pattern-matching

问题 val test1 = [(1,5,3),(3,5,2),(3,4,5)] fun number_in_month dates_and_month = case dates_and_month of (x,y,z)::xs' => y :: number_in_month xs' This code produces the following error when I run in the REPL with test1: uncaught exception Match [nonexhaustive match failure] raised at: hw1pm.sml:28.49 Any clue why? 回答1: It did not know what do when the list was empty. Working code: fun number_in_month dates_and_month = case dates_and_month of [] => [] | (x,y,z)::xs' => y :: number_in_month xs' 回答2:

问题 // Start writing your ScalaFiddle code here sealed trait DSL[A]{ // def run(): A ={ // this match { // case GetLength(something) => // something.length // case ShowResult(number) => // s"the length is $number" // } // } } case class GetLength(something: String) extends DSL[Int] case class ShowResult(number: Int) extends DSL[String] def run[A](fa:DSL[A]): A ={ fa match { case GetLength(something) => something.length case ShowResult(number) => s"the length is $number" } } val dslGetLength =

问题 Suppose we have an enum that looks like this: enum MyEnum { Field1, Field2 {x: f64, y: f64}, /* Maybe some other fields */ MyString(String), } Now I created an instance of this enum of the subtype MyString and after some actions I want to mutate it. For example: fn main() { let mut my_enum = MyEnum::MyString("Hello, world".to_string()); /* Some actions */ // Mutating the string match my_enum { MyEnum::MyString(ref mut content) => { content.push('!'); }, _ => {} } // Printing the string match

问题 Suppose we have an enum that looks like this: enum MyEnum { Field1, Field2 {x: f64, y: f64}, /* Maybe some other fields */ MyString(String), } Now I created an instance of this enum of the subtype MyString and after some actions I want to mutate it. For example: fn main() { let mut my_enum = MyEnum::MyString("Hello, world".to_string()); /* Some actions */ // Mutating the string match my_enum { MyEnum::MyString(ref mut content) => { content.push('!'); }, _ => {} } // Printing the string match

问题 I have a list of strings and I want to split each string on a floating point number. If there is no floating point number in the string, I want to split it on a number. It should only split once and return everything before and after it separated by commas. Input string: ['Naproxen 500 Active ingredient Ph Eur', 'Croscarmellose sodium 22.0 mg Disintegrant Ph Eur', 'Povidone K90 11.0 Binder 56 Ph Eur', 'Water, purifieda, 'Silica, colloidal anhydrous 2.62 Glidant Ph Eur', 'Water purified 49

问题 I have a Regex, which is [\\.|\\;|\\?|\\!][\\s] This is used to split a string. But I don't want it to split . ; ? ! if it is in quotes. 回答1: I'd not use split but Pattern & Matcher instead. A demo: import java.util.regex.Matcher; import java.util.regex.Pattern; public class Main { public static void main(String[] args) { String text = "start. \"in quotes!\"; foo? \"more \\\" words\"; bar"; String simpleToken = "[^.;?!\\s\"]+"; String quotedToken = "(?x) # enable inline comments and ignore

问题 In a Pandas df['Column'] (i.e. a pandas Series) If I use df['company_name'].str.contains('ABC').any() I will get 'True' if an entry is "ABC" But it will also return a (false positive) "True" if some other entry in the Series is "ABC PTY LTD" I only want to match if there is an entry that is exactly "ABC" I've checked about 50 similar questions but none answer this one. I tried a Regex rec_df['recruiters'].str.match( r'^ABC$').any() It works but the problem is I want to pass the 'ABC' part

问题 In a Pandas df['Column'] (i.e. a pandas Series) If I use df['company_name'].str.contains('ABC').any() I will get 'True' if an entry is "ABC" But it will also return a (false positive) "True" if some other entry in the Series is "ABC PTY LTD" I only want to match if there is an entry that is exactly "ABC" I've checked about 50 similar questions but none answer this one. I tried a Regex rec_df['recruiters'].str.match( r'^ABC$').any() It works but the problem is I want to pass the 'ABC' part

问题 What is the purpose of & in the code &i in list ? If I remove the & , it produces an error in largest = i , since they have mismatched types (where i is &32 and i is i32 ). But how does &i convert i into i32 ? fn largest(list: &[i32]) -> i32 { println!("{:?}", list); let mut largest = list[0]; for &i in list { if i > largest { largest = i; } } largest } fn main() { let hey = vec![1, 3, 2, 6, 90, 67, 788, 12, 34, 54, 32]; println!("The largest number is: {}", largest(&hey)); } Playground It

问题 What is the purpose of & in the code &i in list ? If I remove the & , it produces an error in largest = i , since they have mismatched types (where i is &32 and i is i32 ). But how does &i convert i into i32 ? fn largest(list: &[i32]) -> i32 { println!("{:?}", list); let mut largest = list[0]; for &i in list { if i > largest { largest = i; } } largest } fn main() { let hey = vec![1, 3, 2, 6, 90, 67, 788, 12, 34, 54, 32]; println!("The largest number is: {}", largest(&hey)); } Playground It