Struggling to Understand Error Message

Question

I'm trying to run the code below, where Bwavelength, throughput and newflux are lists.

def ABconversion(Bwavelength, throughput):
    ABconstant=[]

    c=3e18
    i=0
    for i in range(0, len(Bwavelength)):
        ABconstant.append(((3e18/((Bwavelength[i])**2))*throughput[i]))
        i+=1
        print len(Bwavelength), len(ABconstant), ABconstant
        a=Bwavelength[0]
        b=Bwavelength[-1]
        h=((b-a)/len(Bwavelength))
        ABflux = numpy.trapz(Bwavelength, ABconstant, h)
        return ABflux

 def ABmagnitude(newflux, ABflux):
    ABmagarray=[]
    for i in range(len(newflux)):
        ABmag = -2.5*log10((newflux[i])/ABflux) - 48.6
        ABmagarray.append(ABmag)
    return ABmagarray

ABflux1 = ABconversion(Bwavelength, throughput)
print ABflux1
ABmagarray = ABmagnitude(z, ABflux1)
print epoch, ABmagarray

z is defined earlier in the file and is also a list.

However, when I run this I get the message:

Traceback (most recent call last):
  File "Rewrite17.11.2014.py", line 196, in <module>
    ABflux1 = ABconversion(Bwavelength, throughput)
  File "Rewrite17.11.2014.py", line 186, in ABconversion
    ABflux = numpy.trapz(Bwavelength, ABconstant, h)
  File "C:\Python27\lib\site-packages\numpy\lib\function_base.py, line 3234, in trapz
    ret = add.reduce(d * (y[slice1]+y[slice2]/2.0, axis)
ValueError: Operands could not be broadcast together with shapes (0,) (444,)

I don't quite understand the error (I'm fairly new to programming), but I think it means the two "shapes" don't have the same dimensions. I'm not sure why this is.

Thanks in advance.

What does the line `print len(Bwavelength), len(ABconstant), ABconstant` print? — ErikR, Nov 22 '14 at 15:27

score 1 · Accepted Answer · edited May 23 '17 at 12:27

According to this documentation the parameters to trapz(y, x, dx, axis) are:

y - Array like - input array to integrate.
x - Optional array - If x is None, then spacing between all y elements is dx.
dx - Optional scalar - If x is None, spacing given by dx is assumed. Default is 1.
axis - Optional Int - specify the axis.

So you shouldn't specify both x and dx - one of them should be None.

Perhaps this is what you want: trapz(Bwavelength, None, h).

See this answer for more details on the error message and NumPy's "braodcasting rule".

score 1 · Answer 2 · answered Nov 22 '14 at 16:21

1

Replace:

numpy.trapz(Bwavelength, ABconstant, h)

with:

numpy.trapz(np.array(Bwavelength)[:,np.newaxis], ABconstant, h)

answered Nov 22 '14 at 16:21

Irshad Bhat

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Struggling to Understand Error Message

2 Answers2