Reverse NSString text - objective-c

Question

I have been googling so much on how to do this, but how would I reverse a NSString? Ex:hi would become: ih
I am looking for the easiest way to do this.
Thanks!
#Vince I made this method:
- (IBAction)doneKeyboard {
// first retrieve the text of textField1
NSString *myString = field1.text;
NSMutableString *reversedString = [NSMutableString string];
NSUInteger charIndex = 0;
while(myString && charIndex < [myString length]) {
NSRange subStrRange = NSMakeRange(charIndex, 1);
[reversedString appendString:[myString substringWithRange:subStrRange]];
charIndex++;
}
// reversedString is reversed, or empty if myString was nil
field2.text = reversedString;
}
I hooked up that method to textfield1's didendonexit. When I click the done button, it doesn't reverse the text, the UILabel just shows the UITextField's text that I entered. What is wrong?

Answer 1

Block version.
NSString *myString = #"abcdefghijklmnopqrstuvwxyz";
NSMutableString *reversedString = [NSMutableString stringWithCapacity:[myString length]];
[myString enumerateSubstringsInRange:NSMakeRange(0,[myString length])
options:(NSStringEnumerationReverse | NSStringEnumerationByComposedCharacterSequences)
usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
[reversedString appendString:substring];
}];
// reversedString is now zyxwvutsrqponmlkjihgfedcba

Answer 2

Write a simple loop to do that:
// myString is "hi"
NSMutableString *reversedString = [NSMutableString string];
NSInteger charIndex = [myString length];
while (charIndex > 0) {
charIndex--;
NSRange subStrRange = NSMakeRange(charIndex, 1);
[reversedString appendString:[myString substringWithRange:subStrRange]];
}
NSLog(#"%#", reversedString); // outputs "ih"
In your case:
// first retrieve the text of textField1
NSString *myString = textField1.text;
NSMutableString *reversedString = [NSMutableString string];
NSInteger charIndex = [myString length];
while (myString && charIndex > 0) {
charIndex--;
NSRange subStrRange = NSMakeRange(charIndex, 1);
[reversedString appendString:[myString substringWithRange:subStrRange]];
}
// reversedString is reversed, or empty if myString was nil
textField2.text = reversedString;

Answer 3

jano’s answer is correct. Unfortunately, it creates a lot of unnecessary temporary objects. Here is a much faster (more complicated) implementation that basically does the same thing, but uses memcpy and unichar buffers to keep memory allocations to a minimum.
- (NSString *)reversedString
{
NSUInteger length = [self length];
if (length < 2) {
return self;
}
unichar *characters = calloc(length, sizeof(unichar));
unichar *reversedCharacters = calloc(length, sizeof(unichar));
if (!characters || !reversedCharacters) {
free(characters);
free(reversedCharacters);
return nil;
}
[self getCharacters:characters range:NSMakeRange(0, length)];
NSUInteger i = length - 1;
NSUInteger copiedCharacterCount = 0;
// Starting from the end of self, copy each composed character sequence into reversedCharacters
while (copiedCharacterCount < length) {
NSRange characterRange = [self rangeOfComposedCharacterSequenceAtIndex:i];
memcpy(reversedCharacters + copiedCharacterCount, characters + characterRange.location, characterRange.length * sizeof(unichar));
i = characterRange.location - 1;
copiedCharacterCount += characterRange.length;
}
free(characters);
NSString *reversedString = [[NSString alloc] initWithCharactersNoCopy:reversedCharacters length:length freeWhenDone:YES];
if (!reversedString) {
free(reversedCharacters);
}
return reversedString;
}
I tested this on 100,000 random multi-byte Unicode strings with lengths between 1 and 128. This version is about 4–5x faster than jano’s.
Enumerate substrings: 2.890528
MemCopy: 0.671090
Enumerate substrings: 2.840411
MemCopy: 0.662882
Test code is at https://gist.github.com/prachigauriar/9739805.
Update: I tried this again by simply converting to a UTF-32 buffer and reversing that.
- (NSString *)qlc_reversedStringWithUTF32Buffer
{
NSUInteger length = [self length];
if (length < 2) {
return self;
}
NSStringEncoding encoding = NSHostByteOrder() == NS_BigEndian ? NSUTF32BigEndianStringEncoding : NSUTF32LittleEndianStringEncoding;
NSUInteger utf32ByteCount = [self lengthOfBytesUsingEncoding:encoding];
uint32_t *characters = malloc(utf32ByteCount);
if (!characters) {
return nil;
}
[self getBytes:characters maxLength:utf32ByteCount usedLength:NULL encoding:encoding options:0 range:NSMakeRange(0, length) remainingRange:NULL];
NSUInteger utf32Length = utf32ByteCount / sizeof(uint32_t);
NSUInteger halfwayPoint = utf32Length / 2;
for (NSUInteger i = 0; i < halfwayPoint; ++i) {
uint32_t character = characters[utf32Length - i - 1];
characters[utf32Length - i - 1] = characters[i];
characters[i] = character;
}
return [[NSString alloc] initWithBytesNoCopy:characters length:utf32ByteCount encoding:encoding freeWhenDone:YES];
}
This is about 3–4x times faster than the memcpy version. The aforementioned gist has been updated with the latest version of the code.
Enumerate substrings: 2.168705
MemCopy: 0.488320
UTF-32: 0.150822
Enumerate substrings: 2.169655
MemCopy: 0.481786
UTF-32: 0.147534
Enumerate substrings: 2.248812
MemCopy: 0.505995
UTF-32: 0.154531

