Automate backward propagation

1 files changed, 51 insertions, 5 deletions

diff --git a/src/scalar.py b/src/scalar.py
index a67b7ae..c8c7601 100644
--- a/src/scalar.py
+++ b/src/scalar.py
@@ -1,3 +1,5 @@
+import math
+
 class Scalar:
     def __init__(self, data, _children=(), _op='', label='') -> None:
         self.label = label
@@ -7,14 +9,58 @@ class Scalar:
 
         self._prev = set(_children)
         self._op = _op
+        
+        self._backward = lambda: None
     
     def __repr__(self) -> str:
-        return f'Scalar({self.data})'
+        return f'Scalar({self.label}: {self.data})'
 
     def __add__(self, y):
-        result = self.data + y.data
-        return Scalar(result, (self, y), _op='+')
+        result = Scalar(self.data + y.data, (self, y), _op='+')
+
+        def _backward():
+            self.grad = result.grad
+            y.grad = result.grad
+
+        self._backward = _backward
+
+        return result
 
     def __mul__(self, y):
-        result = self.data * y.data
-        return Scalar(result, (self, y), _op='*')
+        result = Scalar(self.data * y.data, (self, y), _op='*')
+
+        def _backward():
+            self.grad = y.data * result.grad
+            y.grad = self.data * result.grad
+
+        self._backward = _backward
+
+        return result
+
+    def tanh(self):
+        x = self.data
+        t = (math.exp(2 * x) - 1) / (math.exp(2 * x) + 1)
+        result = Scalar(t, (self, ), 'tanh')
+
+        def _backward():
+            self.grad = (1 - (t ** 2)) * result.grad
+
+        self._backward = _backward
+
+        return result
+
+    def build_children(self):
+        result = []
+
+        result.append(self)
+        for child in self._prev:
+            result += child.build_children()
+
+        return result
+
+    def backward(self):
+        self.grad = 1.0
+        children = self.build_children()
+
+        for child in children:
+            child._backward()