RandomPrime inside Manipulate gets evaluated multiple times

Question

Manipulate[
m = RandomPrime[d];
{m, d},
{d, {20, 50}, SetterBar}
] 

Basically, I expected the code to generate a random prime m (under the upper bound d) for me to make some graphics about m. However, the piece of code evaluates m multiple times, which is bewildering to me. Probably I need to use Dynamic here, but I have not figured it out.

How can I evaluate the RandomPrime function for only once?

Answer 1

whenever you use Manipulate, use TrackedSymbols to tell it which symbols to track

Manipulate[m = RandomPrime[d];
 {m, d}, {d, {20, 50}, SetterBar},
 TrackedSymbols :> {d}]

Without this, it will track each symbol in its expression and if any changes, it will re-evaluate again automatically until no more changes are detected.