Problem with IF-THEN-ELSE constraint

[Marco Leonessi]

Hi!

I have a problem with a constraint with if-then-else.

First of all, I try to explain what I would like to reach.

Index

s - surgical ward;

i - kind of surgical operation;

t -  planning days{1, ..., 20};

Var

N_int_in[i,s,t] - number of operation of kind i belonging to ward s planned for the day t;

N_int_out[i,s,t+deg[i]] - number of patients that leave the hospital after recovery;

Param

deg[i] - deterministic number of days of recovery for the kind of operation;

I need to know how many patients leaves every day the hospital and, if they leave the hospital after 20-days-planning they are added in another variable: N_paz_in_post_pianificazione[s].

subject to CALCOLA_INT_OUT {s in S, t in T, i in I : uo[i] == s} :

if (t+deg[i] <= 20) then { N_int_in[i,s,t] = N_int_out[i,s,t+deg[i]] }

else { N_paz_in_post_pianificazione[s] = N_paz_in_post_pianificazione[s] + N_int_in[i,s,t] };

My problem is that AMPL inform that there is a syntax error in the equal sign:

syntax error

context:  if (t+deg[i] <= 20) then { N_int_in[i,s,t]  >>> =  <<< N_int_out[i,s,t+deg[i]] }

Where is the problem? I made a mistake in writing the constraint?

Thank you for your support!

[AMPL Google Group]

You are defining if-then-else in the constraint, so you are not assigning value of one variable to another. Instead you are telling AMPL to find an optimal solution that satisfies the given constraints.

Regarding syntax error, you don't need curly braces and you need to break your constraint into two constraints:

subject to CALCOLA_INT_OUT {s in S, t in T, i in I : uo[i] == s} :

if (t+deg[i] <= 20) then N_int_in[i,s,t] = N_int_out[i,s,t+deg[i]] ;

subject to c1 {s in S, t in T, i in I : uo[i] == s} : if (t+deg[i] > 20) then N_paz_in_post_pianificazione[s] = N_paz_in_post_pianificazione[s] + N_int_in[i,s,t];


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[Marco Leonessi]

Thank you very much for your help!