Python numpy array calculation

Question

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?

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
– Kobi K, Commented Jul 30, 2014 at 9:17
@KobiK When I do the above I am getting result = [1,1,1,1,1,1] — user3397243
– user3397243, Commented Jul 30, 2014 at 9:20
@user3397243 That's not possible unless you've an array like [1, 2, 3, ...] — Ashwini Chaudhary
– Ashwini Chaudhary, Commented Jul 30, 2014 at 9:20
@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
– user2357112, Commented Jul 30, 2014 at 9:28

Ashwini Chaudhary · Accepted Answer · 2014-07-30 09:22:46Z

3

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]

edited Jul 30, 2014 at 9:22

answered Jul 30, 2014 at 9:12

Ashwini Chaudhary

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5 Comments

Ashwini Chaudhary Over a year ago

@user3397243 Your code works fine for me, but you should not list comprehension if NumPy already provides a function for this.

user3397243 Over a year ago

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)

Ashwini Chaudhary Over a year ago

@user3397243 And what does a contains here?

user3397243 Over a year ago

a contains the numbers of r that have been sliced in 50 bins and b contains the numbers counts within those slices

user3397243 Over a year ago

I have edited my question properly. please take a look

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