// guide[] graph(picture pic=currentpicture, 
//               real[] x, real[] y,
//               bool3[] cond,
//               interpolate join=operator --);
//
// Returns a graph using the scaling information for picture pic of the 
// elements of the arrays (x,y), optionally restricted to those indices 
// for which the elements of the boolean array cond are true, using the 
// given interpolation type.

import graph;
size(7.5cm,0);

real[] x={ -3 , -2 ,  0 ,  2 ,  3 },
       y={  4 ,  3 , -2 ,  4 ,  3 };
                              
real a=2.5;
// TROIS SYNTAXES POSSIBLES POUR ECRIRE LA CONDITION :
// SYNTAXE 1
guide[] courbe=graph(x,y,y>a,join=operator ..);

// SYNTAXE 2
//     bool3[] condition=y>a;               
//     guide[] courbe=graph(x,y,condition,join=operator ..);

// SYNTAXE 3
//     bool3[] condition(real[] x, real[] y){
//         bool3[] cond=y>a;
//         return cond;
//     }                               
//     guide[] courbe=graph(x,y,condition(x,y),join=operator ..);

dot(pairs(x,y),5bp+green);
dot(courbe,3bp+blue);
draw(courbe,red);
draw((-3,a)--(3,a),dashed);

xaxis("$x$",Ticks("%",end=false),Arrow());
yaxis("$y$",Ticks("%",end=false),Arrow());

// Thanks to gk-v and jcbowman for their help.