I have an issue with my n variable. I cannot use n in for loop. Why? n was initialized before for loop. Please, help.
import Foundation
var n: Int
var t: Int
while(true){
var tt = readLine()
t = Int(tt!)!
if (t==0){
break
}
else if ( t < 0){
n = t*(-1)
}
else if(t > 0){
n = t
}
var arr : [[String]] = []
for i in 0..<n*2{
for y in 0..<n*2{
arr[i][y] = "."
}
}
}
A variable may be declared and not immediately initialized, as long as initialization is guaranteed before first use
The error is more subtle than at first glance. You may actually declare a property without initializing it, as long as all program flows leading to its first use ascertain initialization of it.
The issue is with the if, else if and else if block:
var n: Int // declaration
// ...
if (t == 0) {
break
}
else if (t < 0) {
n = t*(-1)
}
else if (t > 0){
n = t
}
// first use
for i in 0..<n*2 { /* ... */ }
Swift cannot not infer that this block is in fact exhaustive, and believes that there is a possibility that none of the above if statements holds, which, in the eyes of the compiler, would lead to the following program state:
program flow has not been broken (break)
and n has not been instantiated
As humans, however, we know that the if - else if - else if block above is indeed exhaustive, and can help the compiler out by simply changing the last if else statement to a simple else statement.
if (t == 0) {
break
}
else if (t < 0) {
n = t*(-1)
}
// if none of the above, t > 0
else {
n = t
}
On another note, the nested array access of non-existing array elements, arr[i][y] = "." will lead to a runtime exception, but this is another issue. In its current form, it looks as if the intent with the nested loops could be replaced with a nested array instantiation:
var arr = [[String]](repeating: [String](repeating: ".", count: 2*n), count: 2*n)
or,
var arr = (0..<2*n).map { _ in [String](repeating: ".", count: 2*n) }
The variable n is only declared, not initialized.
To initialize the variables:
var n: Int = 0
var t: Int = 0
Related
here is my problem:
struct NewStack<S> {
var stacks = [S]()
mutating func addS(_ s: S) {
stacks.append(s)
}
mutating func removeLastS() {
stacks.removeLast()
}
}
var newStacks = NewStack<String>()
newStacks.addS("Eins")
newStacks.addS("Zwei")
newStacks.addS("Drei")
for stack in newStacks.stacks {
var stackCount = 1
repeat {
stackCount + 1
} while stackCount <= newStacks.stacks.count {
print("Stack \(stackCount) = \(stack)")
}
}
At the line with the while-Statement it throughs me the warning: Cannot call value of non-function type 'Int'
Ìt would be very helpful if someone could tell me what I have to do, thank`s
There are several syntax issues with your code.
First, stackCount + 1 doesn't increment stackCount, to do that, you need stackCount += 1. Secondly, you cannot have code executed in the while condition, you need to move the print inside repeat.
for stack in newStacks.stacks {
var stackCount = 1
repeat {
stackCount += 1
print("Stack \(stackCount) = \(stack)")
} while stackCount <= newStacks.stacks.count
}
There is really no need to have an inner loop, you can use a simple for loop like this
for stack in newStacks.stacks {
print("Stack \(stack)")
}
Or if you want the index to be printed as well (I added + 1 so the printed index start at 1)
for (index, stack) in newStacks.stacks.enumerated() {
print("Stack \(index + 1) = \(stack)")
}
Say I have this class:
class EvenNumber {
var num: Int
var stringValue: String
init?(n: Int) {
guard n % 2 == 0 else { return nil }
self.num = n
}
init?(str: String) {
guard let n = Int(str) else { return nil }
self.init(n: n)
//set stringValue?
}
}
In the init that takes a string, I delegate back to the one that takes an Int. How do I know whether it succeeded so I can continue initialization? What's the proper syntax / common pattern here?
You don't need to check that the delegated initialiser has succeeded. If the delegated initialiser has failed, the whole initialisation process fails.
This is evident in this code:
class EvenNumber {
var num: Int
var stringValue: String
init?(n: Int) {
guard n % 2 == 0 else { return nil }
self.num = n
stringValue = "" // you forgot to initialise stringValue in both of your initialisers
}
// you forgot "convenience"
convenience init?(str: String) {
guard let n = Int(str) else { return nil }
self.init(n: n)
print("hello")
stringValue = ""
}
}
EvenNumber(str: "5")
"hello" does not get printed, which means that the rest of the init(str:) does not get executed if init(n:) fails.
