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.

• ErikR: What does the line print len(Bwavelength), len(ABconstant), ABconstant print?
• NXW: It prints 445, 1, [0.0]

Answer 1

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".

Answer 2

Replace:

numpy.trapz(Bwavelength, ABconstant, h)

with:

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