Python error setting an array element with a sequence for different loops

Question

My code is as follows (there's about 100 lines before of setting values for the loop which seem to be working so I've just included necessary values):

fraction=np.array([0.5, 0.3, 0.2])
tauC=np.array([30.,300.,100000.])

dC_memory=np.zeros((1,3))
dC_frac=np.zeros((1,3))    

for j in range(0,ens_num):

    dC_memory=np.zeros((1,3))
    for n in range(0,N-1):


                # C02 Concentration

                for m in range(0,3):

                    dC_frac[m]=fraction[m]*E[j,n+1]-dC_memory[m]/(tauC[m])
                    dC_memory[m]=dC_memory[m]+dC_frac[m]*dt
                    dC[j,n]=dC[j,n]+dC_frac[m]*dt



                C[j,n+1]=C[j,n]+dC[j,n]

                # Temperature

                dT[j,n]=((T2eq*math.log(C[j,n+1]/Cpi)/math.log(2))-T[j,n])*(dt/tauT)
                T[j,n+1]=T[j,n]+dT[j,n]

                # Adaptation

                dTadp[j,n]=(T[j,n]-Tadp[j,n])*dt/tauA
                Tadp[j,n+1]=Tadp[j,n]+dTadp[j,n]
                Tdiff[j,n+1]=0.5*(abs(T[j,n]-Tadp[j,n+1])+T[j,n]-Tadp[j,n+1])



                if yi[j,n+1]+xi0[k]<=mu:


                    count[j]=count[j]+1/N

When I run this I get the error on the line dC[j,n]=dC[j,n]+dC_frac[m]*dt saying

ValueError: setting an array element with a sequence.

I'm new to python but I know that python indexing starts from 0, but I cant understand why this code stops here.

What exactly is `dC`? You never initialize it in the code you posted. I suspect it's an ordinary Python list (instead of a numpy array), in which case `dC[j,n]` is not valid (you'd have to use `dC[j][n]` for multidimensional indexing). — jasonharper, Feb 03 '18 at 15:40
Printing data, (conditional) expression results, anything relevant, is an effective debugging technique. You can print stuff just before the offending line or [catch the error](https://docs.python.org/3/tutorial/errors.html#handling-exceptions) and print in the except suite. It will give you more clues to what is happening. — wwii, Feb 03 '18 at 16:15

score 2 · Accepted Answer · answered Feb 03 '18 at 15:50

Your example code is not complete. But I think the bug is clear.

By defining

 dC_frac=np.zeros((1,3))

You dC_frac is a multidimensional array of shape (1, 3). Use dC_frac.shape you will find it's (1, 3), not, (3,).

Thus in

for m in range(0,3):
    dC_frac[m]=fraction[m]*E[j,n+1]-dC_memory[m]/(tauC[m])
    ...

Your dC_frac[m] is an array of 3 elements, not a scalar.

If your dC[j, n] and dt are scalars,

dC[j,n]=dC[j,n]+dC_frac[m]*dt

This will assign an array of 3 elements to an entry. Thus the error.

To fix, just use

dC_memory=np.zeros(3)
dC_frac=np.zeros(3)

Python error setting an array element with a sequence for different loops

1 Answers1