I'm trying to teach myself Swift via the Stanford iTunes U course (currently working on the calculator app), but I just ran into a super weird issue that I've never seen before and can't figure out how to solve: one of my return statements (and only the return statement itself) is being skipped during runtime.
func evaluateOps(ops: [Op]) -> (result: Double?, remainingOps: [Op]) {
if !ops.isEmpty {
var remainingOps = ops
let op = remainingOps.removeLast()
switch op {
case ...
case ...
case .BinaryOperation(_, let operation):
// check that there are 2 operands before it
if remainingOps.count == 2 {
let op1Evaluation = evaluateOps(remainingOps)
if let operand1 = op1Evaluation.result {
let op2Evaluation = evaluateOps(op1Evaluation.remainingOps)
if let operand2 = op2Evaluation.result {
// PROBLEM AREA...
let x = (operation(operand1, operand2), op2Evaluation.remainingOps)
println("results: \(x.0) remainder: \(x.1)")
return (x.0, x.1) // skipped during runtime...
}
}
}
else { ... }
}
}
println("returning nil")
return (nil, ops)
}
// troublesome method is called here...
let (result, remainder) = evaluateOps(opStack)
println("\(opStrings) = \(result) with \(remainderStrings) leftover")
Everything works so that, if I tried to calculate 5*3 for example, the console would read:
results: 15.0 remainder: []
returning nil
[5.0, ×, 3.0] = nil with [5.0, ×, 3.0] leftover
I think the problem might have something to do with the fact that, in the above code, if I tried to simply return x, I get a compile error that reads Cannot express tuple conversion '(Double, [CalculatorModel.Op])' to '(result: Double?, remainingOps: [CalculatorModel.Op])'. I also have no idea what to do with this.
In my researching the problem, I've discovered the downcasting keyword as (see altered code below), which removed the compile error when returning x rather than (x.0, x.1) but results in the same console result (except that now it says results: Optional(15.0) remainder: []):
let x = (operation(operand1, operand2) as Double?, op2Evaluation.remainingOps as [Op])
println("results: \(x.0) remainder: \(x.1)")
return x // no error but same result as return (x.0, x.1)
I've also tried sticking a println(getClassName(x.0!)) just before the return statement to make sure I even had a double, and it spit out __NSCFNumber... which I also researched, and found to be highly underdocumented (or at least not documented well enough to figure out how that's affecting me or how I can fix it!)
Although as you can see, it should be a double...
enum Op {
case Operand(Double)
case UnaryOperation(String, Double -> Double)
case BinaryOperation(String, (Double, Double) -> Double)
var description: String {
get {
switch self {
case .Operand(let operand):
return "\(operand)"
case .UnaryOperation(let symbol, _):
return "\(symbol)"
case .BinaryOperation(let symbol, _):
return "\(symbol)"
}
}
}
}
As you can see, everything's working perfectly fine as it's performing the calculation and everything...until it gets to that little troublesome area. I've searched StackOverflow and did a bit of Googling, but this seems to be a rather uncommon problem... Any help is appreciated! Let me know if you need any further information. Thanks!
EDIT:
getClassName(_:) is defined below (found it online somewhere...):
func getClassName(obj : AnyObject) -> String
{
let objectClass : AnyClass! = object_getClass(obj)
let className = objectClass.description()
return className
}
Related
So, need you help, I have loops, where I'd like to find first positive element and it will be text of label and exit from all loops, but every time I get last element:
for j in getArrayOfAllTimes[i].timeForGetDifference()
{
switch j - timeNow() {
case let x where x > 0:
nextTimeLabel.text = String(j - timeNow())
break
default:
break
}
}
How get first element > 0?
The first(where:) method of Array
Instead of explicitly breaking out of a loop when a first element that fulfills som predicate is found, you could simply make use of the Array method first(where:).
