macOS Compile is grayed out

I'm using xcode version 3.1.3. Sometimes the Build/Compile option is grayed out and I don't understand why. Sometimes is not though so I know it is in the project properly. For example, if I build then it does compile that file but if I open the file the Compile selection is gray.

Does anyone have a clue as to why this would happen?

I have the .xcodeproj file open. I open the file to be compiled by using the list of files that is in the project. The odd thing is that this most of the time this works but sometimes the compile option is grayed out and apple/K does not work either.

It's back ... all I did was to click on the error message and that took me to the .d file. I made a fix but if I click on Build the Compile option is gray. This is very frustrating. What causes this to happen?

I continue to be haunted by this. Does anyone know why Compile would be grayed out? Note that Preprocess is too.

Never mind ... I found the problem. In order to use Compile and Preprocess you have to have the Groups & Files in focus and highlight the file name in the Groups & Files pane. I was used to Windows where all you need to do is have the file open with the editor (or debugger).