If what you are asking for is to see how many times it crosses the axis when 'accumulating' the values of the top (+1, -1), then the following will do it - this is for 1000 and shows there are 32 crossings of the axis.
> x <- sample(c(-1,1), 1000, TRUE) > plot(cumsum(x), type='l') > # now if you are looking at when it crosses the axis > cumValue <- cumsum(x) > signChanges <- ifelse(cumValue >= 0, 1, -1) > # number of times it crosses the axis > sum(diff(signChanges) != 0) [1] 32 On 10/16/07, azzza <[EMAIL PROTECTED]> wrote: > > Thats what is frustrating me....I've done a lot of reading, but there doesnt > seem to be much info on sign changes..............I dont udnerstand why the > poportion of sign changes is much less than 50%, despite the proportion of > heads being equal to that of tails. > > > Marc Schwartz wrote: > > > > You might want to do some more research on "Bernoulli Trials" and "Fair > > Coins", which would provide some enlightenment on why you would not > > "expect" 500 sign changes in 1,000 tosses and why you should, if the > > coin is "fair", expect to *approach* a 50/50 distribution of heads and > > tails in a large number of tosses, but not actually observe it. > > > > HTH, > > > > Marc Schwartz > > > > > > On Tue, 2007-10-16 at 09:05 -0700, azzza wrote: > >> > >> You are right, I was a bit too vague. I am trying to simulate 1000 coin > >> Tosses. Then to write a code for the number of heads/Tails.....naturally, > >> we > >> would expect the proportion of heads to be 50% so the number of heads > >> would > >> be around 500. Secondly, I'm trying to look at the number of sign changes > >> in > >> 1000 tosses. The example in the book shows that the number of sign > >> changes > >> is WAY less than 50%. so I was trying to look for a code that shows the > >> number of sign changes, which someone pointed out would be to use > >> rle(x).....my concern however is that when the rle(x) is used, the > >> proportion of sign changes is about 50% (around 500), which would be > >> logical > >> to expect, however, real life experiments have shown that the number of > >> sign > >> changes is actually much less than 50%. The other thing I was trying to > >> do > >> is to plot a line graph of number of sign changes versus the number of > >> tosses. Yes, the number of tosses should begin at zero, and at zero coin > >> toss, the sign change is zero. The range of the number of sign changes in > >> the Y-axis should include negative values of Y. So, for toss number zero, > >> the sign change is 0, for the first toss, the sign change may be +3 for > >> instance, for the 50th toss, the sign change may be -5 for instance. The > >> plot shown in the book shows that for most of the tosses, the sign > >> changes > >> were negative, and this would explain how the overall number of sign > >> changes > >> is very low. > >> > >> so basically, i was looking for a code to show the number of sign > >> changes, > >> and a plot of sign changes versus toss number (from toss #0 to toss > >> number > >> 1000). The Y axis (number of sign changes, should include negative values > >> of > >> Y) > >> > >> thanks > >> > >> > >> > >> Daniel Nordlund wrote: > >> > > >> >> -----Original Message----- > >> >> From: [EMAIL PROTECTED] > >> [mailto:[EMAIL PROTECTED] > >> >> On Behalf > >> >> Of azzza > >> >> Sent: Monday, October 15, 2007 6:06 PM > >> >> To: r-help@r-project.org > >> >> Subject: Re: [R] Need some help > >> >> > >> >> > >> >> > >> >> Thanks Jholtman. > >> >> However, the plot didnt come out the way I envisone dit to be. On the > >> Y > >> >> axis, i should have sign changes in 1000 tosses, the range being from > >> >> negative to postitive, and a straight horizontal line across y=0. The > >> >> X-axis should have the toss number, range 0-1000 > >> >> > >> > > >> > You would probably get more prompt and useful help if you would provide > >> a > >> > small, self- contained example of what you wanted (maybe small enough > >> that > >> > you could work it by hand). Also, if you provided some code using the > >> > help that you have already received, that would show that you are > >> trying > >> > to solve the problem yourself and others could help with the specific R > >> > program issues that you are having. For example, it is not clear to me > >> if > >> > you want to count the first run of tosses as 0 or 1 sign change. I > >> will > >> > assume it is zero. So with that assumption, does this get you what you > >> > want? > >> > > >> > n <- 1000 > >> > x <- sample(c(0,1), n, TRUE) > >> > y <- rle(x) > >> > > >> > z <- rep(1:length(y$lengths),y$lengths) > >> > plot(1:n,z-1) > >> > > >> > Hope this is helpful, > >> > > >> > Dan > >> > > >> > Daniel Nordlund > >> > Bothell, WA USA > >> > > >> > ______________________________________________ > >> > 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. > >> > > >> > > >> > > > > ______________________________________________ > > 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. > > > > > > -- > View this message in context: > http://www.nabble.com/Need-some-help-tf4624513.html#a13238567 > 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. > -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem you are trying to solve? ______________________________________________ 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.
