Generation of an array of substring from a string in swift

Question

I have a problem with generation of an array of substrings from a string in the following code

var primers=[String]()

var lengthOfPrimer = 20

var  lentghOfText = str1.characters.count

var rest = lentghOfText - lengthOfPrimer

for var i = 0; i <= str1.characters.count; ++i
{

var temp = rest - i

var substring1 = str1.substringWithRange(Range<String.Index>(start: advance(str1.startIndex, i), end: advance(str1.endIndex, -temp)))

primers.append(substring1)

}

In playground I have the following error at the line with the substring1 code – Execution was interrupted: reason: EXC_BAD_INSTRUCTION (code=EXC 1386_INVOP, subcode=0x0. In spite of error sign, I can see in playground the generated set of substrings in loop, but when I tried to use this code in program this code also did not work, What is wrong? What should I do?

jtbandes · Accepted Answer · 2015-08-16 00:46:04Z

2

Your error is coming from trying to advance the endIndex.

But how about something a little simpler? (Swift 2)

var first = str1.startIndex
var last = advance(first, 20 - 1, str1.endIndex)

while last != str1.endIndex {
    primers.append(str1[first++ ... last++])
}

print("\n".join(primers))

// output:
// ACAAGATGCCATTGTCCCCC
// CAAGATGCCATTGTCCCCCG
// AAGATGCCATTGTCCCCCGG
// AGATGCCATTGTCCCCCGGC
// GATGCCATTGTCCCCCGGCC
// ATGCCATTGTCCCCCGGCCT
// TGCCATTGTCCCCCGGCCTC
// GCCATTGTCCCCCGGCCTCC
// CCATTGTCCCCCGGCCTCCT
// CATTGTCCCCCGGCCTCCTG
// ATTGTCCCCCGGCCTCCTGC
// TTGTCCCCCGGCCTCCTGCT
// ...

Or the for loop way:

for var first = str1.startIndex, last = advance(first, 20 - 1, str1.endIndex);
    last != str1.endIndex;
    ++first, ++last
{
    primers.append(str1[first...last])
}

edited Aug 16, 2015 at 0:46

answered Aug 16, 2015 at 0:18

jtbandes

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7 Comments

VYT Over a year ago

Great, thanks for the working code! I have played with another for loop code and try to modify it as you can see below and it did not work. Sorry, I am new in swift and would like to understand why code did not work. Could you give me the explanation? for var i = 0; i < lengthOfText - 1; ++i { primers.append(str1[advance(str1.startIndex, i)...advance(str1.startIndex, i + 20) ] ) }

jtbandes Over a year ago

I would not recommend doing it that way. ++ for a string may be slow because it has to determine where the character boundary is. advance is like multiple ++s. So it's best to do it as little as possible. In my version above, I use ++ only twice per iteration.

jtbandes Over a year ago

You should also keep in mind what happens when the string is exactly 20 characters or shorter.

VYT Over a year ago

I have used your suggestion with C-like for-loop and now after coming deprecations of both C-like for-loop and ++ , I am using instead while-loop with .successor, but it works two time slower than the same while loop with ++. Is there way to improve the execution time of this while loop with .successor instead of ++ or no ?

jtbandes Over a year ago

Are you building/running the code with optimization on (-O)?

|

w0mbat · Accepted Answer · 2015-08-16 00:21:23Z

2

for var i = 0; i <= str1.characters.count; ++i

Without getting into the rest of this code, this for loop is wrong. You are reading past the end of the string because you are using <= when you should be using <.

Also you are calling str1.characters.count in the loop even though you have that value in a variable, which is slow.

Try:

for var i = 0; i < lengthOfText; ++i

answered Aug 16, 2015 at 0:21

w0mbat

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2 Comments

VYT Over a year ago

I have tried, and it still not working with your correction.

w0mbat Over a year ago

Well yeah, you have other bugs too, I'm sure. I just pointed out the ones that jumped off the screen at me.

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