Added differentiation for expressions

src/expressions.js

@@ -244,6 +244,32 @@ Expression.prototype.summation = function(variable, lower, upper, simplify) {
 	return newExpr;
 };
 
+Expression.prototype.diff = function(variable) {
+    var expr = this.copy();
+    expr.constants = [];
+    epxr = expr.simplify();
+
+    for (var i = 0; i < expr.terms.length; i++) {
+        var thisTerm = expr.terms[i];
+        var termIncludesVar = false;
+        for (var j = 0; j < thisTerm.variables.length; j++) {
+            var thisVar = thisTerm.variables[j];
+            if (thisVar.variable === variable) {
+                thisTerm.coefficients.push(new Fraction(thisVar.degree, 1));
+                thisVar.degree--;
+                if (thisVar.degree === 0 && thisTerm.variables.length === 1) {
+                    expr.constants.push(new Fraction(1, 1));
+                    expr.terms.splice(i, 1);
+                }
+            }
+        }
+    }
+
+    expr = expr.simplify();
+
+    return expr;
+}
+
 Expression.prototype.toString = function(options) {
     var str = "";
 

test/expression-spec.js

@@ -859,3 +859,45 @@ describe("Expression summation", function() {
         expect(answer.toString()).toEqual("4y + 30");
     });
 });
+
+describe("Differentiation of expression", function() {
+    it("should use power rule", function () {
+        var exp = new Expression("x").pow(2);
+
+        var answer = exp.diff("x");
+
+        expect(answer.toString()).toEqual("2x");
+    });
+
+    it("should turn a single power variable of the chosen variable into 1", function () {
+        var exp = algebra.parse("x+x^2");
+
+        var answer = exp.diff("x");
+
+        expect(answer.toString()).toEqual("2x + 1");
+    });
+
+    it("should ignore other variables", function () {
+        var exp = algebra.parse("y+x^2");
+
+        var answer = exp.diff("x");
+
+        expect(answer.toString()).toEqual("2x + y");
+    });
+
+    it("should work for other varibles", function () {
+        var exp = algebra.parse("y+x^2");
+
+        var answer = exp.diff("y");
+
+        expect(answer.toString()).toEqual("x^2 + 1");
+    });
+
+    it("should work for products of main and other variables", function () {
+        var exp = algebra.parse("y*x");
+
+        var answer = exp.diff("x");
+
+        expect(answer.toString()).toEqual("y");
+    });
+});