ampl looping, repeat until loop , syntax error

Question

I just started on ampl and tried some easy algorithm basics . Why is something like this repeat until loop not working ?

param x=8 ;
repeat until x=0;  
    param x=x-1;

display x;

test.mod, line 3 (offset 33): syntax error context: repeat until x=0 >>> ; <<<

Answer 1

The syntax for the repeat until loop in AMPL is the following (see, e.g., page 262 of https://ampl.github.io/ampl-book.pdf):

param x;
let x := 8;
repeat until x == 0 {
    display x;
    let x := x-1;
}
display x;

It will produce the following output:

x = 8

x = 7

x = 6

x = 5

x = 4

x = 3

x = 2

x = 1

x = 0

Note that param x = 8; needs to be replaced by param x; let x := 8; since otherwise you would not be able to change its value.