问在数学表达式中用C的power语法替换^(幂)符号

EN

# helper function which turns getParseData result back to a text expression
recreateExpr <- function(DF,parent=0){
  elements <- DF[DF$parent == parent,]
  s <- ""
  for(i in 1:nrow(elements)){
    element <- elements[i,]
    if(element$terminal)
      s <- paste0(s,element$text)
    else
      s <- paste0(s,recreateExpr(DF,element$id))
  }
  return(s)  
}

expr <- "((2-x+3)^2+(x-5+7)^10)^0.5"

DF <- getParseData(parse(text=expr))[,c('id','parent','token','terminal','text')]

# let's find the parents of all '^' expressions
parentsOfPow <- unique(DF[DF$token == "'^'",'parent'])

# replace all the the 'x^y' expressions with 'pow(x,y)' 
for(p in parentsOfPow){
  idxs <- which(DF$parent == p)
  if(length(idxs) != 3){ stop('expression with '^' is not correct')  }

  idxtok1 <- idxs[1]
  idxtok2 <- idxs[2]
  idxtok3 <- idxs[3]

  # replace '^' token with 'pow'
  DF[idxtok2,c('token','text')] <- c('pow','pow')

  # move 'pow' token as first token in the expression
  tmp <- DF[idxtok1,]
  DF[idxtok1,] <- DF[idxtok2,]
  DF[idxtok2,] <- tmp

  # insert new terminals '(' ')' and ','
  DF <- rbind(
    DF[1:(idxtok2-1),],
    data.frame(id=max(DF$id)+1,parent=p,token=',',terminal=TRUE,text='(',
               stringsAsFactors=FALSE),
    DF[idxtok2,],
    data.frame(id=max(DF$id)+2,parent=p,token=',',terminal=TRUE,text=',',
               stringsAsFactors=FALSE),
    DF[(idxtok2+1):idxtok3,],
    data.frame(id=max(DF$id)+3,parent=p,token=')',terminal=TRUE,text=')',
               stringsAsFactors=FALSE),
    if(idxtok3<nrow(DF)) DF[(idxtok3+1):nrow(DF),] else NULL
  )
}

# print the new expression
recreateExpr(DF)

> [1] "pow((pow((2-x+3),2)+pow((x-5+7),10)),0.5)"