[R] Creating an new variable -or my failure to understand an IF() statement

[John Kane]

I am just starting with R and I am having a stupid 
problem understanding  an if else statement because I
am thinking in terms of something like SAS or Fortran.
 

I am completely misunderstanding what I am reading in
the Intro to R,  the R-Language Definitions and the
Help Mailing List.  I have read references to if not a
vector and ifelse vectored and am even more confused.

Problem :   I want to create some new variables to use
in a data.frame.  The actual data is read into
data.frame (mixed string and numeric data) using a
read.csv() command.

Testdata is:
 a <- c("A", "B","C","D","E")
  Y1 <- c(2, 2 , 400, 500, 600)
  Y2 <- c(2, 4, 4 , 600, 700)
  Y3 <- c(5, 4, 1, 1, 200)
  Y4 <- c(5, 5, 1, 3,  5)

  MyData <- data.frame(a,Y1,Y2,Y3, Y4)

 MyData
 # Results are:
  a  Y1  Y2  Y3 Y4
1 A   2   2   5  5
2 B   2   4   4  5
3 C 400   4   1  1
4 D 500 600   1  3
5 E 600 700 200  5


What I want to do is to add a variable to the
data.frame so that I have:

   a  Y1  Y2   Y3 Y4   P1
1 A    2   2   5   5    4
2 B    2   4   4   5    8
3 C  400   4   1   1    2
4 D  500 600   1   3    4
5 E  600 700 200   5    NA

However when I try this :

 if (a=="A") (P1 <- Y1+ Y2) else
   if (a=="B") (P1 <- Y2+Y3) else
     if (a=="C" ) (P1 <- Y3+ Y4) else
       if (a=="D" (P1 <-  Y4) else
        if (a=="D") (P1<- NA)

Error: syntax error in:
"     if (a=="C" ) (P1 <- Y3+ Y4) else
       if (a=="D" (P1 <-  Y4) else"
>         if (a=="D") (P1<- NA)
Warning message:
the condition has length > 1 and only the first
element will be used in: if (a == "D") (P1 <- NA) 
 
Can anyone help me a) get around it and b) understand
what R is really doing.?

Thanks

John Kane, Kingston ON  Canada