Onur Tuna Onur Tuna - 1 month ago 6
Swift Question

Strange behaviour of for loop in Swift 2

EDIT: As discussed in the comments I don't prefer anyone without solving this question before to look at the answer. This is the idea of Project Euler. Please, only those who solved the question look at the answer.

I can not understand what's wrong with that code?

var sDigits = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"
var iDigits = sDigits.characters.map{Int(String($0)) ?? 0}
var digitsProductList: [Int] = []

for var i = 0; i < iDigits.count - 4; i++ {
var temp = 1
for var j = i; j < 4; j++ {
temp *= iDigits[j]
}
digitsProductList.append(temp)
}

digitsProductList.maxElement()


The output is 126 - the product of the first 4 character.
The digitsProductList is

[126, 18, 6, 6, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, …, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]


It's unbelievable!

Answer

You have a error in your code in the following line:

for var j = i; j < 4; j++ {
    temp *= iDigits[j]
}

As you can see always you set the variable j to reach the max number of 4 and this is wrong, you need to change your code to the following:

var sDigits =  "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"

var iDigits = sDigits.characters.map{Int(String($0)) ?? 0}
var digitsProductList: [Int] = []

for var i = 0; i < iDigits.count - 4; i++ {
   var temp = 1

   for var j = i; j < iDigits.count && j < i + 4; j++ {
       temp *= iDigits[j]
   }
   digitsProductList.append(temp)
}

print(digitsProductList.maxElement()) //5832

In the above code j always check that not reach the limit and always move forward 4 digits from i index: j < iDigits.count && j < i + 4

I hope this help you