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.
How do you get the length of a String? For example, I have a variable defined like:
var test1: String = "Scott"
However, I can't seem to find a length method on the string.
As of Swift 4+
It's just:
test1.count
for reasons.
(Thanks to Martin R)
As of Swift 2:
With Swift 2, Apple has changed global functions to protocol extensions, extensions that match any type conforming to a protocol. Thus the new syntax is:
test1.characters.count
(Thanks to JohnDifool for the heads up)
As of Swift 1
Use the count characters method:
let unusualMenagerie = "Koala 🐨, Snail 🐌, Penguin 🐧, Dromedary 🐪"
println("unusualMenagerie has \(count(unusualMenagerie)) characters")
// prints "unusualMenagerie has 40 characters"
right from the Apple Swift Guide
(note, for versions of Swift earlier than 1.2, this would be countElements(unusualMenagerie) instead)
for your variable, it would be
length = count(test1) // was countElements in earlier versions of Swift
Or you can use test1.utf16count
TLDR:
For Swift 2.0 and 3.0, use test1.characters.count. But, there are a few things you should know. So, read on.
Counting characters in Swift
Before Swift 2.0, count was a global function. As of Swift 2.0, it can be called as a member function.
test1.characters.count
It will return the actual number of Unicode characters in a String, so it's the most correct alternative in the sense that, if you'd print the string and count characters by hand, you'd get the same result.
However, because of the way Strings are implemented in Swift, characters don't always take up the same amount of memory, so be aware that this behaves quite differently than the usual character count methods in other languages.
For example, you can also use test1.utf16.count
But, as noted below, the returned value is not guaranteed to be the same as that of calling count on characters.
From the language reference:
Extended grapheme clusters can be composed of one or more Unicode
scalars. This means that different characters—and different
representations of the same character—can require different amounts of
memory to store. Because of this, characters in Swift do not each take
up the same amount of memory within a string’s representation. As a
result, the number of characters in a string cannot be calculated
without iterating through the string to determine its extended
grapheme cluster boundaries. If you are working with particularly long
string values, be aware that the characters property must iterate over
the Unicode scalars in the entire string in order to determine the
characters for that string.
The count of the characters returned by the characters property is not
always the same as the length property of an NSString that contains
the same characters. The length of an NSString is based on the number
of 16-bit code units within the string’s UTF-16 representation and not
the number of Unicode extended grapheme clusters within the string.
An example that perfectly illustrates the situation described above is that of checking the length of a string containing a single emoji character, as pointed out by n00neimp0rtant in the comments.
var emoji = "👍"
emoji.characters.count //returns 1
emoji.utf16.count //returns 2
Swift 1.2 Update: There's no longer a countElements for counting the size of collections. Just use the count function as a replacement: count("Swift")
Swift 2.0, 3.0 and 3.1:
let strLength = string.characters.count
Swift 4.2 (4.0 onwards): [Apple Documentation - Strings]
let strLength = string.count
Swift 1.1
extension String {
var length: Int { return countElements(self) } //
}
Swift 1.2
extension String {
var length: Int { return count(self) } //
}
Swift 2.0
extension String {
var length: Int { return characters.count } //
}
Swift 4.2
extension String {
var length: Int { return self.count }
}
let str = "Hello"
let count = str.length // returns 5 (Int)
Swift 4
"string".count
;)
Swift 3
extension String {
var length: Int {
return self.characters.count
}
}
usage
"string".length
If you are just trying to see if a string is empty or not (checking for length of 0), Swift offers a simple boolean test method on String
myString.isEmpty
The other side of this coin was people asking in ObjectiveC how to ask if a string was empty where the answer was to check for a length of 0:
NSString is empty
Swift 5.1, 5
let flag = "🇵🇷"
print(flag.count)
// Prints "1" -- Counts the characters and emoji as length 1
print(flag.unicodeScalars.count)
// Prints "2" -- Counts the unicode lenght ex. "A" is 65
print(flag.utf16.count)
// Prints "4"
print(flag.utf8.count)
// Prints "8"
tl;dr If you want the length of a String type in terms of the number of human-readable characters, use countElements(). If you want to know the length in terms of the number of extended grapheme clusters, use endIndex. Read on for details.
