added n <= k condition to combinations function

and also changed x to n in combinations
mramad2 · Jan 6, 2014 · 4d72b3b · 4d72b3b
1 parent 965f423
commit 4d72b3b
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Showing 2 changed files with 14 additions and 10 deletions.
diff --git a/lib/function/probability/combinations.js b/lib/function/probability/combinations.js
@@ -8,29 +8,32 @@ module.exports = function (math) {
       isInteger = util.number.isInteger;
 
   /**
-   * Compute the number of combinations of x items taken k at a time
+   * Compute the number of combinations of n items taken k at a time
    *
-   *     combinations(x, k)
+   *     combinations(n, k)
    *
    * combinations only takes integer arguments
-   * the following condition must be enforced: k <= x
+   * the following condition must be enforced: k <= n
    *
-   * @Param {Number} x
+   * @Param {Number} n
    * @Param {Number} k
    * @return {Number} combinations
    */
-  math.combinations = function combinations (x, k) {
+  math.combinations = function combinations (n, k) {
     var arity = arguments.length;
     if (arity != 2) {
       throw new math.error.ArgumentsError('combinations', arguments.length, 2);
     }
 
-    if (isNumber(x)) {
-      if (!isInteger(x) || x < 0) {
-        throw new TypeError('Positive integer value expected in function combinations');
+    if (isNumber(n)) {
+      if (!isInteger(n) || n < 0) {
+        throw new TypeError('Positive integer value enpected in function combinations');
       }
-      return parseInt(math.factorial(x) / (math.factorial(k) * math.factorial(x-k)));
+      if (k > n) {
+        throw new TypeError('k must be less than or equal to n');
+      }
+      return parseInt(math.factorial(n) / (math.factorial(k) * math.factorial(n-k)));
     }
-    throw new math.error.UnsupportedTypeError('combinations', x);
+    throw new math.error.UnsupportedTypeError('combinations', n);
   };
 };
diff --git a/test/function/probability/combinations.test.js b/test/function/probability/combinations.test.js
@@ -13,6 +13,7 @@ describe('combinations', function() {
 
   it('should not work with non-integer and negative input', function() {
       assert.throws(function() {combinations(0.5, 3)});
+      assert.throws(function() {combinations(3, 5)});
   });
 
   it('should not work with the wrong number or type of arguments', function() {