Answer 4

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;
}

Answer 5

Use method with any objects: NSString,NSNumber,etc..:
NSLog(#"%#",[self reverseObject:#12345]);
NSLog(#"%#",[self reverseObject:#"Hello World"]);
Method:
-(NSString*)reverseObject:(id)string{
string = [NSString stringWithFormat:#"%#",string];
NSMutableString *endString = [NSMutableString new];
while ([string length]!=[endString length]) {
NSRange range = NSMakeRange([string length]-[endString length]-1, 1);
[endString appendString: [string substringWithRange:range]];
}
return endString;}
Log:
2014-04-16 11:20:25.312 TEST[23733:60b] 54321
2014-04-16 11:20:25.313 TEST[23733:60b] dlroW olleH

Answer 6

Swift 2.0:
1) let str = "Hello, world!"
let reversed = String(str.characters.reverse())
print(reversed)
In Short:
String("This is a test string.".characters.reverse())
2)
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())
Tested on Play Ground:
import Cocoa
//Assigning a value to a String variable
var str = "Hello, playground"
//Create empty character Array.
var strArray:Character[] = Character[]()
//Loop through each character in the String
for character in str {
//Insert the character in the Array variable.
strArray.append(character)
}
//Create a empty string
var reversedStr:String = ""
//Read the array from backwards to get the characters
for var index = strArray.count - 1; index >= 0;--index {
//Concatenate character to String.
reversedStr += strArray[index]
}
The shorter version:
var str = “Hello, playground”
var reverseStr = “”
for character in str {
reverseStr = character + reverseStr
}

Answer 7

Would it be faster if you only iterated over half the string swapping the characters at each end? So for a 5 character string, you swap characters 1 + 5, then 2 + 4 and 3 doesn't need swapped with anything.
NSMutableString *reversed = [original mutableCopyWithZone:NULL];
NSUInteger i, length;
length = [reversed length];
for (i = 0; i < length / 2; i++) {
// Store the first character as we're going to replace with the character at the end
// in the example, it would store 'h'
unichar startChar = [reversed characterAtIndex:i];
// Only make the end range once
NSRange endRange = NSMakeRange(length - i, 1);
// Replace the first character ('h') with the last character ('i')
// so reversed now contains "ii"
[reversed replaceCharactersInRange:NSMakeRange(i, 1)
withString:[reversed subStringWithRange:endRange];
// Replace the last character ('i') with the stored first character ('h)
// so reversed now contains "ih"
[reversed replaceCharactersInRange:endRange
withString:[NSString stringWithFormat:#"%c", startChar]];
}
edit ----
Having done some tests, the answer is No, its about 6 times slower than the version that loops over everything. The thing that slows us down is creating the temporary NSStrings for the replaceCharactersInRange:withString method. Here is a method that creates only one NSString by manipulating the character data directly and seems a lot faster in simple tests.
NSUInteger length = [string length];
unichar *data = malloc(sizeof (unichar) * length);
int i;
for (i = 0; i < length / 2; i++) {
unichar startChar = [string characterAtIndex:i];
unichar endChar = [string characterAtIndex:(length - 1) - i];
data[i] = endChar;
data[(length - 1) - i] = startChar;
}
NSString *reversed = [NSString stringWithCharacters:data length:length];
free(data);

