From d83b2606c047966977fe28edceed33ef2d2a6a7e Mon Sep 17 00:00:00 2001
From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Sun, 15 May 2011 23:03:41 +0530
Subject: [PATCH] Added a bunch of solutions to the exercise problems in the
 concurrency section.

---
 src/sicp/ex3_39.rkt | 11 +++++++++++
 src/sicp/ex3_40.rkt | 41 +++++++++++++++++++++++++++++++++++++++++
 src/sicp/ex3_41.rkt | 11 +++++++++++
 src/sicp/ex3_42.rkt |  7 +++++++
 4 files changed, 70 insertions(+)
 create mode 100644 src/sicp/ex3_39.rkt
 create mode 100644 src/sicp/ex3_40.rkt
 create mode 100644 src/sicp/ex3_41.rkt
 create mode 100644 src/sicp/ex3_42.rkt

diff --git a/src/sicp/ex3_39.rkt b/src/sicp/ex3_39.rkt
new file mode 100644
index 0000000..47ee525
--- /dev/null
+++ b/src/sicp/ex3_39.rkt
@@ -0,0 +1,11 @@
+#lang racket
+
+#|
+
+Only the below three cases will remain:
+
+101: 	P1 sets x to 100 and then P2 increments x to 101.
+121: 	P2 increments x to 11 and then P1 sets x to x times x (121).
+100: 	P1 accesses x (twice), then P2 sets x to 11, then P1 sets x.
+
+|#
\ No newline at end of file
diff --git a/src/sicp/ex3_40.rkt b/src/sicp/ex3_40.rkt
new file mode 100644
index 0000000..9329db8
--- /dev/null
+++ b/src/sicp/ex3_40.rkt
@@ -0,0 +1,41 @@
+#lang racket
+
+#|
+
+(define x 10)
+
+(parallel-execute (lambda () (set! x (* x x)))
+                  (lambda () (set! x (* x x x))))
+
+list all possible values of x.
+
+|#
+
+#|
+
+1. 100
+2. 1000
+3. (expt 100 3) = 1000000
+4. (expt (expt 10 3) 2) = 1000000
+5. (* 10 (* 10 10 10)) = 10000
+6. (* 10 (* 10 10) (* 10 10)) = 100000
+7. (* 10 10 (* 10 10)) = 10000
+
+|#
+
+#|
+
+Which of these possibilities remain if we instead use serialized procedures:
+
+(define x 10)
+
+(define s (make-serializer))
+
+(parallel-execute (s (lambda () (set! x (* x x))))
+                  (s (lambda () (set! x (* x x x)))))
+
+ans:
+
+=> 1000000
+
+|#
\ No newline at end of file
diff --git a/src/sicp/ex3_41.rkt b/src/sicp/ex3_41.rkt
new file mode 100644
index 0000000..a20f95e
--- /dev/null
+++ b/src/sicp/ex3_41.rkt
@@ -0,0 +1,11 @@
+#lang racket
+
+#|
+
+I don't agree. 'balance' is only read and for a particular account, it is 
+not usually read while another transaction is in progress. Further, even if
+it is read concurrently with a transaction (deposit or withdrawal), one will
+get either the value before the transaction or the one after. Both are 
+acceptable.
+
+|#
\ No newline at end of file
diff --git a/src/sicp/ex3_42.rkt b/src/sicp/ex3_42.rkt
new file mode 100644
index 0000000..88ba9f5
--- /dev/null
+++ b/src/sicp/ex3_42.rkt
@@ -0,0 +1,7 @@
+#lang racket
+
+#|
+
+Yes, it is safe.
+
+|#
\ No newline at end of file
-- 
2.37.2