In swift, in Xcode, I am making a calculator app. The app was all working fine but then I realised that the function to solve maths was not working. I had used the function shown in this question. It can solve most problems but when you involve decimal places:e.g. "3/2" you would expect it to return "1.5". It will return 1.0.
Here is the function I am using:
func mathsSolver(item: String) -> String {
let result = NSExpression(format: item).expressionValue(with: nil, context: nil) as! Double
return "\(result)"
}
You can call this function with:
print(mathsSolver(item: "3/2")) //this will print 1.0
Any Ideas?
You have to pass the double value so that you can get result 1.5
print(mathsSolver(item: "3.0/2.0"))
Above you are trying to pass Int value so that it is not giving you the proper result because integer division will always gives you the answer in integer.
Try:
NSExpression(format: "3.0 / 2.0");
this issue has plenty answears on stackoverflow. Check them out. ^_^
How to stop NSExpression from rounding
NSExpression 1/2
Its very popular problem. In swift wench you try to divide one Int number by another Int its automatically cleans up everything what must be after doth. For example if you want to divide 5/2 instead 2.5 you will get = 2. In you example swift has deleted everything after doth and instead 1.5 you got just 1.
To resolve this issue try to use Double or Float instead of Int and everything will be fine!
Related
just a short question. In Swift it is possible to solve the following code:
var a: String;
a = "\(3*3)";
The arithmetic operation in the string will be solved. But i can´t figure out, why this following variation doesn´t work.
var a: String;
var b: String;
b = "3*3";
a = "\(b)";
In this case the arithmetic operation in var a will not be resolved. Any ideas why and how i can this get to work. Some things would be much more easier if this would work. Thanks for your answers.
In the second case, you are interpolating a string, not an arithmetic expression. In your example, it's a string you chose at compile time, but in general it might be a string from the user, or loaded from a file or over the web. In other words, at runtime b could contain some arbitrary string. The compiler isn't available at runtime to parse an arbitrary string as arithmetic.
If you want to evaluate an arbitrary string as an arithmetic formula at runtime, you can use NSExpression. Here's a very simple example:
let expn = NSExpression(format:"3+3")
println(expn.expressionValueWithObject(nil, context: nil))
// output: 6
You can also use a third-party library like DDMathParser.
Swift 4.2
let expn = "3+3"
print(expn.expressionValue(with: nil, context: nil))
But I also have a solution thats not the most effective way but could be used in some cases if your sure it's only "y+x" and not longer string.
var yNumber: Int!
var xNumber: Int!
let expn: String? = "3+3"
// Here we take to first value in the expn String.
if let firstNumber = expo?.prefix(1), let myInt = Int(firstNumber){
// This will print (Int : 3)
print("Int : \(myInt)")
// I set the value to yNumber
yNumber = myInt
}
// Here we take the last value in the expn string
if let lastNumber = optionalString?.suffix(1), let myInt = Int(lastNumber){
// This will print (Int : 3)
print("Int : \(myInt)")
// I set the value to xNumber
xNumber = myInt
}
// Now you can take the two numbers and add
print(yNumber + xNumber)
// will print (6)
I can't recommend this but it works in some cases
This won't be solved because this is not an arithmetic operation, this is a string:
"3*3"
the same as this
"String"
Everything you put in " it's a string.
The second example lets you construct a new String value from a mix of constants, variables, literals, and expressions:
"\(3*3)"
this is possible because of string interpolation \()
You inserted a string expression which swing convert and create expected result.
You can try to use evaluatePostfixNotationString method from that class.
The whole project is about recognizing math expression from camera image and calculating it after.
I am asking this hesitantly as I know this is probably a dumb question.
I am returning a Realm result and then have gone ahead and tried to cast it to a String as normal (to put in a text label).
However I'm getting an error 'init' has been renamed to 'init(describing:)'.
When I try use the describing method instead, the label prints "Optional" inside it which obviously isn't what I want.
Is there a reason I can't use :
previousTimeLabel.text = String(lastRecord?.time)
I'm sure I've done this before and it's been fine, am I missing something? (lastRecord.time is an Int).
I've checked the answer here about Interpolation Swift String Interpolation displaying optional? and tried changing to something like this :
if let previousRounds = String(lastRecord?.rounds) {
previousRoundsLabel.text = previousRounds
}
but get the same error + Initializer for conditional binding must have Optional type, not 'String'
The issue isn't String(lastRecord?.time) being Optional. The issue is lastRecord being Optional, so you have to unwrap lastRecord, not the return value of String(lastRecord?.time).
if let lastRecord = lastRecord {
previousRoundsLabel.text = "\(lastRecord.time)"
}
To summarize Dávid Pásztor's answer, here's a way you can fix it:
previousTimeLabel.text = String(lastRecord?.time ?? 0)
This may not be the best way for your application. The point Dávid was making is that you need to deal with lastRecord possibly being nil before trying to pass its time Int into the String initializer. So the above is one simple way to do that, if you're ok with having "0" string as your previousTimeLabel's text if there was no lastRecord.
