how do we make a 180 Degree aiming and shooting?

[UnicornMan]

Hi, I have been trying to make this for a while and found a few tutorials for 360 aim with mouse and tried to modify them to make it 180 aim on the right side of the character. I also tried to modify the 360 degree aiming and shooting to not be following a mouse but instead the up and down arrow keys. I know this is possible but I don't understand how to make it?
Can someone please help me on this?
Thanks in advance!

[UltimateWalrus]

Let's assume your player has an alterable value called AimAngle, and your bullets have alterable values called AimAngle, XOrigin, YOrigin, and Radius.

Pseudocode:

//Aiming
//------
"Up" key held
    --> Add 1 to AimAngle of "Player"
"Down" key held
    --> Sub 1 to AimAngle of "Player"
AimAngle of "Player" is greater than 90
    --> Set AimAngle of "Player" to 90
AimAngle of "Player" is less than -90
    --> Set AimAngle of "player" to -90
Always
    --> Set Angle of "Player" to AimAngle("Player")

//Shooting
//--------
"Space" key pressed
    --> Create "Bullet" at (0,0) from "Player"
    --> Set AimAngle of "Bullet" to AimAngle of "Player"
    --> Set XOrigin of "Bullet" to X("Player")
    --> Set YOrigin of "Bullet" to Y("Player")
Always
    --> Add 10 to Radius of "Bullet"
    --> Set X Position of "Bullet" to XOrigin("Bullet") + Radius("Bullet") * Cos( AimAngle("Bullet") )
    --> Set Y Position of "Bullet" to YOrigin("Bullet") + Radius("Bullet") * Sin( AimAngle("Bullet") )