Implemented select() in custom expressions

tests/TestParser.cpp

@@ -178,6 +178,8 @@ void verifyDerivative(const string& expression, const string& expectedDeriv) {
     verifySameValue(computed, expected, 2.0, 3.0);
     verifySameValue(computed, expected, -2.0, 3.0);
     verifySameValue(computed, expected, 2.0, -3.0);
+    verifySameValue(computed, expected, 0.0, -3.0);
+    verifySameValue(computed, expected, 2.0, 0.0);
 }
 
 /**
@@ -249,6 +251,8 @@ int main() {
         verifyEvaluation("step(x-3)+y*step(x)", 2.0, 3.0, 3.0);
         verifyEvaluation("floor(x)", -2.1, 3.0, -3.0);
         verifyEvaluation("ceil(x)", -2.1, 3.0, -2.0);
+        verifyEvaluation("select(x, 1.0, y)", 0.3, 2.0, 1.0);
+        verifyEvaluation("select(x, 1.0, y)", 0.0, 2.0, 2.0);
         verifyInvalidExpression("1..2");
         verifyInvalidExpression("1*(2+3");
         verifyInvalidExpression("5++4");
@@ -282,6 +286,7 @@ int main() {
         verifyDerivative("max(5, x^2)", "(1-step(5-x^2))*2*x");
         verifyDerivative("abs(3*x)", "step(3*x)*3+(1-step(3*x))*-3");
         verifyDerivative("floor(x)+0.5*x*ceil(x)", "0.5*ceil(x)");
+        verifyDerivative("select(x, x^2, 3*x)", "select(x, 2*x, 3)");
         testCustomFunction("custom(x, y)/2", "x*y");
         testCustomFunction("custom(x^2, 1)+custom(2, y-1)", "2*x^2+4*(y-1)");
         cout << Parser::parse("x*x").optimize() << endl;