Writing a whole game from scratch by yourself is a big task for all but the simplest of games.
A progression might be for tic-tac-toe
- simple console program (no clrscr() or anything) with input for 2 players
- Add platform-specific screen handling to make it look better
- Add single player with dumb (picks random location) computer AI
- Add average AI (blocks opponents winning move)
- Add good AI (seeks winning moves)
- Add menu option for the above as "difficulty" levels
- Add sound effects
- Add proper graphics (still 2D)
- Add 3D graphics or animations
- Add network play for two remote players
Each step should build on the previous step (a lot of code re-use). You might end up re-writing it over the entire sequence, but each transition should have recognisable re-use. This will test your ability to plan ahead and design accordingly.
Or get yourself onto an open source games project.
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