Isn't c(0, cumsum(diff(theoP)) ) just theoP - theoP[1] ? -- David
-----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of David Winsemius Sent: Monday, March 07, 2011 10:52 AM To: rivercode Cc: r-help@r-project.org Subject: Re: [R] Replace for loop when vector calling itself On Mar 7, 2011, at 11:12 AM, rivercode wrote: > > Hope this clarifies my Q. > > Creating a vector where each element is (except the first which is > 0) is: > the previous element + a calculation from another vector theoP[i] - > theoP[i-1] > > I could not figure out how to do this without a for loop, as the > vector had > to reference itself for the next element...I am missing something > obvious, > but not too sure what. > > d = vector(mode = "numeric", length= len) > d[1] = 0 > if (len>1) for (i in 2:len) { d[i] = d[i-1] + theoP[i] - theoP[i-1] } so why not: c(0, cumsum(diff(theoP)) ) > theoP <- sample(1:10, 15, replace=TRUE); len=length(theoP); d = vector(mode = "numeric", length= len) > d[1] = 0 > if (len>1) for (i in 2:len) { d[i] = d[i-1] + theoP[i] - theoP[i-1] } > > d [1] 0 1 -6 -5 -4 0 1 0 1 -4 3 -5 -5 1 -3 > c(0, cumsum(diff(theoP)) ) [1] 0 1 -6 -5 -4 0 1 0 1 -4 3 -5 -5 1 -3 > all.equal(d, c(0, cumsum(diff(theoP)) ) ) [1] TRUE -- David. > > Thanks, > Chris > >> Hi, >> >> I am missing something obvious. >> >> Need to create vector as: >> >> (0, i-1 + TheoP(i) - TheoP(i-1), repeat....) Where i is the index >> position >> in the vector and i[1] is always 0. > > I think your prototype is not agreeing with the code below. Is "i" > suppose to be the index (as suggested above) or the prior term (as > implied below)? >> >> Found myself having to use a For Loop because I could not get sapply >> working. Any suggestions ? > > Assuming the code below, you construct the first three or four values > by hand I think you will find that the intermediate values of TheoP > will have alternating signs. > > term2 = 2-1 + TheoP(2) - TheoP(1) > term3 = 3-1 + TheoP(3) - (2-1 + TheoP(2) - TheoP(1)) > term4 = 4-1 + TheoP(4) - (3-1 + TheoP(3) - (2-1 + TheoP(2) - > TheoP(1)) ) > > The answer to the first question will determine how you proceed. If > the index is being used then there are two series of cumulative sums > and perhaps you can construct an expression that can be fed to the > cumsum function. > > The diff function is also available and if the index version is > correct, then it might even be as simple as c(0, > ((1:len)-1)+diff(TheoP) ) > > So clarify what is intended. > -- > David. > >> >> delta <- function(x) { >> >> start = index[x] >> end = index[x+1] - 1 >> iTheo = start:end >> len = length(iTheo) >> theoP = as.numeric(TheoBA$Bid[iTheo]) >> d = vector(mode = "numeric", length= len) >> d[1] = 0 >> if (len>1) for (i in 2:len) { d[i] = d[i-1] + theoP[i] - theoP[i-1] } >> return(d) >> } >> >> Thanks, >> Chris >> >> -- > > > > -- > View this message in context: > http://r.789695.n4.nabble.com/Replace-for-loop-when-vector-calling-itself-tp3338383p3339351.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. 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