Since you haven't provided us with a minimal, complete and verifiable example (which you should) we'll construct such an example:
/* Example setup */
struct Foo: CustomStringConvertible {
private let bar: Int
init(_ bar: Int) { self.bar = bar }
func timeForGetDifference() -> Int {
return bar
}
var description: String {
return String(bar)
}
}
func timeNow() -> Int { return 10 }
let getArrayOfAllTimes = [Foo(6), Foo(2), Foo(9), Foo(4), Foo(11), Foo(3), Foo(13)]
// nextTimeLabel: some UILabel
For the example as per above, we could set the text property of the nextTimeLabel as follows, using first(where:) to find the first element fulfilling our predicate, given that it exists (otherwise; will return nil in which case we will not enter the optional binding block below).
if let firstNonNegativeFoo = getArrayOfAllTimes
.first(where: { $0.timeForGetDifference() - timeNow() > 0 }) {
nextTimeLabel.text = String(describing: firstNonNegativeFoo) // 11
}
As to why your own approach does not work as intended: a break statement within a case of a switch statement will simply end the execution of the switch statement (not the loop which is one level above the switch statement.
From the Language Reference - Statements:
Break Statement
A break statement ends program execution of a loop, an if statement,
or a switch statement.
In you case, you've added the break statements as the last statements of each case: here, particularly, the break has truly no effect (since the switch statement would break out anyway, after exiting the case which it entered).
for i in 1...3 {
switch i {
case is Int: print(i); break // redundant 'break'
case _: ()
}
} // 1 2 3
// ... the same
for i in 1...3 {
switch i {
case is Int: print(i)
case _: ()
}
} // 1 2 3
for j in getArrayOfAllTimes[i].timeForGetDifference() {
bool a = NO;
switch j - timeNow() {
case let x where x > 0:
a = YES
nextTimeLabel.text = String(j - timeNow())
break
default:
break
}
if a == YES {
break;
}
}
I'm currently trying to complete the Swift Course on iTunes U and we are building a calculator. I'm having trouble understanding part of the code.
I added the code below that I thought was relevant from the file.
Here is what confuses me: why does operation(operand) compute the value for the UnaryOperation (i.e. the square root)? I see that when the CalculatorBrain class is called the dictionary is initialized, but when I print the dictionary out I just get something that looks like this: [✕: ✕, -: -, +: +, ⌹: ⌹, √: √]. So where/when does the program compute the square root when I click on the square root button?
Class CalculatorBrain
{
private enum Op: Printable
{
case Operand(Double)
case UnaryOperation(String, Double -> Double)
case BinaryOperation(String, (Double, Double) -> Double)
var description: String {
get {
switch self {
case .Operand(let operand):
return "\(operand)"
case .UnaryOperation(let symbol, _):
return symbol
case .BinaryOperation(let symbol, _):
return symbol
}
}
}
}
private var opStack = [Op]()
private var knownOps = [String: Op]()
init() {
func learnOp(op: Op) {
knownOps[op.description] = op
}
learnOp(Op.BinaryOperation("✕", *))
learnOp(Op.BinaryOperation("⌹") { $1 / $0 })
learnOp(Op.BinaryOperation("+", +))
learnOp(Op.BinaryOperation("-") { $0 - $1 })
learnOp(Op.UnaryOperation ("√", sqrt))
}
private func evaluate(ops: [Op]) -> (result: Double?, remainingOps: [Op])
{
if !ops.isEmpty {
var remainingOps = ops
let op = remainingOps.removeLast()
switch op {
case .Operand(let operand):
return (operand, remainingOps)
case .UnaryOperation(_, let operation):
let operandEvaluation = evaluate(remainingOps)
if let operand = operandEvaluation.result {
**return (operation(operand), operandEvaluation.remainingOps)**
}
// case.BinaryOperation(.....)
}
}
return (nil, ops)
}
func evaluate() -> Double? {
let (result, remainder) = evaluate(opStack)
return result
}
func pushOperand(operand: Double) -> Double? {
opStack.append(Op.Operand(operand))
return evaluate()
}
func performOperation(symbol: String) -> Double? {
if let operation = knownOps[symbol] {
opStack.append(operation)
}
return evaluate()
}
}
The Op enum implements the Printable protocol, which means it has a description: String property. When you print the Dictionary, you are sending [String : Op] to the println function which then tries to print the Op using its description.
