Issue #64

src/main/java/net/objecthunter/exp4j/operator/Operators.java

@@ -24,8 +24,9 @@ public abstract class Operators {
     private static final int INDEX_MODULO = 5;
     private static final int INDEX_UNARYMINUS = 6;
     private static final int INDEX_UNARYPLUS = 7;
+    private static final int INDEX_FACTORIAL = 8;
 
-    private static final Operator[] builtinOperators = new Operator[8];
+    private static final Operator[] builtinOperators = new Operator[9];
 
     static {
         builtinOperators[INDEX_ADDITION]= new Operator("+", 2, true, Operator.PRECEDENCE_ADDITION) {
@@ -82,6 +83,23 @@ public double apply(final double... args) {
                 return args[0] % args[1];
             }
         };
+        builtinOperators[INDEX_FACTORIAL]= new Operator("!", 1, true, Operator.PRECEDENCE_POWER + 1) {
+            @Override
+            public double apply(double... args) {
+                final int arg = (int) args[0];
+                if ((double) arg != args[0]) {
+                    throw new IllegalArgumentException("Operand for factorial has to be an integer");
+                }
+                if (arg < 0) {
+                    throw new IllegalArgumentException("The operand of the factorial can not be less than zero");
+                }
+                double result = 1;
+                for (int i = 1; i <= arg; i++) {
+                    result *= i;
+                }
+                return result;
+            }
+        };
     }
 
     public static Operator getBuiltinOperator(final char symbol, final int numArguments) {
@@ -106,6 +124,10 @@ public static Operator getBuiltinOperator(final char symbol, final int numArgume
                 return builtinOperators[INDEX_POWER];
             case '%':
                 return builtinOperators[INDEX_MODULO];
+            case '!':
+                if (numArguments != 1) {
+                    return builtinOperators[INDEX_FACTORIAL];
+                }
             default:
                 return null;
         }

src/main/java/net/objecthunter/exp4j/tokenizer/Tokenizer.java

@@ -226,6 +226,8 @@ private Operator getOperator(String symbol) {
 
             }
             op = Operators.getBuiltinOperator(symbol.charAt(0), argc);
+
+            return op;
         }
         return op;
     }

src/test/java/net/objecthunter/exp4j/ExpressionBuilderTest.java

@@ -2720,4 +2720,58 @@ public void testSignum() {
                 .build();
         assertEquals(-1, e.evaluate(), 0d);
     }
+
+    @Test
+    public void testOperatorFactorial() throws Exception {
+        Expression exp = new ExpressionBuilder("3!").build();
+        assertEquals(6, exp.evaluate(), 0);
+        assertTrue(exp.validate().isValid());
+        exp = new ExpressionBuilder("3!!").build();
+        assertEquals(720, exp.evaluate(), 0);
+        assertTrue(exp.validate().isValid());
+        exp = new ExpressionBuilder("4 + 3!").build();
+        assertEquals(10, exp.evaluate(), 0);
+        assertTrue(exp.validate().isValid());
+        exp = new ExpressionBuilder("3! * 2").build();
+        assertEquals(12, exp.evaluate(), 0);
+        assertTrue(exp.validate().isValid());
+        exp = new ExpressionBuilder("2 * 3!").build();
+        assertEquals(12, exp.evaluate(), 0);
+        assertTrue(exp.validate().isValid());
+        exp = new ExpressionBuilder("4 + (3!)").build();
+        assertEquals(10, exp.evaluate(), 0);
+        assertTrue(exp.validate().isValid());
+        exp = new ExpressionBuilder("4 + 3! + 2 * 6").build();
+        assertEquals(22, exp.evaluate(), 0);
+        assertTrue(exp.validate().isValid());
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testOperatorFactorial2() throws Exception {
+        //Issue 64
+        Expression exp = new ExpressionBuilder("!3").build();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testOperatorFactorial3() throws Exception {
+        Expression exp = new ExpressionBuilder("!!3").build();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testOperatorFactorial4() throws Exception {
+        Expression exp = new ExpressionBuilder("1.5!").build();
+        exp.evaluate();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testOperatorFactorial5() throws Exception {
+        Expression exp = new ExpressionBuilder("(-1)!").build();
+        exp.evaluate();
+    }
+
+    @Test
+    public void testFactorialIssue75() {
+        Expression exp = new ExpressionBuilder("3!-2!").build();
+        assertEquals(4, exp.evaluate(), 1e-12);
+    }
  }