Merge branch 'master' into ex-3-23-sol

Author: RichDom2185

xml/chapter2/section5/subsection1.xml

@@ -994,6 +994,54 @@ function install_complex_package() {
     arithmetic package.  This operation should work for ordinary numbers,
     rational numbers, and complex numbers.
     <LABEL NAME="ex:=zero?"/>
+    <SOLUTION>
+      <SNIPPET EVAL="no">
+	<JAVASCRIPT>
+// provided by GitHub user clean99
+
+function is_equal_to_zero(x) {
+    return apply_generic("is_equal", list(x));
+}
+
+function install_javascript_number_package() {
+    // ...
+
+    put("is_equal_to_zero", "javascript_number",
+          x => x === 0);
+
+    // ...
+}
+
+function install_rational_package() {
+    // ...
+
+    function is_equal_to_zero(x) {
+        return numer(x) === 0;
+    }
+
+    put("is_equal_to_zero", "rational",
+           is_equal_to_zero);
+
+    // ...
+}
+
+function install_complex_package() {
+    // ...
+
+    function is_equal_to_zero(z) {
+        return real_part(z) === 0
+               ? imag_part(z) === 0
+               : false;
+    }
+
+    put("is_equal_to_zero", "complex",
+            is_equal_to_zero);
+
+    //...
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <INDEX>generic arithmetic operations<CLOSE/></INDEX>

xml/chapter3/section3/subsection1.xml

@@ -1522,6 +1522,56 @@ display(count_pairs(cycle));
     distinct pairs in any structure.  (Hint: Traverse the structure, maintaining
     an auxiliary data structure that is used to keep track of which pairs have
     already been counted.)
+    <SOLUTION>
+      <SNIPPET>
+	<EXAMPLE>exercise_3_17_solution_example</EXAMPLE>
+	<JAVASCRIPT>
+// solution provided by GitHub user clean99
+	  
+function count_pairs(x) {
+    let counted_pairs = null;
+    function is_counted_pair(current_counted_pairs, x) {
+        return is_null(current_counted_pairs)
+               ? false
+               : head(current_counted_pairs) === x
+               ? true
+               : is_counted_pair(tail(current_counted_pairs), x);
+    }
+    function count(x) {
+        if(! is_pair(x) || is_counted_pair(counted_pairs, x)) {
+            return 0;
+        } else {
+            counted_pairs = pair(x, counted_pairs);
+            return count(head(x)) +
+                   count(tail(x)) +
+                   1;
+        }
+    }
+    return count(x);
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+      <SNIPPET HIDE="yes">
+         <NAME>exercise_3_17_solution_example</NAME>
+         <JAVASCRIPT>
+const three_list = list("a", "b", "c");
+const one = pair("d", "e");
+const two = pair(one, one);
+const four_list = pair(two, "f");
+const seven_list = pair(two, two);
+const cycle = list("g", "h", "i");
+set_tail(tail(tail(cycle)), cycle);
+
+// return 3; return 3; return 3;
+display(count_pairs(three_list));
+display(count_pairs(four_list));
+display(count_pairs(seven_list));
+
+// return 3
+display(count_pairs(cycle));
+	 </JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>
@@ -1542,6 +1592,50 @@ display(count_pairs(cycle));
     would go into an infinite loop. Exercise<SPACE/><REF NAME="ex:make-cycle"/>
     constructed such lists.
     <LABEL NAME="ex:find-cycle"/>
+   <SOLUTION>
+      <SNIPPET>
+	<EXAMPLE>exercise_3_18_solution_example</EXAMPLE>
+	<JAVASCRIPT>
+// solution provided by GitHub user clean99
+
+function contains_cycle(x) {
+    let counted_pairs = null;
+    function is_counted_pair(counted_pairs, x) {
+        return is_null(counted_pairs)
+               ? false
+               : head(counted_pairs) === x
+               ? true
+               : is_counted_pair(tail(counted_pairs), x);
+    }
+    function detect_cycle(x) {
+        if (is_null(x)) {
+            return false;
+        } else if (is_counted_pair(counted_pairs, x)) {
+            return true;
+        } else {
+            counted_pairs = pair(x, counted_pairs);
+            return detect_cycle(tail(x));
+        }
+    }
+    return detect_cycle(x);
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+      <SNIPPET HIDE="yes">
+         <NAME>exercise_3_18_solution_example</NAME>
+         <JAVASCRIPT>
+const three_list = list("a", "b", "c");
+const cycle = list("g", "h", "i");
+set_tail(tail(tail(cycle)), cycle);
+
+// displays false
+display(contains_cycle(three_list));
+
+// displays true
+display(contains_cycle(cycle));
+	 </JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>