How do I get Xcode 4.1 code completion to work automatically?

I just 开发者_如何学运维bought a MBP and trying out Xcode. I know code completion works fine when pressing 'Escape', but how can it work automatically with Xcode 4.1? For example, pressing down '.' in C++?

I read several posts on this. Some suggest going to preferences and turning it on, but I didn't see any option. Can you please provide a list of steps?

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In the preferences > Text editing > Editing > Code completion > Suggest completion as I type.