The reason the description of the operators is the same as its key in the Dictionary is because the learnOp(op: Op) function sets the key to be op.description (knownOps[op.description] = op)
To see the effects of this, you could add a new operator learnOp(Op.UnaryOperation ("#", sqrt)) which will be printed as #:# inside of the knownOps Dictionary. (And if you add a new button for the # operator, it will also perform the square root operation)
Since the calculator is stack based, the operands get pushed on, then the operations. When evaluate() gets called, it calls evaluate(opStack) passing the entire stack through.
evaluate(ops: [Op]) then takes the to item off of the stack and evaluates the function after having calculated the operands.
As an example, lets say you want to calucalte sqrt(4 + 5).
You would push the items onto the stack, and it would look like: [ 4, 5, +, sqrt ]
Then evaluate(ops: [Op]) sees the sqrt and evaluates the operand with a recursive call. That call then evaluates + with two more recursive calls which return 5 and 4.
The tree of calls would look like this:
ops: [4, 5, +, sqrt] // Returns sqrt(9) = 3
|
ops: [4, 5, +] // Returns 4 + 5 = 9
____|_____
| |
ops: [4, 5] ops: [4]
return 5 return 4
I strongly recommend you put a breakpoint on the evaluate() -> Double? function and step through the program to see where it goes with different operands and operations.
learnOp(Op.UnaryOperation ("√", sqrt))
sqrt is a built in function, so you're teaching the calculator that "√" means it should perform the sqrt operation.
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Closed 3 years ago.
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I am following along with the Stanford Swift course on iTunes. I am in lesson 3 where the professor writes a recursive function to evaluate a stack of operators and operands.
Here is the code as the professor has typed it:
class CalculatorBrain
{
private enum Op {
case Operand(Double)
case UnaryOperation(String, Double -> Double)
case BinaryOperation(String, (Double, Double) -> Double)
}
private var opStack = [Op]()
private var knownOps = [String:Op]()
init() {
knownOps["×"] = Op.BinaryOperation("×", *)
knownOps["+"] = Op.BinaryOperation("+", +)
knownOps["-"] = Op.BinaryOperation("-") { $1 - $0 }
knownOps["÷"] = Op.BinaryOperation("÷") { $1 / $0 }
knownOps["√"] = Op.UnaryOperation("√", sqrt)
}
func pushOperand(operand: Double) {
opStack.append(Op.Operand(operand))
}
func performOperation(symbol: String) {
if let operation = knownOps[symbol] {
opStack.append(operation)
}
}
private func evaluate(ops: [Op]) -> (result: Double?, remainingOps: [Op])
{
var remainingOps = ops
if !remainingOps.isEmpty {
let op = remainingOps.removeLast()
switch op {
case .Operand(let operand):
return (operand, remainingOps)
case .UnaryOperation(_, let operation):
let operandEvaluation = evaluate(remainingOps)
if let operand = operandEvaluation.result {
return (operation(operand), operandEvaluation.remainingOps)
}
case .BinaryOperation(_, let operation):
// code snipped
}
}
return (nil, ops)
}
}
I get a compiler error but he doesn't.
The error I get is on the line let operandEvaluation = evaluate(remainingOps). I get the error "Use of local variable 'evaluate' before its declaration"
Does anyone know why I'm getting this error? It's supposed to be a recursive call to the function evaluate, but instead the compiler thinks evaluate is a local variable.
I am using Xcode 6.4 on OS X 10.10.4
Thanks!
I got this error, too.
However, I fixed it by adding my bracket to the if clause several lines below that error.
You're missing the } to close:
if let operand = operandEvaluation.result {
And the the switch is not exhaustive, you need to add the missing case .BinaryOperation.
There was a bracketing typo after all. Sorry, and thanks for the assistance.
The API response I am working with returns a total amount. It supports multiple different currencies, so sometimes the currency symbol will be at the front (ex. $20.00) or at the end (ex. 20.00€). I am doing a check to see if the first char of the value is an int. In this specific case, the value "20.00€" is being returned. firstChar is "2" :
DOES NOT WORK:
let firstNumOpt: Int? = String(firstChar).toInt()
if let num = firstNumOpt { //20.00€
NSLog("Total: \(total)")
}
WORKS:
if let num = String(firstChar).toInt() { //20.00€
NSLog("Total: \(total)")
}
Can someone explain why the first code block does not work? Both ways seem identical to me.
