netcdf - How to get value of one variable based on another variable in r? -

for netcdf file while includes precipitation values @ many grid cells. grid cells represented latitude , longitude. how precipitation value @ specific grid, say, latitude , longitude? thanks. example, used code open file:

library('ncdf4') precip.nc = nc_open('obs.pr.nc') 

when typed precip.nc, shows 3 variables:

float longitude_bnds float latitude_bnds float pr 

how open pr @ specific longitude_bnds , latitude_bnds? help.