Here's some supporting documentation (you need to scroll down a bit, under "Propagation of Initialization Failure"):
In either case, if you delegate to another initializer that causes initialization to fail, the entire initialization process fails immediately, and no further initialization code is executed.
(This section is included for completeness' sake.)
Now you (or whoever comes to this question in the future) might ask, "but what if I want to do something else if the delegated initialiser fails?" In that case, you must check for the condition that causes the initialiser to fail:
if n % 2 == 1 {
self.init(n: n)
} else {
// do something else
}
Why is this so "clumsy"? Well, let's say you could do this (warning: made-up syntax):
if self.init(n: n) {
// success!
} else {
// fail
}
To get to the "fail" branch, we must have already run self.init(n:). self.init(n:), before it failed, might have already initialised some let properties. Recall that let properties can only be initialised once. So now self.init(n: n) has been executed, but the compiler doesn't know which let properties have been initialised. See the problem? How is the compiler going to verify that you have initialised every property exactly once in the "fail" branch?
I created a function that recieves input and compare it to a list, when find a match it return the match, in this case this match is the attribute of a class that i created.
I understand that the problem is with the return statement, so in the beginning of the function I declare the return as "Any", further more than that I'm kinda lost.
The error is this: A 'return' expression required in a function with a block body ('{...}')
class Class1(var self: String)
var test_class = Class1("")
fun giver(){
test_class.self = "Anything"
}
class Funciones(){
fun match_finder(texto: String): Any{
var lista = listOf<String>(test_class.self)
var lista_de_listas = listOf<String>("test_class.self")
var count = -1
for (i in lista_de_listas){
count = count + 1
if (texto == i){
lista_de_listas = lista
var variable = lista_de_listas[count]
return variable
}
}
}
}
fun main(){
giver()
var x = "test_class.self"
var funcion = Funciones()
var y = funcion.match_finder(x)
println(y)
}
To explain you what the problem is, let's consider the following code:
class MyClass {
fun doSomething(): String {
val numbers = listOf(1, 2, 3)
for (number in numbers) {
if (number % 2 == 0) {
return "There is at least one even number in the list"
}
}
}
}
If you try compiling it you'll get the same error message as in your question: A 'return' expression required in a function with a block body ('{...}'). Why is that?
Well, we defined a function doSomething returning a String (it could be any other type) but we're returning a result only if the list of numbers contains at least one even number. What should it return if there's no even number? The compiler doesn't know that (how could it know?), so it prompts us that message. We can fix the code by returning a value or by throwing an exception:
class MyClass {
fun doSomething(): String {
val numbers = listOf(1, 2, 3)
for (number in numbers) {
if (number % 2 == 0) {
return "There is at least one even number in the list"
}
}
// return something if the list doesn't contain any even number
return "There is no even number in the list"
}
}
The same logic applies to your original code: what should the function return if there is no i such that texto == i?
Please also note that the solution you proposed may be syntactically correct - meaning it compiles correctly - but will probably do something unexpected. The for loop is useless since the if/else statement will always cause the function to return during the first iteration, so the value "There is no match" could be returned even if a match actually exists later in the list.
I searched online, if someone has the same problem, the correct code is as follows:
class Funciones(){
fun match_finder(texto: String): Any{
var lista = listOf<String>(test_class.self)
var lista_de_listas = listOf<String>("test_class.self")
var count = -1
var variable = " "
for (i in lista_de_listas){
count = count + 1
if (texto == i){
lista_de_listas = lista
var variable = lista_de_listas[count]
return variable
} else {
return "There is no match"
}
}
return variable
}
}
I need to implement an algorithm to check if an input is valid by calculating a modulo of a String.
The code in Kotlin:
private val facteurs = arrayOf(7, 3, 1)
private fun modulo(s: String): Int {
var result = 0
var i = -1
var idx = 0
for (c in s.toUpperCase()) {
val value:Int
if (c == '<') {
value = 0
} else if (c in "0123456789") {
value = c - '0'
} else if (c in "ABCDEFGHIJKLMNOPQRSTUVWXYZ") {
value = c.toInt() - 55
} else {
throw IllegalArgumentException("Unexpected character: $c at position $idx")
}
i += 1
result += value * facteurs[i % 3]
idx += 1
}
return result % 10
}
This implies doing math operations on the characters.