I am a little confused about a conversion error.
I migrated my project form Swift 2.3 to Swift 3.0
func updateCelsiusLabel() {
if let value = celsiusValue {
//This was the original code (that worked but is) failing after migration
//due to: Argument labels do not match any available overloads
celsiusLabel.text = numberFormatter.string(from: NSNumber(value))
//This is my code trying to fix this issue and the project is now compiling
//and everything is fine
celsiusLabel.text = numberFormatter.string(from: value as NSNumber)
}
else { celsiusLabel.text = "???"
}
}
At first I thought that in Swift 3.0 the cast Type(value) was now forbidden, but I checked and I get absolutely no compiler warning. Can somebody tell me what the problem with NSNumber(value) is?
As far as I understand value as NSNumber and NSNumber(value) should be the same thing.
In Swift 3, NSNumber(value) won't work. Let's say that your value is an Int. In that case, you'd need NSNUmber(value: yourIntValue). In Swift 3, you must have the name of the first (and in this case the only) parameter in the function call. So, your usage of
value as NSNumber
works, but
NSNumber(value: yourNumberValue)
works too.
First of all I have taken some assumption here, I have assumed that -
numberFormatter = NSNumberFormatter() // Now it has been renamed to NumberFormatter
celsiusLabel.text I am taking text as optional string just for example you can use label.text for same.
After the above assumption please see below code which will work in Swift 3 -
var celsiusValue:Double?
var numberFormatter = NumberFormatter()
var text:String?
func updateCelsiusLabel() {
if let value = celsiusValue {
//This was the original code (that worked but is) failing after migration due to: Argument labels do not match any available overloads
text = numberFormatter.string(from: NSNumber(value: value))!
}
else {
text = "???"
}
}
Hope it help feel free to leave comment in case you have any doubt.
I can't figure out why this 'optional' isn't working in scenario 1, but without the optional ? it works in scenario 2.
Using Swift v 1.2, xCode 6.2
var stuff = "6t"
// SCENARIO 1
// Why is this failing when stuff contains non-digit characters?
// i.e. it works if stuff = "45".
if let value: Int? = stuff.toInt() {
println("value \(value!)")
}
// SCENARIO 2
// This works!
if let value = stuff.toInt() {
println("val3 \(value)")
}
For Reference also see these SO Answers:
* I wonder if the Sift 1.2 example/Answer here is just plain wrong?
Swift - Converting String to Int
Converting String to Int in Swift
The first IF is always true.
Infact in both cases:
when the toInt() returns a valid Int
when returns nil
the if let will succeed (and yes, the first IF is useless).
Specifically in your code toInt() returns nil in both scenarios.
But in your first scenario you are simply accepting nil as a valid value to enter the THEN block.
There is no point of using if let value: Int?. If the if let works, then the value is an Int. There is no way that it could be nil. Therefore, you do not need to declare it as an optional.
Heys guys,
I am pretty new into programming and therefore I've followed I course on Udemy to teach me Swift 2.2.
For learning purpose I have been trying to program a BMI-calculator where I have a textfield (which just displays the value) and a slider where I can put my weight in kg. After dragging the slider the value is visible in the textfield. I cannot put a value into the textfield so that it is displayed on the slider!
The same textfield-slider relation is used with the height in cm. Now I created an IBAction the bring my kgSlider.value into my kgField.text and it looks like this:
#IBAction func kgSet(sender: AnyObject) {
kgField.text! = String(Int(kgSlider.value))
}
Thats works fine but I unwrapped (like the teacher in the course) without knowing, if there really is a value. Okay, I know in this case that there will be a value, but I would like to go more real and therefore I tried to use an Optional-Binding to find out, if there is a value instead of directly unwrap it.
Therefore I used the cm.Field and the cm.Slider in my code but it doesn't work for now and I don't know why. The code is the following:
#IBAction func cmSet(sender: AnyObject) {
if let tempCm = String(Int(cmSlider.value)) as String! {
cmField.text = tempCm
}
}
So I created the constant called tempCM which will got the value from the cmSlider, if there is a value. Therefore I casted the cmSlider.value like in the other IBAction into an Int and then into a String. If there is the value it will carry it into the cmField.text. This didn't work, therefore I tried to use the "as String!" statement but know I get always 0 instead of the correct value.
So what am I doing wrong there?
So, this should compile fine and provide you with your desired result.
#IBAction func cmSet(sender: AnyObject) {
if let tempCm = String(Int(cmSlider.value)) {
cmField.text = tempCm
}
}
You could also try this
cmField.text = String(Int(cmSlider.value)) ?? " "
in the second example, you are using the optional operator to say if you can convert this to an Int then that Int to a string set the cmField.text property to its value, otherwise use a blank space as the default value.