Answer 8

Reverse the string using recursion:
#implementation NSString (Reversed)
+ (NSString *)reversedStringFromString:(NSString *)string
{
NSUInteger count = [string length];
if (count <= 1) { // Base Case
return string;
} else {
NSString *lastLetter = [string substringWithRange:NSMakeRange(count - 1, 1)];
NSString *butLastLetter = [string substringToIndex:count - 1];
return [lastLetter stringByAppendingString:[self reversedStringFromString:butLastLetter]];
}
}
#end

Answer 9

Google is your friend:
-(NSString *) reverseString
{
NSMutableString *reversedStr;
int len = [self length];
// Auto released string
reversedStr = [NSMutableString stringWithCapacity:len];
// Probably woefully inefficient...
while (len > 0)
[reversedStr appendString:
[NSString stringWithFormat:#"%C", [self characterAtIndex:--len]]];
return reversedStr;
}

Answer 10

None of the answers seem to consider multibyte characters so here is my sample code. It assumes you only ever pass in a string longer than one character.
- (void)testReverseString:(NSString *)string
{
NSMutableString *rString = [NSMutableString new];
NSInteger extractChar = [string length] - 1;
while (extractChar >= 0)
{
NSRange oneCharPos = [string rangeOfComposedCharacterSequenceAtIndex:extractChar];
for (NSUInteger add = 0; add < oneCharPos.length; ++ add)
{
unichar oneChar = [string characterAtIndex:oneCharPos.location + add];
[rString appendFormat:#"%C", oneChar];
}
extractChar -= oneCharPos.length;
}
NSLog(#"%# becomes %#", string, encryptedString );
}

Answer 11

NSString into char utf32 (always 32 bits (unsigned int))
Reverse
char utf32 into NSString
+ (NSString *)reverseString3:(NSString *)str {
unsigned int *cstr, buf, len = [str length], i;
cstr = (unsigned int *)[str cStringUsingEncoding:NSUTF32LittleEndianStringEncoding];
for (i=0;i < len/2;i++) buf = cstr[i], cstr[i] = cstr[len -i-1], cstr[len-i-1] = buf;
return [[NSString alloc] initWithBytesNoCopy:cstr length:len*4 encoding:NSUTF32LittleEndianStringEncoding freeWhenDone:NO];
}
Example : Apple_is  --->  si_elppA

Answer 12

NSMutableString *result = [NSMutableString stringWithString:#""];
for (long i = self.length - 1; i >= 0; i--) {
[result appendFormat:#"%c", [self characterAtIndex:i]];
}
return (NSString *)result;

Answer 13

Here is a collection of categories in Objective-C that will reverse both NSStrings and NSAttributedStrings (while preserving character attributes): TextFlipKit
For example:
NSString *example = #"Example Text";
NSString *reversed = example.tfk_reversed;
NSLog(#"Reversed: %#", reversed);
//prints 'Reversed: txeT elpmaxE'

Answer 14

Swift:
let string = "reverse"
let reversedStringCollection = string.characters.reversed()
for character in reversedStringCollection {
reversedString.append(character)
print(reversedString)
}

Answer 15

We can also achieve the reverse string as follows.
NSString *originalString = #"Hello";
NSString *reverseString;
for (NSUInteger index = originalString.length; index > 0; index--) {
char character = [originalString characterAtIndex:index];
reverseString = [reverseString stringByAppendingString:[NSString stringWithFormat:#"%c", character]];
}
or
NSString *originalString = #"Hello";
NSString *reverseString;
for (NSUInteger index = originalString.length; index > 0; index--) {
char *character = [originalString characterAtIndex:index];
reverseString = [reverseString stringByAppendingString:[NSString stringWithFormat:#"%s", character]];
}

Answer 16

Add a category to NSString so you can call reverse on any NSString in the future like this:
#import "NSString+Reverse.h"
#implementation NSString (Reverse)
-(NSString*)reverse {
char* cstring = (char*)[self UTF8String];
int length = [self length]-1;
int i=0;
while (i<=length) {
unichar tmp = cstring[i];
cstring[i] = cstring[length];
cstring[length] = tmp;
i++;
length--;
}
return [NSString stringWithCString:cstring encoding:NSUTF8StringEncoding];
}
#end

