#1




*ANSWER* HW5 Q8/9  Help with R code
Hi, can anybody send me their R code for Q 8/9 or look at mine (appended below)? My gradient descent never runs for more than 1 epoch. I can't figure out what I'm doing wrong, I've pored over my code for so long I'm feeling crosseyed
# Learning from Data HW5 (items 8 and 9) # Logistic Regression with Stochastic Gradient Descent nIter = 100; nIn = 100; nOut = 10000; eta = 0.01; eOut = rep(NA, nIter); nEpochs = rep(NA, nIter); set.seed(123); for (i in (1:nIter)) { # Initialization of f(x) and of matching Insample nEpoch = nEpoch +1; }} # E(Eout) avgEOut = mean(eOut); # E(NEpoch) avgNEpoch = mean(nEpochs); Last edited by catherine; 05082013 at 12:55 AM. Reason: code formatting 
#2




Re: *ANSWER* HW5 Q8/9  Help with R code
Hi Catherine,
It would help if you can wrap your code up in the code tags. The tags can be found with the other formatting options when posting a message on the forum. This will make it easier for us to read and remove the sad face, although I thought it was appropriate. 
#3




Re: *ANSWER* HW5 Q8/9  Help with R code
Sorry, I hadn't noticed the advanced editing option. First time I'm posting something to the forum.

#4




Re: *ANSWER* HW5 Q8/9  Help with R code
Welcome aboard.
__________________
Where everyone thinks alike, no one thinks very much 
#5




Re: *ANSWER* HW5 Q8/9  Help with R code
I think the following bit of code has a bug. You are updating w and w0 every time through the loop and then comparing the two w's at the end. I believe you should be comparing the value of w from before the loop to the value of w at the end of the loop.
So the first time you have w=[0,0,0] then you loop through all points updating the weight. Then after that you compare w=[0,0,0] with weight you have at the end of the loop. Then you use the updated weight as the initial weight for the next time through the loop. Code:
for (j in permuted.ind) { w0 = w; v = (YIn[j] * XIn[j,]) / (1 + exp(YIn[j] * (XIn[j,] %*% w0))); w = w0  eta * v; } # Stop gradient descent as soon as delta(w, w0) < 0.01 if (sqrt(sum((w  w0)^2)) < 0.01) break() 
#6




Re: *ANSWER* HW5 Q8/9  Help with R code
Good spotting! Thank you so much arcticblue! I had simply misread the definition of the exit condition

#7




Re: *ANSWER* HW5 Q8/9  Help with R code
Hello all:
I answered questions 8 and 9 incorrectly. My R code ran, but it is giving wrong values. Here it is: Code:
#Q8 #we need a function for determining where a point falls #given the 2 points p1 and p2 that determine the line out < function(p1,p2,p) { m < (p2p1)[2]/(p2p1)[1] if (p[2] < m*(p[1]p1[1])+p1[2]) return(1) else return(+1) } #function for generating the training data. Size=N generate_training_set < function(p1,p2,N) { tm < matrix(data=runif(min=1,max=1,n=2*N),nrow=N,ncol=2) y < sapply(1:N,function(i) out(p1,p2,tm[i,])) return( as.matrix(data.frame(x0=rep(1,N),x1=tm[,1],x2=tm[,2],y=y)) ) } N < 100 eta < 0.1 runs < 100 #the number of epochs and the Eout for each run #will be stored in the respective vectors epochs < numeric(0); Eouts < numeric(0) for (r in 1:runs) { #Generate the 2 points for the line p1 < runif(min=1,max=+1,n=2) p2 < runif(min=1,max=+1,n=2) #generate the training set of size N training_data < generate_training_set(p1,p2,N) w < c(0,0,0); wp < c(1,1,1); epoch < 0 while (sqrt(sum((wwp)^2)) > 0.01) { wp < w perm < sample(1:N,size=N) for (j in 1:N) { i < perm[j] grad < (training_data[i,4])*training_data[i,1:3]/ (1+exp(training_data[i,4]*w%*%training_data[i,1:3])) w < w  eta*grad } epoch < epoch + 1 } epochs < c(epochs,epoch) #Evaluate Eout, generate a new test set 10 times larger test_data < generate_training_set(p1,p2,N*10) s < sapply(1:(N*10),function(i) log(1+exp(test_data[i,1:3]%*%w*(test_data[i,4]))) ) Eout < mean(s) Eouts < c(Eouts,Eout) print(paste(r,epoch,Eout)) #so I can see what the program is doing } 
#8




Re: *ANSWER* HW5 Q8/9  Help with R code
Yes, I now get average number of epochs = 340.17 and average error out = 0.1036295 after correcting the exit condition for the gradient descent:
Code:
# Gradient descent repeat { wbase = w; nEpoch = nEpoch +1; # Permutate Insample and update weight vector with each data point in turn permuted.ind = sample(nIn, nIn, replace=FALSE); for (j in permuted.ind) { v = (YIn[j] * XIn[j,]) / (1 + exp(YIn[j] * (XIn[j,] %*% w))); w = w  eta * v; } # Stop gradient descent as soon as delta(w, wbase) < 0.01 if (sqrt(sum((w  wbase)^2)) < 0.01) break() } 
#9




Re: *ANSWER* HW5 Q8/9  Help with R code
OOps! Thanks for pointing that out, catherine! That's how I blew away 2 points on that quiz!

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