[R] Correct Syntax for a Loop

[ebashi]

I'll appreciate if some one can help me with the
following loop. This is the logic of the loop,
if we have the following data;
> x.df
    x.dif
 .    .
 .    .
102  0.00
103  0.42
104  0.08
105  0.00
106  0.00
107  0.00
108 -0.16
109 -0.34
110  0.00
111 -0.17
112 -0.33
113  0.00
114  0.00
115  0.00
116  0.33
117  0.17
118  0.00 
 .    .
 .    .
I'm trying to find i's where 
  for (i in 2:length(x.dif))
  if (x.dif[i-1]<=0 and x.dif[i]>0 and x.dif[i+2]>0)
  it would return i+2 to me,
How can I turn this to a right format as a loop.(I
can't figure out the syntax)

Cheers,

Sean