Then you need to switch resistors to get your different currents.
No he wants to programmatically control the size of current produced by a constant current generator.
Nothing to do with switching resistors.
For a constant current supply project see this:-
Look at the first diagram. This has a pot that is adjusting the current by changing the voltage on pin 2 of the op amp.
What you need to do is to remove the pot and get your programmable variable voltage onto this pin.
Note it will have to go up to the 35V (or whatever ) supply. The simplest way to do that is to put a PWM signal through a transistor pulling up to this voltage. Then filter it with an RC filter before applying it to this pin.
However with only 8 bits from a PWM output the resolution might be too coarse for your needs.