Coercion of items in vector: peril and promise

### Filename: plotMathProblem.R
### Paul Johnson July 7, 2010
### email me

### This is a tail of “coercion” of items in vectors.

### Run this code through and see if you can explain the
### puzzle at the end.

sigma <- 10.0 mu <- 4.0 myx <- seq( mu - 3.5*sigma, mu+ 3.5*sigma, length.out=500) myDensity <- dnorm(myx,mean=mu,sd=sigma) ### xpd needed to allow writing outside strict box of graph ### Need big bottom margin to add several x axes par(xpd=TRUE, ps=10, mar=c(18,2,2,2)) plot(myx, myDensity, type="l", xlab="", ylab="Probability Density ", main="Normal", axes=FALSE) axis(2, pos= mu - 3.6*sigma) axis(1, pos=0) lines(c(myx[1],myx[length(myx)]),c(0,0)) ### closes off axes addInteriorLine <- function(x, m, sd){ for (i in 1:(length(x))){ lines( c(x[i],x[i]), c(0, dnorm(x[i],m=m,sd=sd)), lty= 14, lwd=.2) } } dividers <- c(qnorm(0.025), -1, 0, 1, qnorm(0.975)) addInteriorLine(mu+sigma*dividers, mu,sigma) # bquote gets the value of mu from the environment with .(mu). t1 <- bquote( mu == .(mu)) mtext(t1, 1, at=mu, line=-1) addInteriorLabel <- function(pos1, pos2, m, s){ area <- abs(100*( pnorm(m+pos1*s,m,s)-pnorm(m+pos2*s, m,s))) mid <- m+0.5*(pos1+pos2)*s text(mid, 0.5*dnorm(mid,m,s),label=paste(round(area,2),"%")) } addInteriorLabel(dividers[1],dividers[2], mu, sigma) addInteriorLabel(dividers[2],dividers[3], mu, sigma) addInteriorLabel(dividers[3],dividers[4], mu, sigma) addInteriorLabel(dividers[4],dividers[5], mu, sigma) b1 <- substitute( mu ~ d*sigma, list(d=round(dividers[1],2))) b2 <- substitute( mu ~ d*sigma, list(d=round(dividers[2],2))) b3 <- substitute( mu ) b4 <- substitute( mu + d*sigma, list(d=round(dividers[4],2))) b5 <- substitute( mu + d*sigma, list(d=round(dividers[5],2)) ) ## 1. Fails, labels not ready yet. R needs expressions to do plotmath axis(1, line=4, at=mu+dividers*sigma, labels=c(b1,b2,b3,b4,b5), padj=-1) ## 2. Also fails, but in an interesting way. Puzzle. Why do all ## but b1 display properly. axis(1, line=7, at=mu+dividers*sigma, labels=c(expression(b1),b2,b3,b4,b5), padj=-1) ## 3. Works, probably the recommended way axis(1, line=10, at=mu+dividers*sigma, labels=as.expression(c(b1,b2,b3,b4,b5)), padj=-1) ### Duncan Murdoch explained that as.expression is needed in r-help. ### The reason that "as." is required is difficult, I think. ## 4. Also Works, but hard to understand why. axis(1, line=13, at=mu+dividers*sigma, labels=c(as.expression(b1),b2,b3,b4,b5), padj=-1) ## Why does it work with the as.expression applied only to first ## element? ### Puzzle: Why does 2 fail, and why does 4 work? ### Several r-help members helped me understand this. ### It comes back to this: ### a vector in R has to be a homogeneous collection of items. ### All elements have to be numbers, or characters, or expressions. ### Putting things into a vector with c() signals R ### you want to "coerce" all of the things into the same type. ### What happens if you do ### x <- c("a", 1, 2, 3, 4) ##? ### You don't end up with one character and 4 numbers. You get 5 ### characters. ### > x
### [1] “a” “1” “2” “3” “4”
### Behind the scenes, R applies “as.character”.

### Try #2 sets the first thing as expression(b1), and
### then in the effort to build the vector, R runs “as.expression”
### on b2, b3, b4, b5 automatically. Because “as.expression” is the
### required fix, then we get the correct output for those items.

### Why does Try #4 succeed? Same answer. After the first item
### is seen as an expression, then R coerces the rest with as.expression.

### So the approach in #4 works, but only luckily, and when #2 failed,
### it creates a very hard to debug problem (for me, at least).

### Anyway, now that this “works” I can concentrate on putting this to
### use in another project.

About pauljohn

Paul E. Johnson is a Professor of Political Science at the University of Kansas. He is an avid Linux User, an adequate system administrator and C programmer, and humility is one of his greatest strengths.
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