if not conditions in R?

Try:

if(!condition) { do something }

The problem is in how you are defining the condition. It should be

    if(!(x > 0)){ 

instead of

    if(!x > 0){ 

This is because !x converts the input (a numeric) to a logical - which will give TRUE for all values except zero. So:

> fun <- function(x){
+   if (!(x > 0)) {print ("not bigger than zero")}
+ }
> fun(1)
> fun(0)
[1] "not bigger than zero"
> fun(-1)
[1] "not bigger than zero"