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Commit 6bd8af5

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Incorporated @twobitlabs style suggestions; added some comments; fixed bug in Eight Queens problem
1 parent 195f697 commit 6bd8af5

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5 files changed

+35
-33
lines changed

5 files changed

+35
-33
lines changed

‎Classic Computer Science Problems in Swift.playground/Pages/Chapter 2.xcplaygroundpage/Contents.swift

Lines changed: 4 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -265,11 +265,11 @@ func dfs<StateType: Hashable>(initialState: StateType, goalTestFn: (StateType) -
265265

266266
func nodeToPath<StateType>(_ node: Node<StateType>) -> [StateType] {
267267
var path: [StateType] = [node.state]
268-
var currentNode = node.parent
268+
var node = node // local modifiable copy of reference
269269
// work backwards from end to front
270-
while currentNode !=nil {
271-
path.insert(currentNode!.state, at: 0)
272-
currentNode = currentNode!.parent
270+
while letcurrentNode = node.parent {
271+
path.insert(currentNode.state, at: 0)
272+
node = currentNode
273273
}
274274
return path
275275
}

‎Classic Computer Science Problems in Swift.playground/Pages/Chapter 3.xcplaygroundpage/Contents.swift

Lines changed: 20 additions & 20 deletions
Original file line numberDiff line numberDiff line change
@@ -74,21 +74,21 @@ public func backtrackingSearch<V, D>(csp: CSP<V, D>, assignment: Dictionary<V, D
7474
// assignment is complete if it has as many assignments as there are variables
7575
if assignment.count == csp.variables.count { return assignment } // base case
7676

77-
// get an unassigned variable
78-
var variable: V = csp.variables.first! // temporary
79-
for x in csp.variables where assignment[x] == nil {
80-
variable = x
81-
}
77+
// what are the unassigned variables?
78+
let unassigned = csp.variables.lazy.filter({ assignment[0ドル] == nil })
8279

83-
// get the domain of it and try each value in the domain
84-
for value in csp.domains[variable]! {
85-
var localAssignment = assignment
86-
localAssignment[variable] = value
87-
// if the value is consistent with the current assignment we continue
88-
if isConsistent(variable: variable, value: value, assignment: localAssignment, csp: csp) {
89-
// if as we go down the tree we get a complete assignment, return it
90-
if let result = backtrackingSearch(csp: csp, assignment: localAssignment) {
91-
return result
80+
// get the domain of the first unassigned variable
81+
if let variable: V = unassigned.first, let domain = csp.domains[variable] {
82+
// try each value in the domain
83+
for value in domain {
84+
var localAssignment = assignment
85+
localAssignment[variable] = value
86+
// if the value is consistent with the current assignment we continue
87+
if isConsistent(variable: variable, value: value, assignment: localAssignment, csp: csp) {
88+
// if as we go down the tree we get a complete assignment, return it
89+
if let result = backtrackingSearch(csp: csp, assignment: localAssignment) {
90+
return result
91+
}
9292
}
9393
}
9494
}
@@ -97,7 +97,7 @@ public func backtrackingSearch<V, D>(csp: CSP<V, D>, assignment: Dictionary<V, D
9797

9898
/// check if the value assignment is consistent by checking all constraints of the variable
9999
func isConsistent<V, D>(variable: V, value: D, assignment: Dictionary<V, D>, csp: CSP<V,D>) -> Bool {
100-
for constraint in csp.constraints[variable]! {
100+
for constraint in csp.constraints[variable]??[] {
101101
if !constraint.isSatisfied(assignment: assignment) {
102102
return false
103103
}
@@ -160,14 +160,14 @@ final class QueensConstraint: Constraint <Int, Int> {
160160
}
161161

