使用Python计算向量的中点

Question

I started to learn python from scratch. I got some issues while doing the following problem.

I have the following vector , x_vector = (0,1,2,3,4,5,6,7,8,9) . Using this vector, I need to create this new vector x1 = (-0.5,0.5,1.5,2.5,3.5,4.5,5.5,6.5,7.5,8.5,9.5) .

Basically the desired vector should have first element -0.5, mid points between each elements and the last element +0.5.

The code I tried so far as follows:

import numpy as np
x_vector=np.array([0,1,2,3,4,5,6,7,8,9])
x=len(x_vector)
mid=np.zeros(x+1)

for i in range (0,x):
    if i==0 :
        mid[i]= x_vector[i]-0.5
    else :
        mid[i]=(x_vector[i] + x_vector[i+1])/2
        i +=1

Seems like this doesn't give the desired output. Can you one help me to figure out what can I do to get correct output?

Answer 1

Using itertools.pairwise :

from itertools import tee

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return zip(a, b)

res = []
res.append(min(x_vector)-0.5)
res.append(max(x_vector)+0.5)
res.extend([np.mean(z) for z in pairwise(x_vector)])
sorted(res)

Output:

[-0.5, 0.5, 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5]

Answer 2

Consider, what will happen for i = 0 and i = 1 in your loop:

mid[0] = x_vector[0] - 0.5 # = -0.5
mid[1] = (x_vector[1] + x_vector[2]) / 2 # (1 + 2) / 2 = 3 / 2 = 1 (or 1.5 if python3)

you mismatched indexes. Try this:

for i in range (0,x):
    if i == 0:
        mid[i] = x_vector[i]-0.5
    else :
        mid[i] = (x_vector[i - 1] + x_vector[i]) / 2.0

Note, that i changed division to divide by 2.0 instead of 2 - this will make sure, that division result will be double (number with fraction) instead of integer (number without fraction, in python 2 division two integers will round to integer). Also i += 1 is redundant, i variable in for loop will updated (overwriting your += 1 statement) every loop iteration.

Answer 3

It is not clear whether this is a homework, but given that you are using numpy I think it is fair game to use it as its whole potential, in this case you can just do:

import numpy as np
x_vector=np.array([0,1,2,3,4,5,6,7,8,9]) 
a = np.insert(x, 0, x[0] - 1)
b = np.append(x, x[-1] + 1)
mid = (a + b) / 2