Reverse a string without using temporary variable : string, reverse, using, without

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06.01.2004 at 01:06AM PDT, ID: 21008644

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6.2

Reverse a string without using temporary variable

Zone: C Programming Language

Tags: string, reverse, using, without

Hi,

Is there any way I can reverse a string without using a temporary variable ?

Thanks,
Parag.

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Question Stats

Zone: Programming

Question Asked By: paragm78

Solution Provided By: Computer101

Participating Experts: 11

Solution Grade: A

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06.01.2004 at 01:16AM PDT, ID: 11200986

Rank: Master

akshayxx:

if you know how to swap two integers and hence characters w/o using temp variable .. then its straight forward..
here is how u can swap two integers

int a,b;
a=45;
b=54;

b=a+b;
a=b-a;
b=b-a;

one more a 'bit' cryptic way to swap is
a ^= b;
b ^= a;
a ^= b;

well .. u got it or need more help?

06.01.2004 at 01:18AM PDT, ID: 11200989

Rank: Guru

PaulCaswell:

Parag,

Its possible to 'hide' the temporary variable in recursion or an assignment within an expression or using 'xor' but the temporary variable will still exist at some time or another in some guise.

Here's the classic one but there are others:

void Reverse ( char * str )
{
int i, j;

for ( i = 0, j = strlen(str); i < len; i++, j-- )
{
// Using XOR:
str[i] ^= str[j];
// The temorary variable is actually in the '0' bits of str[j] and the '1' bits of str[i].
str[j] ^= str[i];
}
}

06.01.2004 at 01:18AM PDT, ID: 11200990

sjef_bosman:

What have you tried in the meantime? Suggested solution:
- make a function that accepts a string (char *s) as parameter
- you need a local variable (e.g. p) here, type char *
- you also need a temporary char variable (unfortunately)
- set p to s + length(s)-1, it then points to the last position
- then swap *s and *p, increment s and decrement p, until s>=p

What temp. variable you don't want? How to swap chars without one?

06.01.2004 at 01:21AM PDT, ID: 11201002

Rank: Guru

PaulCaswell:

Parag,

Akshay is right. I missed an ^= but I hope you got the essence of it which is that the temporary variable is still there, its just 'hidden' by various techniques.

Paul.

06.01.2004 at 07:35AM PDT, ID: 11203130

Rank: Master

Axter:

You can use strrev() function.

#include <string.h>

char data[]="Hello World!";
strrev(data);

strrev is not part of the C standard, and it's a C-Extension function which is supported by many compilers.

06.01.2004 at 07:42AM PDT, ID: 11203193

Rank: Guru

PaulCaswell:

I just noticed another problem with my code sample above (I should stop when i >= j too). Here it is corrected.

void Reverse ( char * str )
{
int i, j;

for ( i = 0, j = strlen(str); i < j; i++, j-- )
{
// Using XOR: The temorary variable is actually in the '0' bits of str[j] and the '1' bits of str[i].
str[i] ^= str[j];
str[j] ^= str[i];
str[i] ^= str[j];
}
}

This still uses a temporary variable! You just dont notice it because its stored in the '0' bits of the two bytes you are swapping.

Paul

06.01.2004 at 07:44AM PDT, ID: 11203211

Rank: Guru

jaime_olivares:

heterodoxe method:
all C strings terminates with 0
you can use this position as your "temporary variable"
Obviously you have to put back the 0 again in the same position.

06.01.2004 at 07:44AM PDT, ID: 11203212

Rank: Guru

PaulCaswell:

Actually, j should start at strlen(str) - 1. Sorry! :(

06.01.2004 at 08:17AM PDT, ID: 11203528

Rank: Guru

jaime_olivares:

>Actually, j should start at strlen(str) - 1. Sorry! :(
Who imposed this limitation? Maybe my alternative is not the best for paragm78, but it is totally possible.

06.02.2004 at 12:08AM PDT, ID: 11209904

vasureddym:

Hi all!
Hi paragm78!
i can suggest the solution using single statement..

try this , it is tested..

x^=y^=x^=y;

u can find more on this at http://www.codeproject.com/tips/stupidxortrick.asp

let me know your comments
bye
vasureddym

06.02.2004 at 02:09AM PDT, ID: 11210430

Rank: Master

akshayxx:

x^=y^=x^=y;
this is equivalent of
three step XOR earlier mentioned..
just that all three XOR steps are condensed in single..
otherwise both are equally effiecient/inefficient.

06.02.2004 at 02:26AM PDT, ID: 11210506

Rank: Guru

PaulCaswell:

My point still stands. It is impossible to swap two values of an array without temporary values. All the above solutions, including mine, use temporary variables in some form.

