Reversing a string in iOS [closed] - ios

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I am writing code which should reverse a string. When I run the following code it throws an error:
- (NSString*) reversingName:(NSString *)myNameText
{
NSString *result;
int len = [myNameText length];
NSMutableString *reverseName = [[NSMutableString alloc] initWithCapacity:len];
for(int i=len;i>0;i--)
{
[reverseName appendFormat:[NSString stringWithFormat:#"%c",[myNameText characterAtIndex:i]]];
}
result = reverseName;
return result;
}

Answer 1

The for loop line should be as follows:
for(int i=len-1;i>=0;i--)
So your method should be
- (NSString*) reversingName:(NSString *)myNameText
{
int len = [myNameText length];
NSMutableString *reverseName = [[NSMutableString alloc] initWithCapacity:len];
for(int i=len-1;i>=0;i--)
{
[reverseName appendString:[NSString stringWithFormat:#"%c",[myNameText characterAtIndex:i]]];
}
return [reverseName autorelease];
}

Answer 2

I thought I'd throw another version out there in case anyone's interested.. personally, I like the cleaner approach using NSMutableString but if performance is the highest priority this one is faster:
- (NSString *)reverseString:(NSString *)input {
NSUInteger len = [input length];
unichar *buffer = malloc(len * sizeof(unichar));
if (buffer == nil) return nil; // error!
[input getCharacters:buffer];
// reverse string; only need to loop through first half
for (NSUInteger stPos=0, endPos=len-1; stPos < len/2; stPos++, endPos--) {
unichar temp = buffer[stPos];
buffer[stPos] = buffer[endPos];
buffer[endPos] = temp;
}
return [[NSString alloc] initWithCharactersNoCopy:buffer length:len freeWhenDone:YES];
}
I also wrote a quick test as well to compare this with the more traditional NSMutableString method (which I also included below):
// test reversing a really large string
NSMutableString *string = [NSMutableString new];
for (int i = 0; i < 10000000; i++) {
int digit = i % 10;
[string appendFormat:#"%d", digit];
}
NSTimeInterval startTime = [[NSDate date] timeIntervalSince1970];
NSString *reverse = [self reverseString:string];
NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSince1970] - startTime;
NSLog(#"reversed in %f secs", elapsedTime);
Results were:
using NSMutableString method (below) - "reversed in 3.720631 secs"
using unichar *buffer method (above) - "reversed in 0.032604 secs"
Just for reference, here's the NSMutableString method used for this comparison:
- (NSString *)reverseString:(NSString *)input {
NSUInteger len = [input length];
NSMutableString *result = [[NSMutableString alloc] initWithCapacity:len];
for (int i = len - 1; i >= 0; i--) {
[result appendFormat:#"%c", [input characterAtIndex:i]];
}
return result;
}
(NOTE: I don't have enough reputation points yet to vote or comment on answers so I'd appreciate if anyone could vote on this answer for me. I've been a long time reader but now want to start contributing more!)

Answer 3

try this sample code :
NSString *name = #"abcdefghi" ;
int len = [name length];
NSMutableString *reverseName = [[NSMutableString alloc] initWithCapacity:len];
for(int i=len-1;i>=0;i--)
{
[reverseName appendFormat:[NSString stringWithFormat:#"%c",[name characterAtIndex:i]]];
}
NSLog(#"%#",reverseName);

Answer 4

Reverse a String in Swift 2.0:
let string = "This is a test string."
let characters = string.characters
let reversedCharacters = characters.reverse()
let reversedString = String(reversedCharacters)
The short way :
String("This is a test string.".characters.reverse())
OR
let string = "This is a test string."
let array = Array(string)
let reversedArray = array.reverse()
let reversedString = String(reversedArray)
The short way :
String(Array("This is a test string.").reverse())