Constraint help: proc optmodel

[SAScowboy]

I need help adding a constraint to the below model that restricts the sum of "buy" for even numbered adwords to be at most 10.

proc optmodel;
set ADWORDS = 1..100;
num cost_per_click{ADWORDS} = ranuni(123);
num clicks{ADWORDS} = ranuni(567);
num conversions{ADWORDS} = ranuni(789);
num revperconversion=10;
var buy{ADWORDS} >=0;
max effectiveness =sum{a in ADWORDS}((revperconversion*conversions[a])-(clicks[a]*cost_per_click[a]))*buy[a];
con buyAtMost50: sum{a in ADWORDS}buy[a]<=50;
solve;
Quit;

[RobPratt · Accepted solution]

Here are two ways:

con buyAtMost10even: sum {a in ADWORDS: mod(a,2) = 0} buy[a] <= 10;

con buyAtMost10even: sum {a in 2..100 by 2} buy[a] <= 10;

[SAScowboy]

Thank you so much. I’m still learning.