a = frame_query("select ....",db)
With my code, the var 'c' is a numpy.ndarray which I would like to plot. However when I execute the following code, i get an empty plot!
for i in a.values:
c = (i[:1]-a.values[-1:])/a.values[-1:]*100
plt.plot(c)
plt.show()
print c
gives:
[[ 28.57142857]]
[[ 27.27272727]]
[[ 27.92207792]]
[[ 28.57142857]]
[[ 22.07792208]]
[[ 22.07792208]]
[[ 22.07792208]]
Where exactly am I going wrong?
Thank you in advance.
The simplest way to adapt your code is probably to start an empty list before the loop, append the values from c onto it within the loop and then plot it after the loop. For example:
c_series = []
for i in a.values:
c = (i[:1]-a.values[-1:])/a.values[-1:]*100
c_series.append(c[0])
plt.plot(c)
plt.show()
Note using c[0] is safe here, because the logic of the line above guarantees that the ndarray will only have one member.
However, that's a bit of an odd way to deal with your data structure. As it seems that a.values is an ndarray, you could also simply do this by using the facilities that numpy provides to perform arithmetical operations on arrays (I can't test it as I don't have a verbatim copy of your a.values):
const = a.values[-1:]
c_series = (a.values - const) / const*100
plt.plot(c_series)
plt.show()
In general - when using numpy arrays, it's often better to keep them as arrays (called vectorising the code), rather than dealing with them element by element in a loop.
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