The String type is implemented as an ordered collection (i.e., sequence) of Unicode characters, and it conforms to the CollectionType protocol, which conforms to the _CollectionType protocol, which is the input type expected by countElements(). Therefore, countElements() can be called, passing a String type, and it will return the count of characters.
However, in conforming to CollectionType, which in turn conforms to _CollectionType, String also implements the startIndex and endIndex computed properties, which actually represent the position of the index before the first character cluster, and position of the index after the last character cluster, respectively. So, in the string "ABC", the position of the index before A is 0 and after C is 3. Therefore, endIndex = 3, which is also the length of the string.
So, endIndex can be used to get the length of any String type, then, right?
Well, not always...Unicode characters are actually extended grapheme clusters, which are sequences of one or more Unicode scalars combined to create a single human-readable character.
let circledStar: Character = "\u{2606}\u{20DD}" // ☆⃝
circledStar is a single character made up of U+2606 (a white star), and U+20DD (a combining enclosing circle). Let's create a String from circledStar and compare the results of countElements() and endIndex.
let circledStarString = "\(circledStar)"
countElements(circledStarString) // 1
circledStarString.endIndex // 2
In Swift 2.0 count doesn't work anymore. You can use this instead:
var testString = "Scott"
var length = testString.characters.count
Here's something shorter, and more natural than using a global function:
aString.utf16count
I don't know if it's available in beta 1, though. But it's definitely there in beta 2.
Updated for Xcode 6 beta 4, change method utf16count --> utf16Count
var test1: String = "Scott"
var length = test1.utf16Count
Or
var test1: String = "Scott"
var length = test1.lengthOfBytesUsingEncoding(NSUTF16StringEncoding)
As of Swift 1.2 utf16Count has been removed. You should now use the global count() function and pass the UTF16 view of the string. Example below...
let string = "Some string"
count(string.utf16)
For Xcode 7.3 and Swift 2.2.
let str = "🐶"
If you want the number of visual characters:
str.characters.count
If you want the "16-bit code units within the string’s UTF-16 representation":
str.utf16.count
Most of the time, 1 is what you need.
When would you need 2? I've found a use case for 2:
let regex = try! NSRegularExpression(pattern:"🐶",
options: NSRegularExpressionOptions.UseUnixLineSeparators)
let str = "🐶🐶🐶🐶🐶🐶"
let result = regex.stringByReplacingMatchesInString(str,
options: NSMatchingOptions.WithTransparentBounds,
range: NSMakeRange(0, str.utf16.count), withTemplate: "dog")
print(result) // dogdogdogdogdogdog
If you use 1, the result is incorrect:
let result = regex.stringByReplacingMatchesInString(str,
options: NSMatchingOptions.WithTransparentBounds,
range: NSMakeRange(0, str.characters.count), withTemplate: "dog")
print(result) // dogdogdog🐶🐶🐶
You could try like this
var test1: String = "Scott"
var length = test1.bridgeToObjectiveC().length
in Swift 2.x the following is how to find the length of a string
let findLength = "This is a string of text"
findLength.characters.count
returns 24
Swift 2.0:
Get a count: yourString.text.characters.count
Fun example of how this is useful would be to show a character countdown from some number (150 for example) in a UITextView:
func textViewDidChange(textView: UITextView) {
yourStringLabel.text = String(150 - yourStringTextView.text.characters.count)
}
In swift4 I have always used string.count till today I have found that
string.endIndex.encodedOffset
is the better substitution because it is faster - for 50 000 characters string is about 6 time faster than .count. The .count depends on the string length but .endIndex.encodedOffset doesn't.
But there is one NO. It is not good for strings with emojis, it will give wrong result, so only .count is correct.
In Swift 4 :
If the string does not contain unicode characters then use the following
let str : String = "abcd"
let count = str.count // output 4
If the string contains unicode chars then use the following :
let spain = "España"
let count1 = spain.count // output 6
let count2 = spain.utf8.count // output 7
In Xcode 6.1.1
extension String {
var length : Int { return self.utf16Count }
}
I think that brainiacs will change this on every minor version.
