Vector of cumulative sums in R

Question

I'm trying to get a vector of cumulative sums, that is, I have:

    # 500 Samples from the U(0,1) Distribution
U<-runif(500,0,1)

# Empty Vector of length 500
F<-rep(0,500)

# Fill the vector with f(U(k))
for ( i in 1:500 ){
  F[i] <- sqrt(1-U[i]^2)
}

# Another Empty Vector of length 500
I<-rep(0,500)

# Fill the second empty vector with the sums of F
for ( i in 1:500 ){
  I[i]<-cumsum(F[1]:F[i])
}

The last line of code is the problem, I want 'I' to be a vector such that I[1] = F[1], I[n] = F[1] + F[2] +.....+ F[n]. The cumsum function doesn't work for this for some reason. What is wrong with trying to do it like this ?

score 9 · Accepted Answer · answered May 24 '15 at 12:33

9

Please correct me if I'm misunderstanding, but I believe you simply want this:

I <- cumsum(sqrt(1 - U^2))

It is unclear why you want to use for loops.

answered May 24 '15 at 12:33

Roland

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oh.. I thought it wouldn't apply it to each element. I thought the for loop would ensure each element was transformed... seems silly now – WeakLearner May 24 '15 at 12:36
What if I wanted each element of I to be the average up to that point? ie I[3] = (F[1] + F[2] + F[3]) /3, must I use a for loop then ? – WeakLearner May 24 '15 at 12:38
2

No, just do `I/(seq_along(I))`. – Roland May 24 '15 at 12:41

Vector of cumulative sums in R

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