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I have calculated a histogram slice using numpy histogram by N,a = np.histogram(z,bins=50). Now my a contains the values of the 50 slices of z and N contains the number counts within those slices.

I would like to calculate R-r for a

I have tried 

result = [R-r for R,r in zip(a[1:],a)]

but it doesn't seem to work. What I am doing wrong here?

share|improve this question
    
The code is fine, output: [2, 3, 2, 9, 6, 22], "but it doesn't seem to work", can you be more specific? –  Kobi K Jul 30 '14 at 9:17
    
@KobiK When I do the above I am getting result = [1,1,1,1,1,1] –  user3397243 Jul 30 '14 at 9:20
    
@user3397243 That's not possible unless you've an array like [1, 2, 3, ...] –  Ashwini Chaudhary Jul 30 '14 at 9:20
    
@user3397243 can you attach the code? –  Kobi K Jul 30 '14 at 9:22
    
@user3397243: Why are you working with the second return value? That's the bin edges. If this were a graphical histogram, you'd be doing your math with the little ticks on the bottom of your graph instead of looking at the height of the bars. –  user2357112 Jul 30 '14 at 9:28

1 Answer 1

up vote 3 down vote accepted

You simply need to use numpy.diff for this:

>>> a = np.array([1,3,6,8,17,23,45])
>>> np.diff(a)
array([ 2,  3,  2,  9,  6, 22])

Edit:

Your code is working fine too, but you should not use list comprehension for this as NumPy already provides a function for this because it is going be both fast and efficient.

>>> a = np.array([1,3,6,8,17,23,45])
>>> [R-r for R,r in zip(a[1:],a)]
[2, 3, 2, 9, 6, 22]
share|improve this answer
    
Thanks, it works! But what is wrong with my code? –  user3397243 Jul 30 '14 at 9:13
    
@user3397243 Your code works fine for me, but you should not list comprehension if NumPy already provides a function for this. –  Ashwini Chaudhary Jul 30 '14 at 9:16
    
The same does not work for a histogram slice!! i.e if I have b,a = np.histogram(r,bins=50) and I say result = np.diff(a) –  user3397243 Jul 30 '14 at 9:21
    
@user3397243 And what does a contains here? –  Ashwini Chaudhary Jul 30 '14 at 9:24
    
a contains the numbers of r that have been sliced in 50 bins and b contains the numbers counts within those slices –  user3397243 Jul 30 '14 at 9:26

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