If you have those files installed to a standard libraries folder then you can use either syntax because both cause the standard libraries folders to be searched. However, if you know the file is not in the local folder then using the quotes syntax will perhaps slow compilation a bit as an extra folder is unnecessarily searched. Conversely if you use the angle brackets syntax to try to include a file that is not in the standard libraries search path then it won't work (as hblair discovered).
So if you wanted to be safe you could always use the quotes syntax. This would only be problematic in the rare case where there is a local file matching the included file name that you don't want to include, preferring a file of the same name located in the standard search path. For a beginner that might be a reasonable approach but also a beginner will practically never have a local file include so angle brackets will serve them just as well.
For anyone more experienced, I say tke a few minutes to understand the difference and then always use the most correct syntax. Unfortunately it's very common to see angle brackets used for including local files in Arduino libraries. This works fine when the library is installed normally but if you want to bundle the library with a sketch so you can distribute it as a self-contained package you need to fix all the instances of incorrect use of angle brackets. This is a good example of why it's best to just understand which syntax is correct and use it.
GolamMostafa:
LiquidCrystal_I2C.h : Could not trace the directory!?
If you can include it then it must be somewhere on your computer.
To find any library that has example sketches do this:
- Tools > Examples > open any example from the library you want to find
- Sketch > Show Sketch Folder - this will open the example folder inside of the library folder.
Another way to find it is to enabling verbose output during compilation in File > Preferences and then examining the contents of the console after a compilation of a sketch that includes the library.