Code from YouTube video.
This is the example code used in the video Coding Primes by GroundhogGrafix. There are 3 functions that are significant in this code: isprime1, isprime2 and isprime3.
isprime4 was a test that I never put in the video and is probably buggy and shouldn't be used for anything more than testing.
Code was written and compiled using Microsoft Quick C 2.5 under MS-DOS.
Link to the video: https://youtu.be/6m4CRK7pF-4