Question

I want to iterate a loop only for some values so I am using this:

present <- c(3,5,7,8)
for(i in present)
{
   print(i)
}

which gives me

[1] 3
[1] 5
[1] 7
[1] 8

however I need to jump to the next value within the loop, say I dont want 5 to be printed in above example.

I cannot use next since I want it in nested for like this

present <- c(3,5,7,8)
for(i in present)
{
    k <- i
    "Jump to next value of present"
    while(k < "The next value for i should come here")
    {
        k <- k + 1
        print(k)
    }
}

The output would be 3 4 5 6 7 8 but the condition must check value of k if it exceeds next value of i.

Is there anyway to accomplish this?

I’ll take help of C to explain further,

for(i=0; i < 10; i++)
{
   for(k=i;k <= i+1;k++)
   {
       printf("%d", k);
   }
}

The link contains output of above code
http://codepad.org/relkenY3

It is easy in C since next value is in sequence, but here next value is not known, hence the problem.

Answer 1

What you should do is loop through two vectors:

x <- head(present, -1)
# [1] 3 5 7
y <- tail(present, -1)
# [1] 5 7 8

and the function to do that is mapply (have a look at ?mapply). A close translation of your pseudo-code would be:

invisible(mapply(function(x, y) while(x < y) {x <- x + 1; print(x)}, x, y))

but maybe you’ll find this more interesting:

mapply(seq, x + 1, y)