Generation of an array of substring from a string in swift - arrays

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?

Answer 1

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])
}

Answer 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