Getting an unexpected generator object ?

[mcgrim]

thank you.
I have changed line 24-25
with this code

error=(abs(trapezoidal(f,a,b,n)-c[0]) for n in range (300,900))
print(list(error))

and it works, however, I am not understanding the meaning of c[0], I just notice that if I write just 'c' I get an error. What does the 0 stand for? It looks like is the first element of a list, but c is the outcome of the integral so I am not sure what is the relationship between the integral and the list.