Get string value from your textview or textfield:
let textlengthstring = (yourtextview?.text)! as String
Find the count of the characters in the string:
let numberOfChars = textlength.characters.count
Here is what I ended up doing
let replacementTextAsDecimal = Double(string)
if string.characters.count > 0 &&
replacementTextAsDecimal == nil &&
replacementTextHasDecimalSeparator == nil {
return false
}
Swift 4 update comparing with swift 3
Swift 4 removes the need for a characters array on String. This means that you can directly call count on a string without getting characters array first.
"hello".count // 5
Whereas in swift 3, you will have to get characters array and then count element in that array. Note that this following method is still available in swift 4.0 as you can still call characters to access characters array of the given string
"hello".characters.count // 5
Swift 4.0 also adopts Unicode 9 and it can now interprets grapheme clusters. For example, counting on an emoji will give you 1 while in swift 3.0, you may get counts greater than 1.
"👍🏽".count // Swift 4.0 prints 1, Swift 3.0 prints 2
"👨❤️💋👨".count // Swift 4.0 prints 1, Swift 3.0 prints 4
Swift 4
let str = "Your name"
str.count
Remember: Space is also counted in the number
You can get the length simply by writing an extension:
extension String {
// MARK: Use if it's Swift 2
func stringLength(str: String) -> Int {
return str.characters.count
}
// MARK: Use if it's Swift 3
func stringLength(_ str: String) -> Int {
return str.characters.count
}
// MARK: Use if it's Swift 4
func stringLength(_ str: String) -> Int {
return str.count
}
}
Best way to count String in Swift is this:
var str = "Hello World"
var length = count(str.utf16)
String and NSString are toll free bridge so you can use all methods available to NSString with swift String
let x = "test" as NSString
let y : NSString = "string 2"
let lenx = x.count
let leny = y.count
test1.characters.count
will get you the number of letters/numbers etc in your string.
ex:
test1 = "StackOverflow"
print(test1.characters.count)
(prints "13")
Apple made it different from other major language. The current way is to call:
test1.characters.count
However, to be careful, when you say length you mean the count of characters not the count of bytes, because those two can be different when you use non-ascii characters.
For example;
"你好啊hi".characters.count will give you 5 but this is not the count of the bytes.
To get the real count of bytes, you need to do "你好啊hi".lengthOfBytes(using: String.Encoding.utf8). This will give you 11.
Right now (in Swift 2.3) if you use:
myString.characters.count
the method will return a "Distance" type, if you need the method to return an Integer you should type cast like so:
var count = myString.characters.count as Int
my two cents for swift 3/4
If You need to conditionally compile
#if swift(>=4.0)
let len = text.count
#else
let len = text.characters.count
#endif
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.
"The Swift Programming Language" contains the following sample code:
let label = "The width is "
let width = 94
let widthLabel = label + String(width)
When I changed width implicitly to Double:
let width = 94.4
a compiler error is created for the line let widthLabel = label + String(width):
Cannot invoke '+' with an argument list of type (String, String)
While I can convert the Double var to String by calling width.description, I want to know:
Why String(Integer) works but String(Double) doesn't?
Is there any difference between calling String(var) and var.description on a numerical type (Integer, Float, Double, etc)?
The reason why you can't do that is because String doesn't have an initializer accepting a double or a float, whereas it implements initializers for all integer types (Int, Uint, Int32, etc.).
So #derdida's solution is the right way to do it.
I discourage using the description property. It is not meant to convert a double to a string, but to provide a human readable textual representation of a double. So, whereas today's representation coincides with the conversion:
let width = 94.5
println(width) // Prints "94.5"
tomorrow it can be changed to something different, such as:
ninety four point five
That's still human readable, but it's not exactly a conversion of double to string.
This rule is valid for all types (structs, classes, etc.) implementing the Printable protocol: the description property should be used to describe, not to convert.
Addendum
Besides using string interpolation, we can also use the old c-like string formatting:
let pi = 3.1415
let piString = String(format: "%0.2f", arguments:[pi])
let message = "Pi is " + String(format: "%0.2f", arguments:[pi])
println(message)
I would use this when you like to create a string value:
let width:Double = 94.4
let widthLabel = "The width is \(width)"