Multiple for loops issue

I need to run this code and ultimately generate the numbers for SNRatio. Did I use the for loop in the correct way?

It gives me infinity at the end. Is there a way to store the number generated in each for loop and carry it to the final step?

W=1000; %Initial average excitation power
W1=W+rand(1,W)*10; %Power fluctuation
for M = [10 100] %Magnification factor
    W2=W1*M;
end
    for i=1:W2 %power fluctuation
end
S_i = 100; %Initial Signal
S=S_i+rand(1,S_i)*10; %Generate a set of multiple numbers for signal
for S_f=1:S %final signal
meanS=mean(S_f);
stdeviationS=std(S_f);
SNRatio = mean(S_f)/std(S_f)
end