162162
override func isSatisfied(assignment: Dictionary<Int, Int>) -> Bool {
163-
for (q1c, q1r) in assignment {
163+
for (q1c, q1r) in assignment { // q1c = queen 1 column, q1r = queen 1 row
164164
if (q1c >= vars.count) {
165165
break
166166
}
167-
for q2c in (q1c + 1)..<vars.count {
168-
if let q2r = assignment[q2c] {
169-
if q1r == q2r { return false }
170-
if abs(q1r - q2r) == abs(q1c - q2c) { return false }
167+
for q2c in (q1c + 1)...vars.count { // queen 2 column
168+
if let q2r = assignment[q2c] { // queen 2 row
169+
if q1r == q2r { return false } // rows same?
170+
if abs(q1r - q2r) == abs(q1c - q2c) { return false } // same diagonal?
171171
}
172172
}
173173
}

‎Classic Computer Science Problems in Swift.playground/Pages/Chapter 4.xcplaygroundpage/Contents.swift

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -52,7 +52,7 @@ extension Graph {
5252
if let i = vertices.index(of: vertex) {
5353
return i
5454
}
55-
return nil;
55+
return nil
5656
}
5757

5858
/// Find all of the neighbors of a vertex at a given index.
@@ -339,11 +339,11 @@ open class WeightedGraph<V: Equatable & Hashable, W: Comparable & Summable>: Gra
339339
/// - parameter index: The index for the vertex to find the neighbors of.
340340
/// - returns: An array of tuples including the vertices as the first element and the weights as the second element.
341341
public func neighborsForIndexWithWeights(_ index: Int) -> [(V, W)] {
342-
var distanceTuples: [(V, W)] = [(V, W)]();
342+
var distanceTuples: [(V, W)] = [(V, W)]()
343343
for edge in edges[index] {
344344
distanceTuples += [(vertices[edge.v], edge.weight)]
345345
}
346-
return distanceTuples;
346+
return distanceTuples
347347
}
348348

349349
/// This is a convenience method that adds a weighted edge.

‎Classic Computer Science Problems in Swift.playground/Pages/Chapter 5.xcplaygroundpage/Contents.swift

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -27,7 +27,7 @@ extension Array {
2727
if count < 2 { return shuffledArray } // already shuffled
2828
for i in (1..<count).reversed() { // count backwards
2929
let position = Int(arc4random_uniform(UInt32(i + 1))) // random to swap
30-
if i != position { // swap with the end, don't bother with selp swaps
30+
if i != position { // swap with the end, don't bother with self swaps
3131
shuffledArray.swapAt(i, position)
3232
}
3333
}

‎Classic Computer Science Problems in Swift.playground/Pages/Chapter 8.xcplaygroundpage/Contents.swift

Lines changed: 7 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -26,16 +26,18 @@ struct Item {
2626
let value: Float
2727
}
2828

29-
// based on sedgewick second edition p 596
29+
// originally based on Algorithms in C by Sedgewick, Second Edition, p 596
3030
func knapsack(items: [Item], maxCapacity: Int) -> [Item] {
3131
//build up dynamic programming table
3232
var table: [[Float]] = [[Float]](repeating: [Float](repeating: 0.0, count: maxCapacity + 1), count: items.count + 1) //initialize table - overshooting in size
33-
for iin 1...items.count {
33+
for (i, item)in items.enumerated() {
3434
for capacity in 1...maxCapacity {
35-
if capacity - items[i - 1].weight >= 0 { // still room in knapsack
36-
table[i][capacity] = max(table[i - 1][capacity - items[i - 1].weight] + items[i - 1].value, table[i - 1][capacity]) // only take if more valuable than previous combo
35+
let previousItemsValue = table[i][capacity]
36+
if capacity >= item.weight { // item fits in knapsack
37+
let valueFreeingWeightForItem = table[i][capacity - item.weight]
38+
table[i + 1][capacity] = max(valueFreeingWeightForItem + item.value, previousItemsValue) // only take if more valuable than previous combo
3739
} else { // no room for this item
38-
table[i][capacity]=table[i -1][capacity] //use prior combo
40+
table[i+1][capacity]= previousItemsValue //use prior combo
3941
}
4042
}
4143
}

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