# [Numpy-discussion] interpolating arrays (?)

Tue Mar 23 22:29:11 EST 2004

```The nd_image module in numarray has a spline intrpolation function
(spline_filter1d). I've tried it, and it is OK.

-----Original Message-----
From:	Ray Schumacher [mailto:rays at blue-cove.com]
Sent:	Tue 23-Mar-04 03:11
To:	numpy-discussion at lists.sourceforge.net
Cc:
Subject:	[Numpy-discussion] interpolating arrays (?)
Howdy,

I'd like to resize an array (1-dimensional) a of length n into an array of
n+x, and interpolate the values as necessary.
x might be+/-200, len(a) is usually ~500

I tried it in pure Python but it's not quite right yet. (rubberBand, below)
Someone out there must have done this... and linear interpolation for
values is probably sufficient (instead of bi-cubic or some such)

Cheers,
Ray

==================================================================
startSlice = 10000
endOfSlice =
10000+int(round(self.samplesPerRotation))    #self.samplesPerRotation=661
## self.dataArray is about len=200,000 of Int

for delta in range(-2,3):
## interpolate the different slices to the original length
##  0 should return the same array, this is a test
newArray = self.rubberBand(
self.dataArray[startSlice:endOfSlice+delta],
round(self.samplesPerRotation))
print len(self.dataArray[startSlice:endOfSlice+delta]),
len(newArray),
## show the result
print delta, Numeric.sum(self.dataArray[startSlice:endOfSlice]
- newArray)

def rubberBand(self, data, desiredLength):
""" alias/interpolate the data so that it is adjusted to the
desired array length"""
currentLength = float(len(data))

## positive if the new array is to be longer
difference = desiredLength - currentLength
#print  difference, desiredLength, currentLength

## set up the desired length array
newData = Numeric.zeros(desiredLength, Numeric.Float)

## accounts for binary rounding errors
smallNum = 2**-14

for index in range(desiredLength):
## find the ratio of the current index to the desired length
ratio = index / float(desiredLength)

## find the same (float) position in the old data
currIndex = int(ratio * currentLength)
## find the decimal part
decimalPart = (ratio * currentLength) - currIndex
print index, currIndex, decimalPart,
if(decimalPart>smallNum):
## interpolate
newData[index] = ((1 - decimalPart) * data[currIndex] +
decimalPart * data[currIndex+1])
print data[currIndex], data[currIndex+1], newData[index]
else:
newData[index] = data[currIndex]
print 'else',data[currIndex], newData[index]

return newData

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