Numpy: ValueError:用序列设置数组元素。

I am just playing with python's scientific libraries expecially the examples from here: http://faculty1.coloradocollege.edu/~sburns/toolbox/ODE_II.html

我只是在使用python的科学库，特别是在这里的例子:http://adminty1.coloradocollege.edu/~ sburns/toolbox/ODE_II.html。

I modified it by defining a "force function" and tried to make a simple (but non physical example) where the force depends on the x and y position of the object. Additionally I want to plot the force field with quiever. The problem is that I don't understand how to make the force function properly depend on (a function of) the x and y components of the position of the object, especially I get the error cited below.

我通过定义一个“力函数”来修改它，并尝试做一个简单的(但非物理的例子)，在那里力取决于物体的x和y位置。另外，我想用quiever来绘制力场。问题是，我不理解如何使力函数正确地依赖于(一个函数)对象位置的x和y分量，特别是我得到了下面提到的错误。

from pylab import *
from scipy.integrate import odeint
import numpy as np

## set initial conditions and parameters
g = 9.81            # acceleration due to gravity
th = 91            # set launch angle
th = th * pi/180.   # convert launch angle to radians
v0 = 100           # set speed

x0=0                # specify initial conditions
y0=0
vx0 = v0*sin(th)
vy0 = v0*cos(th)



## define force Funktion

def F_func(x):
    F = zeros(2)
    F[0] = x[1] # 
    F[1] = x[0] #
    return F


## define function to compute f(X,t)
def f_func(state,time):
    f = zeros(4)    # create array to hold f vector
    f[0] = state[2] # f[0] = x component of velocity
    f[1] = state[3] # f[1] = x component of velocity
    f[2] = F_func(state[:2])[0]        # f[2] = acceleration in x direction
    f[3] = F_func(state[:2])[1]      # f[3] = acceleration in y direction
    return f

## set initial state vector and time array
X0 = [ x0, y0, vx0, vy0]        # set initial state of the system
t0 = 0.
tf = 10
tau = 0.1
#tf = input("Enter final time: ")
#tau = input("Enter time step: ")

# create time array starting at t0, ending at tf with a spacing tau
t = arange(t0,tf,tau)   

## solve ODE using odeint
X = odeint(f_func,X0,t) # returns an 2-dimensional array with the 
                        # first index specifying the time and the
                        # second index specifying the component of
                        # the state vector

print X
# putting ':' as an index specifies all of the elements for
# that index so x, y, vx, and vy are arrays at times specified 
# in the time array
x = X[:,0]  
y = X[:,1] 
vx = X[:,2] 
vy = X[:,3]


## plot the trajectory
fig = figure()
ax = fig.add_subplot(1,1,1)

## Enlarge Limits by en Percent
en = 0.05


#xMin,xMax = 0,10
xMin,xMax = min(x),max(x)
yMin,yMax = min(y),max(y)

xMin,xMax = xMin - (xMax-xMin)*en,xMax + (xMax-xMin)*en
yMin,yMax = yMin - (yMax-yMin)*en,yMax + (yMax-yMin)*en


#plot(x,y,[xMin,xMax],[yMin,yMax])
#plot(x,y,[0,10],[0,10])
ax.plot(x,y)
ax.axis('tight')
xlim([xMin,xMax])
ylim([yMin,yMax])


xG,yG = meshgrid(linspace(xMin,xMax,10),linspace(yMin,yMax,5))

ax.quiver(xG,yG,F_func(zip(xG,yG))[0],F_func(zip(xG,yG))[1],pivot='middle',minshaft=5,minlength=1,alpha=0.1)


xlabel('x')
ylabel('y')

show()

With this code I get the following error:

通过这段代码，我得到了以下错误:

ValueError                                Traceback (most recent call last)
/usr/lib/python2.7/dist-packages/IPython/utils/py3compat.pyc in execfile(fname, *where)
    176             else:
    177                 filename = fname
--> 178             __builtin__.execfile(filename, *where)

/home/myuser/python/test.py in <module>()
     87 xG,yG = meshgrid(linspace(xMin,xMax,10),linspace(yMin,yMax,5))
     88 
---> 89 ax.quiver(xG,yG,F_func(zip(xG,yG))[0],F_func(zip(xG,yG))[1],pivot='middle',minshaft=5,minlength=1,alpha=0.1)
     90 
     91 

/home/myuser/python/test.py in F_func(x)
     20 def F_func(x):
     21     F = zeros(2)
---> 22     F[0] = x[1] #
     23     F[1] = x[0] #
     24     return F

ValueError: setting an array element with a sequence.

Can somebody explain this and how to fix it?

有人能解释一下吗?

1 个解决方案

#1

Try using

试着用

def F_func(x):
    F1 = x[1]
    F2 = x[0]
    return array([F1, F2])

instead of your F_func. And replace the invocation of quiver with

而不是你的F_func。并替换抖动的调用。

ax.quiver(xG,yG,F_func(array([xG,yG]))[0],F_func(array([xG,yG]))[1],pivot='middle',minshaft=5,minlength=1,alpha=0.1)

You can find out why numpy is giving you an error by placing a print x on the first line of your F_func.

您可以通过在F_func的第一行上放置一个print x来了解为什么numpy会给您一个错误。

F is just a two-element array and you are trying to assign arrays to each of its two elements (Hence the error "setting an array element with a sequence").

F只是一个二元数组，并且您正在尝试将数组分配给它的两个元素(因此错误“设置一个带有序列的数组元素”)。

This happens because zip(xG, yG) does something you probably didn't intend it to do. (Try playing with it in ipython).

这是因为zip(xG, yG)做了一些您可能不打算做的事情。(试着在ipython中玩它)。

This is the resulting plot of the ODE solution I get when running your code:

这是我运行代码时得到的ODE解决方案的结果图: