friends,
as we have geocoder getfromlocation(locationname,maximumResults) function of google api in android.
i dont see such function in iphone sdk to obtain lat
You can either use the Google geocoder API as desribed in some of the other answers. Alternatively you can use the geocoding services provided by geonames.org.
You can download my ILGeoNames wrapper classes from GitHub. They provide an Objective C API for using the geonames.org search functions. Among other things you can search for cities by name and get their coordinates.