obvious   np concatenate  a  a 0      But I get an error saying ValueError  arrays must have same number of dimensions  I don t understand this - the arrays are both just 1d arrays

User · Answer

If you want to add an element use append     a   numpy append a  1  in this case add the 1 at the end of the array  If you want to insert an element use insert    a   numpy insert a  index  1  in this case you can put the 1 where you desire  using index to set the position in the array

User · Answer

When appending only once or once every now and again  using np append on your array should be fine  The drawback of this approach is that memory is allocated for a completely new array every time it is called  When growing an array for a significant amount of samples it would be better to either pre-allocate the array  if the total size is known  or to append to a list and convert to an array afterward   Using np append   b   np array  0   for k in range int 10e4        b   np append b  k  1 2 s    16 1 ms per loop  mean    std  dev  of 7 runs  1 loop each    Using python list converting to array afterward   d    0  for k in range int 10e4        d append k  f   np array d  13 5 ms    277   s per loop  mean    std  dev  of 7 runs  100 loops each    Pre-allocating numpy array   e   np zeros  n    for k in range n       e k    k 9 92 ms    752   s per loop  mean    std  dev  of 7 runs  100 loops each    When the final size is unkown pre-allocating is difficult  I tried pre-allocating in chunks of 50 but it did not come close to using a list   85 1 ms    561   s per loop  mean    std  dev  of 7 runs  10 loops each

User · Answer

Try this   np concatenate  a  np array  a 0        http   docs scipy org doc numpy reference generated numpy concatenate html  concatenate needs both elements to be numpy arrays  however  a 0  is not an array  That is why it does not work

User · Answer

append   creates a new array which can be the old array with the appended element   I think it s more normal to use the proper method for adding an element   a   numpy append a  a 0

User · Answer

This might be a bit overkill  but I always use the the np take function for any wrap-around indexing    gt  gt  gt  a   np array  1  2  3    gt  gt  gt  np take a  range 0  len a  1   mode  wrap   array  1  2  3  1     gt  gt  gt  np take a  range -1  len a  1   mode  wrap   array  3  1  2  3  1

User · Answer

Let s say a  1 2 3  and you want it to be  1 2 3 1    You may use the built-in append function  np append a 1    Here 1 is an int  it may be a string and it may or may not belong to the elements in the array  Prints   1 2 3 1

User · Answer

a 0  isn t an array  it s the first element of a and therefore has no dimensions   Try using a 0 1  instead  which will return the first element of a inside a single item array

User · Answer

t   np array  2  3   t   np append t   4

User · Answer

This command   numpy append a  a 0     does not alter a array  However  it returns a new modified array   So  if a modification is required  then the following must be used   a   numpy append a  a 0

[python] Add single element to array in numpy

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