Numpy ValueError: setting an array element with a sequence. This message may appear without the existing of a sequence?

Question

Why do I get this error message? ValueError: setting an array element with a sequence. Thank you

Z=np.array([1.0,1.0,1.0,1.0])    def func(TempLake,Z):     A=TempLake     B=Z     return A*B  Nlayers=Z.size N=3 TempLake=np.zeros((N+1,Nlayers))  kOUT=np.zeros(N+1) for i in xrange(N):     kOUT[i]=func(TempLake[i],Z) 

Answer 1

You're getting the error message

ValueError: setting an array element with a sequence. 

because you're trying to set an array element with a sequence. I'm not trying to be cute, there -- the error message is trying to tell you exactly what the problem is. Don't think of it as a cryptic error, it's simply a phrase. What line is giving the problem?

kOUT[i]=func(TempLake[i],Z) 

This line tries to set the ith element of kOUT to whatever func(TempLAke[i], Z) returns. Looking at the i=0 case:

In [39]: kOUT[0] Out[39]: 0.0  In [40]: func(TempLake[0], Z) Out[40]: array([ 0.,  0.,  0.,  0.]) 

You're trying to load a 4-element array into kOUT[0] which only has a float. Hence, you're trying to set an array element (the left hand side, kOUT[i]) with a sequence (the right hand side, func(TempLake[i], Z)).

Probably func isn't doing what you want, but I'm not sure what you really wanted it to do (and don't forget you can usually use vectorized operations like A*B rather than looping in numpy.) That should explain the problem, anyway.

Answer 2

It's a pity that both of the answers analyze the problem but didn't give a direct answer. Let's see the code.

Z = np.array([1.0, 1.0, 1.0, 1.0])    def func(TempLake, Z):     A = TempLake     B = Z     return A * B Nlayers = Z.size N = 3 TempLake = np.zeros((N+1, Nlayers)) kOUT = np.zeros(N + 1)  for i in xrange(N):     # store the i-th result of     # function "func" in i-th item in kOUT     kOUT[i] = func(TempLake[i], Z) 

The error shows that you set the ith item of kOUT(dtype:int) into an array. Here every item in kOUT is an int, can't directly assign to another datatype. Hence you should declare the data type of kOUT when you create it. For example, like:

Change the statement below:

kOUT = np.zeros(N + 1) 

into:

kOUT = np.zeros(N + 1, dtype=object) 

or:

kOUT = np.zeros((N + 1, N + 1)) 

All code:

import numpy as np Z = np.array([1.0, 1.0, 1.0, 1.0])  def func(TempLake, Z):     A = TempLake     B = Z     return A * B  Nlayers = Z.size N = 3 TempLake = np.zeros((N + 1, Nlayers))  kOUT = np.zeros(N + 1, dtype=object) for i in xrange(N):     kOUT[i] = func(TempLake[i], Z) 

Hope it can help you.