CODEWITHSUNDEEP
r
Pedro Alonso
Pedro Alonso

Reputation: 103

Why isn't the mean() function in R giving me the right result?

I've been practicing basics in R (3.6.3) and I'm stuck trying to understand this problem for hours already. This was the exercise:

Step 1: Generate sequence of data between 1 and 3 of total length 100; #use the jitter function (with a large factor) to add noise to your data Step 2: Compute the vector of rolling averages roll.mean with the average of 5 consecutive points. This vector has only 96 averages. Step 3: add the vector of these averages to your plot Step 4: generalize step 2 and step 3 by making a function with parameters consec (default=5) and y.

y88 = seq(1,3,0.02)
y = jitter(y88, 120, set.seed(1))
y = y[-99] # removed one guy so y can have 100 elements, as asked

roll.meanT = rep(0,96)
for (i in 1:length(roll.meanT)) # my 'reference i' is roll.mean[i], not y[i]
{
roll.meanT[i] = (y[i+4]+y[i+3]+y[i+2]+y[i+1]+y[i])/5  
}

plot(y)
lines(roll.meanT, col=3, lwd=2)

This produced this plot: enter image description here

Then, I proceed to generalize using a function (it asks me to generalize steps 2 and 3, so the data creation step was ignored) and I consider y to remain constant):

fun50 = function(consec=5,y)
{
  roll.mean <- rep(NA,96) # Apparently, we just leave NA's as NA's, since lenght(y) is always greater than lenght(roll.means)
  for (i in 1:96)
  {
    roll.mean[i] <- mean(y[i:i+consec-1]) # Using mean(), I'm able to generalize.
  } 
  plot(y)
  lines(roll.mean, col=3, lwd=2)
}

Which gave me a completely different plot:

enter image description here

When I manually try too see if mean(y[1:5]) produces the right mean, it does. I know I could have already used the mean() function in the first part, but I would really like to get the same results using (y[i+4]+y[i+3]+y[i+2]+y[i+1]+y[i])/5 or mean(y[1:5],......).

Upvotes: 0

Views: 471

Answers (1)

Dason
Dason

Reputation: 62003

You have the line

    roll.mean[i] <- mean(y[i:i+consec-1]) # Using mean(), I'm able to generalize.

I believe your intention is to grab the values with indices i to (i+consec-1). Unfortunately for you - the : operator takes precedence over arithmetic operations.

> 1:1+5-1  #(this is what your code would do for i=1, consec=5)
[1] 5
> (1:1)+5-1 # this is what it's actually doing for you
> 5
> 2:2+5-1 #(this is what your code would do for i=2, consec=5)
[1] 6
> 3:3+5-1 #(this is what your code would do for i=3, consec=5)
[1] 7
> 3:(3+5-1) #(this is what you want your code to do for i=3, consec=5)
[1] 3 4 5 6 7

so to fix - just add some parenthesis

roll.mean[i] <- mean(y[i:(i+consec-1)]) # Using mean(), I'm able to generalize.

Upvotes: 2

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