Answer 17

str=#"india is my countery";
array1=[[NSMutableArray alloc] init];
for(int i =0 ;i<[str length]; i++) {
NSString *singleCharacter = [NSString stringWithFormat:#"%c", [str characterAtIndex:i]];
[array1 addObject:singleCharacter];
}
NSMutableString* theString = [NSMutableString string];
for (int i=[array1 count]-1; i>=0;i--){
[theString appendFormat:#"%#",[array1 objectAtIndex:i]];
}

Answer 18

I have written a category ove that one :D
//NSString+Reversed.h
#import
//
// NSString+Reversed.h
// HTMLPageFormatter
// Created by beit46 on 21.06.13.
//
#interface NSString (Reversed)
- (NSString *)reversedString;
#end
//NSString+Reversed.m
//
// NSString+Reversed.m
// HTMLPageFormatter
// Created by beit46 on 21.06.13.
#import "NSString+Reversed.h"
#implementation NSString (Reversed)
- (NSString *)reversedString {
NSMutableString *reversedString = [NSMutableString stringWithCapacity:[self length]];
[self enumerateSubstringsInRange:NSMakeRange(0,[self length])
options:(NSStringEnumerationReverse | NSStringEnumerationByComposedCharacterSequences)
usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
[reversedString appendString:substring];
}];
return [reversedString copy];
}
#end

Answer 19

I have two simple solutions for that purpose:
+(NSString*)reverseString:(NSString *)str
{
NSMutableString* reversed = [NSMutableString stringWithCapacity:str.length];
for (int i = (int)str.length-1; i >= 0; i--){
[reversed appendFormat:#"%c", [str characterAtIndex:i]];
}
return reversed;
}
+(NSString*)reverseString2:(NSString *)str
{
char* cstr = (char*)[str UTF8String];
int len = (int)str.length;
for (int i = 0; i < len/2; i++) {
char buf = cstr[i];
cstr[i] = cstr[len-i-1];
cstr[len-i-1] = buf;
}
return [[NSString alloc] initWithBytes:cstr length:len encoding:NSUTF8StringEncoding];
}
Now, lets test it!
NSString* str = #"Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language";
NSLog(#"REV 1: %#", [Util reverseString:str]);
start = [NSDate date];
for (int i = 0 ; i < 1000; ++i)
[Util reverseString:str];
end = [NSDate date];
NSLog(#"Time per 1000 repeats: %f", [end timeIntervalSinceDate:start]);
NSLog(#"REV 2: %#", [Util reverseString2:str]);
start = [NSDate date];
for (int i = 0 ; i < 1000; ++i)
[Util reverseString2:str];
end = [NSDate date];
NSLog(#"Time per 1000 repeats: %f", [end timeIntervalSinceDate:start]);
Results:
ConsoleTestProject[68292:303] REV 1: egaugnal gnimmargorp C eht ot gnigassem elyts-klatllamS sdda taht egaugnal gnimmargorp detneiro-tcejbo ,esoprup-lareneg a si C-evitcejbO
ConsoleTestProject[68292:303] Time per 1000 repeats: 0.063880
ConsoleTestProject[68292:303] REV 2: egaugnal gnimmargorp C eht ot gnigassem elyts-klatllamS sdda taht egaugnal gnimmargorp detneiro-tcejbo ,esoprup-lareneg a si C-evitcejbO
ConsoleTestProject[68292:303] Time per 1000 repeats: 0.002038
And more chars result was:
ConsoleTestProject[68322:303] chars: 1982
ConsoleTestProject[68322:303] Time 1 per 1000 repeats: 1.014893
ConsoleTestProject[68322:303] Time 2 per 1000 repeats: 0.024928
The same text with above functions:
ConsoleTestProject[68366:303] Time 1 per 1000 repeats: 0.873574
ConsoleTestProject[68366:303] Time 2 per 1000 repeats: 0.019300
ConsoleTestProject[68366:303] Time 3 per 1000 repeats: 0.342735 <-Vladimir Gritsenko
ConsoleTestProject[68366:303] Time 4 per 1000 repeats: 0.584012 <- Jano
So, choose performance!