difficulty in understanding numpy array behavior

the following is a (very)shorterned vesrion of my program

the normvecs vector is not getting overwritten , you woud be able to see for yourself just once you run this code snippet , the parameters are correct geom function takes an n+1 X 2 array and n as input, i guess i'm doing something really stupid (which i think i am) or i don't understand this behavior

import numpy as np

#geometry calculations
def geom(pts,n):
    r = np.zeros(n)
    normvecs = np.zeros((n,2))
    tgtvecs  = np.zeros((n,2))
    alphap = np.zeros(n)
    cpts = np.zeros((n,2))
    #collocation points
    cpts[:,0] = (pts[0:n,0]+pts[1:n+1,0])/2  
    cpts[:,1] = (pts[0:n,1]+pts[1:n+1,1])/2
    #length of panels
    r[:] = np.sqrt((pts[0:n,0]-pts[1:n+1,0])**2 + (pts[0:n,1]-pts[1:n+1,1])**2)
    #angle of each panel with the horizantal(chord)/refernce axis
    alphap[:] = np.arctan2(pts[1:n+1,1]-pts[0:n,1],pts[1:n+1,0]-pts[0:n,0])
    #normal vectors
    normvecs[:,0] = -np.sin(alphap[:])
    normvecs[:,1] =  np.cos(alphap[:])
    xx = np.cos(alphap[:])
    print(normvecs[:,0],normvecs[:,1],np.cos(alphap[:]),xx)
    return

geom(np.random.rand(31,2),30)

normvecs[:,1]开发者_如何转开发 is showing just negative of nomvecs[:,0] i wasn't able to overwrite like normvecs[:,1] = xx (no errors .. but just printed the same thing)

I maynot sound clear,once if you run you may see what i'm trying to tell.

what's the problem?(if it is so!)

I see no problem at all. Running a more concise version of your code:

import numpy as np

#geometry calculations
def geom(pts,n):
    normvecs = np.zeros((n,2))
    alphap = np.zeros(n)
    alphap[:] = np.arctan2(pts[1:n+1,1]-pts[0:n,1],pts[1:n+1,0]-pts[0:n,0])
    normvecs[:,0] = -np.sin(alphap[:])
    normvecs[:,1] =  np.cos(alphap[:])
    print np.abs(normvecs[:,0])-np.abs(normvecs[:,1])
    return

geom(np.random.rand(31,2),30)

produces something like this:

[ 0.47500019 -0.03182906 -0.46597523  0.7479451   0.12580804 -0.36311644
  0.06406616 -0.29641905 -0.39982319 -0.98493049 -0.4431009  -0.29506693
 -0.25931983  0.67831564 -0.80676608 -0.53007712  0.63448069  0.67457029
  0.25457744 -0.82095266 -0.27461275 -0.91244341 -0.69530798 -0.69023852
  0.18840807  0.49891863  0.52417747  0.06833423  0.83449866  0.47608894]

Which suggests to me that the elements of two rows of normvecs you are saying are identical in magnitude are not.