Is there an elegant way to do this in Swift 3 and 4?
I tried some cumbersome constructs like this :
value = Int(c.unicodeScalars) - Int("0".first!.unicodeScalars)
But it does not even compile.
I'm currently using Swift 4 with XCode9, but Swift3 answer is welcome too.
You can enumerate the unicodeScalars view of a string together
with the running index, use switch/case pattern matching,
and access the numeric .value of the unicode scalar:
func modulo(_ s: String) -> Int? {
let facteurs = [7, 3, 1]
var result = 0
for (idx, uc) in s.uppercased().unicodeScalars.enumerated() {
let value: UInt32
switch uc {
case "<":
value = 0
case "0"..."9":
value = uc.value - UnicodeScalar("0").value
case "A"..."Z":
value = uc.value - UnicodeScalar("A").value + 10
default:
return nil
}
result += Int(value) * facteurs[idx % facteurs.count]
}
return result % 10
}
This compiles with both Swift 3 and 4. Of course you could also
throw an error instead of returning nil for invalid input.
Note that "<", "0", "9" etc.
in the switch statement are inferred from the context as UnicodeScalar,
not as String or Character, therefore "0"..."9" (in this context)
is a ClosedRange<UnicodeScalar> and uc can be matched against
that range.
Something like this works for me:
"A".utf16.first! + 2 //comes out to 67
Careful with the forced unwrap "!"
If you need the scalars value you can do
"A".unicodeScalars.first!.value + 2
More reading can be done on this here in the SPL.
For the c Character type value you could do this:
String(c).unicodeScalars.first!.value + 2
Here is an attempt to mod the function:
func modulo(s: String) -> Int? {
var result = 0
var factors = [7,3,1]
for (i, c) in s.uppercased().characters.enumerated() {
let char = String(c)
var val: Int
if char == "<" {
val = 0
} else if "0123456789".contains(char) {
val = Int(char.unicodeScalars.first!.value - "0".unicodeScalars.first!.value)
} else if "ABCDEFGHIJKLMNOPQRSTUVWXYZ".contains(char) {
val = Int(char.unicodeScalars.first!.value - 55)
} else {
return nil
}
result += val * factors[(i) % 3]
}
return result % 10
}
This is in swift 3...in 4 I believe you can just iterate over the string without converting to Chars
In Swift, I need to create a simple for-condition-increment loop with all the multiples of 3 from 3-100. So far I have:
var multiplesOfThree: [String] = []
for var counter = 0; counter < 30; ++counter {
multiplesOfThree.append("0")
if counter == 3 {
multiplesOfThree.append("3")
} else if counter == 6 {
multiplesOfThree.append("6")
} else if counter == 9 {
multiplesOfThree.append("9")
}
println("Adding \(multiplesOfThree[counter]) to the Array.")
}
I would like to replace all the if and else if statements with something like:
if (index %3 == 0)
but I’m not sure what the proper syntax would be? Also, if I have a single IF statement do I need a .append line to add to the Array?
You are very much on the right track. A few notes:
Swift provides a more concise way to iterate over a fixed number of integers using the ..< operator (an open range operator).
Your if statement with the modulus operator is exactly correct
To make a string from an Int you can use \(expression) inside a string. This is called String Interpolation
Here is the working code:
var multiplesOfThree: [String] = []
for test in 0..<100 {
if (test % 3 == 0) {
multiplesOfThree.append("\(test)")
}
}
However, there is no reason to iterate over every number. You can simply continue to add 3 until you reach your max:
var multiplesOfThree: [String] = []
var multiple = 0
while multiple < 100 {
multiplesOfThree.append("\(multiple)")
multiple += 3
}
As rickster pointed out in the comments, you can also do this in a more concise way using a Strided Range with the by method:
var multiplesOfThree: [String] = []
for multiple in stride(from: 0, to: 100, by: 3) {
multiplesOfThree.append("\(multiple)")
}
Getting even more advanced, you can use the map function to do this all in one line. The map method lets you apply a transform on every element in an array:
let multiplesOfThree = Array(map(stride(from: 0, to: 100, by: 3), { "\($0)" }))
Note: To understand this final code, you will need to understand the syntax around closures well.