Some debug info:
(lldb) po firstNumOpt
2
{
value = 2
}
(lldb) po num
411432864
Note: This isn't really an answer, but I needed to post code to describe what was working.
I'm not sure that your error is in the code that you've posted. Here is some code that I just ran in an empty playground:
func test(str: String) {
let firstNumOpt: Int? = String(str[str.startIndex]).toInt()
if let anum = firstNumOpt {
print("First worked on \(str)")
}
if let bnum = String(str[str.startIndex]).toInt() {
print("Second worked on \(str)")
}
}
test("20.00€") // prints "First worked on 20.00€" and "Second worked on 20.00€"
test("$20.00") // doesn't print anything
Im trying to ask for some values from a variable.
The variable is going to have the description of the weather and i want to ask for specific words in order to show different images (like a sun, rain or so)
The thing is i have code like this:
if self.descriptionWeather.description.rangeOfString("Clear") != nil
{
self.imageWeather.image = self.soleadoImage
}
if self.descriptionWeather.description.rangeOfString("rain") != nil
{
self.imageWeather.image = self.soleadoImage
}
if self.descriptionWeather.description.rangeOfString("broken clouds") != nil
{
self.imageWeather.image = self.nubladoImage
}
Because when i tried to add an "OR" condition xcode gives me some weird errors.
Is it possible to do a swich sentence with that? Or anyone knows how to do add an OR condition to the if clause?
I had a similar problem today and realized this question hasn't been updated since Swift 1! Here's how I solved it in Swift 4:
switch self.descriptionWeather.description {
case let str where str.contains("Clear"):
print("clear")
case let str where str.contains("rain"):
print("rain")
case let str where str.contains("broken clouds"):
print("broken clouds")
default:
break
}
Swift 5 Solution
func weatherImage(for identifier: String) -> UIImage? {
switch identifier {
case _ where identifier.contains("Clear"),
_ where identifier.contains("rain"):
return self.soleadoImage
case _ where identifier.contains("broken clouds"):
return self.nubladoImage
default: return nil
}
}
You can do this with a switch statement using value binding and a where clause. But convert the string to lowercase first!
var desc = "Going to be clear and bright tomorrow"
switch desc.lowercaseString as NSString {
case let x where x.rangeOfString("clear").length != 0:
println("clear")
case let x where x.rangeOfString("cloudy").length != 0:
println("cloudy")
default:
println("no match")
}
// prints "clear"
Swift language has two kinds of OR operators - the bitwise ones | (single vertical line), and the logical ones || (double vertical line). In this situation you need a logical OR:
if self.descriptionWeather.description.rangeOfString("Clear") != nil || self.descriptionWeather.description.rangeOfString("clear") != nil {
self.imageWeather.image = self.soleadoImage
}
Unlike Objective-C where you could get away with a bitwise OR in exchange for getting a slightly different run-time semantic, Swift requires a logical OR in the expression above.
If you do this a lot, you can implement a custom ~= operator that defines sub-string matching. It lends itself to this nice syntax:
switch "abcdefghi".substrings {
case "def": // calls `"def" ~= "abcdefghi".substrings`
print("Found substring: def")
case "some other potential substring":
print("Found \"some other potential substring\"")
default: print("No substring matches found")
}
Implementation:
import Foundation
public struct SubstringMatchSource {
private let wrapped: String
public init(wrapping wrapped: String) {
self.wrapped = wrapped
}
public func contains(_ substring: String) -> Bool {
return self.wrapped.contains(substring)
}
public static func ~= (substring: String, source: SubstringMatchSource) -> Bool {
return source.contains(substring)
}
}
extension String {
var substrings: SubstringMatchSource {
return SubstringMatchSource(wrapping: self)
}
}
I'd recommend using a dictionary instead, as a mapping between the substring you're searching for and the corresponding image:
func image(for weatherString: String) -> UIImage? {
let imageMapping = [
"Clear": self.soleadoImage,
"rain": self.soleadoImage,
"broken clouds": self.nubladoImage]
return imageMapping.first { weatherString.contains($0.key) }?.value
}
A dictionary gives you flexibility, adding new mappings is easy to do.
This link also describes overloading operator ~= which is actually used by the switch statement for matching cases to allow you to match regular expressions.