[Numpy-discussion] Re: Method to shift elements in an array?

Tim Hochberg tim.hochberg at cox.net
Tue Feb 28 09:53:03 CST 2006


Alan G Isaac wrote:

>Tim wrote:
>  
>
>>import numpy 
>>def roll(A, n):
>>    "Roll the array A in place. Positive n -> roll right, negative n -> 
>>roll left"
>>    if n > 0:
>>        n = abs(n)
>>        temp = A[-n:]
>>        A[n:] = A[:-n]
>>        A[:n] = temp
>>    elif n < 0:
>>        n = abs(n)
>>        temp = A[:n]
>>        A[:-n] = A[n:]
>>        A[-n:] = temp
>>    else:
>>        pass
>>    
>>
>
>This probably counts as a gotcha:
>  
>
>>>>a=N.arange(10)
>>>>temp=a[-6:]
>>>>a[6:]=a[:-6]
>>>>a[:6]=temp
>>>>a
>>>>        
>>>>
>array([4, 5, 0, 1, 2, 3, 0, 1, 2, 3])
>  
>

Ack! Right, those temp variables needed to be copies. That's why I added 
the caveat about only rolling a few elements, since otherwise it gets 
expensive.  Then I forgot to make the copies in the code, doh!

-tim


>Cheers,
>Alan Isaac
>
>PS Here's something close to the rotater functionality.
>
>#rotater: rotate row elements
># Format:    y = rotater(x,r,copydata)
># Input:     x           RxC array
>#            rotateby    size R integer array, or integer (rotation amounts)
>#            inplace     boolean (default is False -> copies data)
># Output:    y           RxC array:
>#                          rows rotated by rotateby
>#                        or None (if inplace=True)
># Remarks:   Intended for use with 2D arrays.
>#            rotateby values are positive for rightward rotation,
>#	               negative for leftward rotation
># :author: Alan G Isaac (aisaac AT american DOT edu)
># :date:   24 Feb 2006
>def rotater(x,rotateby,inplace=False) :
>	assert(len(x.shape)==2), "For 2-d arrays only."
>	xrotate = numpy.array(x,copy=(not inplace)) 
>	xrows = xrotate.shape[0]
>	#make an iterater of row shifts
>	if isinstance(rotateby,int):
>		from itertools import repeat
>		rowshifts = repeat(rotateby,xrows)
>	else:
>		rowshifts = numpy.asarray(rotateby)
>		assert(rowshifts.size==xrows)
>		rowshifts = rowshifts.flat
>	#perform rotation on each row
>	for row in xrange(xrows):
>		rs=rowshifts.next()
>		#do nothing if rs==0
>		if rs>0:
>			xrotate[row] = numpy.concatenate([xrotate[row][-rs:],xrotate[row][:-rs]])
>		elif rs<0:
>			xrotate[row] = numpy.concatenate([xrotate[row][:-rs],xrotate[row][-rs:]])
>	if inplace:
>		return None
>	else:
>		return xrotate
>
>
>
>
>
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