Matlab与Python：切片

Question

I am trying to covert some Matlab code to Python. I am having problems with slicing.

Matlab Code:

demod_1_b=-1*mod_noisy*2.*sin(2*pi*Fc*t+phi);
y = filter(Hd,demod_1_b);
y2=conv(y,raised)/ConvFac;
%% till this line the length in python and Matlab are same
y2=y2(sa/2:end-(sa/2));
%%%% when i write this line in Python it gives me wrong answer it should come out as   26     but in python it gives me 33 i think i havnt converted it in a rigth way 
demod_3_b=y2(sa/2:sa:end);

Python Code:

demod_1_b=-1*mod_noisy*2*sin((2*pi*Fc*t)+phi)

N=10
Fc=40
Fs=1600
d=firwin(numtaps=N,cutoff=40,nyq=Fs/2)
print(len(d))
Hd=lfilter( d, 1.0, demod_1_b)
y2=(convolve(Hd,raised))/Convfac
print(len(y2))
y2=y2[(sa/2)-1:-sa/2]
print(len(y2))
 # problem starts here
demod_3_b=y2[(sa/2)-1:sa:,]
print(len(demod_3_a))

I just want ask, are demod_3_b=y2(sa/2:sa:end); in Matlab and demod_3_v=y2[(sa/2)-1:sa:,] in Python the same?

Answer 1

Yes, your indexing is wrong. In NumPy, the following indexing applies:

The basic slice syntax is i:j:k where i is the starting index, j is the stopping index, and k is the step (k≠0).

Hence, what you are looking for in Python is:

y2[(sa/2)-1::sa]

Unlike in Matlab, the stepsize is the last input. As you want to process the entire length of your array, don't put anything between the two : .