Select which Webcam to use in OpenCV

I have a built in webcam (HP) and an USB webcam. Whenever I use webcams in OpenCV, the built in webcam is used. However I want use the USB webcam. I tried disabling the built in webcam and even uninstalling the drivers, but after doing that, I get a message saying that no webcams were detected, even though the USB webcam is connected. How can I use the USB webcam instead of the built in webcam? I have searched the net top to bottom and have found no fix.开发者_如何学C

Thanks

The command CaptureFromCAM accepts an integer as its only argument which specifies the device number to use. Change this number and it will use a different device.