Re: [R] generating a vector of y_t = \sum_{i = 1}^t (alpha^i * x_{t - i + 1})

[Michael Kao]

Dear Enrico,

Brilliant! Thank you for the improvements, not sure what I was thinking 
using rev. I will test it out to see whether it is fast enough for our 
implementation, but your help has been SIGNIFICANT!!!

Thanks heaps!
Michael

On 27/11/2011 10:43 a.m., Enrico Schumann wrote:
Hi Michael

here are two variations of your loop, and both seem faster than the 
original loop (on my computer).


require("rbenchmark")

## your function
loopRec <- function(x, alpha){
    n <- length(x)
    y <- double(n)
    for(i in 1:n){
        y[i] <- sum(cumprod(rep(alpha, i)) * rev(x[1:i]))
    }
    y
}
loopRec(c(1, 2, 3), 0.5)

loopRec2 <- function(x, alpha){
    n <- length(x)
    y <- numeric(n)
    for(i in seq_len(n)){
        y[i] <- sum(cumprod(rep.int(alpha, i)) * x[i:1])
    }
    y
}
loopRec2(c(1, 2, 3), 0.5)

loopRec3 <- function(x, alpha){
    n <- length(x)
    y <- numeric(n)
    aa <- cumprod(rep.int(alpha, n))
    for(i in seq_len(n)){
        y[i] <- sum(aa[seq_len(i)] * x[i:1])
    }
    y
}
loopRec3(c(1, 2, 3), 0.5)


## Check whether value is correct
all.equal(loopRec(1:1000, 0.5), loopRec2(1:1000, 0.5))
all.equal(loopRec(1:1000, 0.5), loopRec3(1:1000, 0.5))


## benchmark the functions.
benchmark(loopRec(1:1000, 0.5), loopRec2(1:1000, 0.5),
 loopRec3(1:1000, 0.5),
 replications = 50, order = "relative")


... I get
                   test replications elapsed relative user.self sys.self
2 loopRec2(1:1000, 0.5)           50    0.77 1.000000      0.76     0.00
3 loopRec3(1:1000, 0.5)           50    0.86 1.116883      0.85     0.00
1  loopRec(1:1000, 0.5)           50    1.84 2.389610      1.79     0.01


Regards,
Enrico


Am 27.11.2011 01:20, schrieb Michael Kao:
Dear R-help,

I have been trying really hard to generate the following vector given
the data (x) and parameter (alpha) efficiently.

Let y be the output list, the aim is to produce the the following
vector(y) with at least half the time used by the loop example below.

y[1] = alpha * x[1]
y[2] = alpha^2 * x[1] + alpha * x[2]
y[3] = alpha^3 * x[1] + alpha^2 * x[2] + alpha * x[3]
.....

below are the methods I have tried and failed miserably, some are just
totally ridiculous so feel free to have a laugh but would appreciate if
someone can give me a hint. Otherwise I guess I'll have to give RCpp a
try.....


## Bench mark the recursion functions
loopRec <- function(x, alpha){
n <- length(x)
y <- double(n)
for(i in 1:n){
y[i] <- sum(cumprod(rep(alpha, i)) * rev(x[1:i]))
}
y
}

loopRec(c(1, 2, 3), 0.5)

## This is a crazy solution, but worth giving it a try.
charRec <- function(x, alpha){
n <- length(x)
exp.mat <- matrix(rep(x, each = n), nc = n, byrow = TRUE)
up.mat <- matrix(eval(parse(text = paste("c(", paste(paste(paste("rep(0,
", 0:(n - 1), ")", sep = ""),
paste("cumprod(rep(", alpha, ",", n:1, "))", sep = "") , sep = ","),
collapse = ","), ")", sep = ""))), nc = n, byrow = TRUE)
colSums(up.mat * exp.mat)
}
vecRec(c(1, 2, 3), 0.5)

## Sweep is slow, shouldn't use it.
matRec <- function(x, alpha){
n <- length(x)
exp.mat <- matrix(rep(x, each = n), nc = n, byrow = TRUE)
up.mat <- sweep(matrix(cumprod(rep(alpha, n)), nc = n, nr = n,
byrow = TRUE), 1,
c(1, cumprod(rep(1/alpha, n - 1))), FUN = "*")
up.mat[lower.tri(up.mat)] <- 0
colSums(up.mat * exp.mat)
}
matRec(c(1, 2, 3), 0.5)

matRec2 <- function(x, alpha){
n <- length(x)
exp.mat <- matrix(rep(x, each = n), nc = n, byrow = TRUE)
up.mat1 <- matrix(cumprod(rep(alpha, n)), nc = n, nr = n, byrow = TRUE)
up.mat2 <- matrix(c(1, cumprod(rep(1/alpha, n - 1))), nc = n, nr = n)
up.mat <- up.mat1 * up.mat2
up.mat[lower.tri(up.mat)] <- 0
colSums(up.mat * exp.mat)
}

matRec2(c(1, 2, 3), 0.5)

## Check whether value is correct
all.equal(loopRec(1:1000, 0.5), vecRec(1:1000, 0.5))
all.equal(loopRec(1:1000, 0.5), matRec(1:1000, 0.5))
all.equal(loopRec(1:1000, 0.5), matRec2(1:1000, 0.5))

## benchmark the functions.
benchmark(loopRec(1:1000, 0.5), vecRec(1:1000, 0.5), matRec(1:1000, 
0.5),
matRec2(1:1000, 0.5), replications = 50,
order = "relative")

Thank you very much for your help.

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and provide commented, minimal, self-contained, reproducible code.