Before anyone starts shouting, the triple xor method DOES use a temporary variable, it's just hidden in the two real variables. Thats why you need three operations instead of one.

Paul

06.03.2004 at 02:02AM PDT, ID: 11220309

ajay_kanse:

Hi,

I guess u can do it without using temp variable.
Try to get the strlen() of the string.
Find out the middle character position
Use to counters : one for the array index from start and other for the array index from end.
In the for loop swap the characters from the first to last position.
Increment first counter and decrement second counter.

I hope this will work.

--Ajay

06.10.2004 at 05:48AM PDT, ID: 11278756

aib_42:

Why do I hate these silly assignments? Why do academic programmers make me mad? Oops sorry, went a bit OT there. Here's the answer:

No. Without using a temporary variable, you cannot hold the length of the string, without which you have nothing to swap the first character with. Without using a temporary variable, you cannot even access the array's contents.

And here's the mocking answer since I'm still mad:

No. Use permanent variables instead.

06.10.2004 at 08:31AM PDT, ID: 11280421

sjef_bosman:

My statement: every variable is temporary ;)

Which makes it more a NO.

06.13.2004 at 04:01AM PDT, ID: 11299369

x4u:

It is possible, it just looks a little stupid:

switch( strlen( str ) )
{
case 0: case 1:
break;
case 2:
str[ 0 ] ^= str[ 1 ] ^= str[ 0 ] ^= str[ 1 ];
break;
case 3:
str[ 0 ] ^= str[ 2 ] ^= str[ 0 ] ^= str[ 2 ];
break;
case 4:
str[ 0 ] ^= str[ 3 ] ^= str[ 0 ] ^= str[ 3 ];
str[ 1 ] ^= str[ 2 ] ^= str[ 1 ] ^= str[ 2 ];
break;
case 5:
str[ 0 ] ^= str[ 4 ] ^= str[ 0 ] ^= str[ 4 ];
str[ 1 ] ^= str[ 3 ] ^= str[ 1 ] ^= str[ 3 ];
break;
// ... implement all other cases up to your maximum string length
}

Or if you think strlen might use temporary variables you can do it like this:

if( str[ 0 ] && str[ 1 ] )
{
if( !str[ 2 ] )
str[ 0 ] ^= str[ 1 ] ^= str[ 0 ] ^= str[ 1 ];
else if( !str[ 3 ] )
str[ 0 ] ^= str[ 2 ] ^= str[ 0 ] ^= str[ 2 ];
else if( !str[ 4 ] )
{
str[ 0 ] ^= str[ 3 ] ^= str[ 0 ] ^= str[ 3 ];
str[ 1 ] ^= str[ 2 ] ^= str[ 1 ] ^= str[ 2 ];
}
else if( !str[ 5 ] )
{
str[ 0 ] ^= str[ 4 ] ^= str[ 0 ] ^= str[ 4 ];
str[ 1 ] ^= str[ 3 ] ^= str[ 1 ] ^= str[ 3 ];
}
// ... implement all other cases up to your maximum str length
}

06.13.2004 at 12:58PM PDT, ID: 11301006

Rank: Guru

jaime_olivares:

Where is the author?

06.20.2004 at 07:43AM PDT, ID: 11354431

manav_mathur:

Hey,
This is the most popular example for recursion
declare a function like this

getchr()
{
c=getch();
if(c != '*') getchr();
putchar(c);
}

What this function does is recursively call itself and read a character. If the character is not a termination character (* in our case), it again calls itself and accepts a character.

When the termination character is entered, it recursively spirals thru the last call to the first call of the function. This specific behaviour is obtained by the placement of the getchr() and putchar() functions.

Try this and give me your feedback

Manav

06.21.2004 at 01:47AM PDT, ID: 11357780

sjef_bosman:

If your string is 20 characters long, there will be 20 variables c on the stack. Not temporary??

06.22.2004 at 12:00AM PDT, ID: 11366694

manav_mathur:

No. Because those 20 variables on the stack store the string just like a 20 character string which is not a temporary variable.

This is one of the standard questions you will find in amny books under the recursion chapter. This question is more about lateral thinking than technical.
The trick here is how to read the input. Rather than storing it in an array, store it in a fashion where u can inherently access it backwards. There are 2 methods, a link list or recursion. For link list, u again need a temp variable, but not for recursion.

So thats your answer. FOr 20 bytes string, u will be using 20 'c' variables in the STACK RATHER THAN THE HEAP. Thats the trick!!

Manav

Th

09.02.2004 at 10:42PM PDT, ID: 11970430

jmcg:

Paragm78,

This old question needs to be finalized accept an answer, split points, or get a refund. For information on your options, please see
http://www.experts-exchange.com/help.jsp#hs5

EXPERTS:
Please help with this cleanup process! Post closing recommendations within 4 days.

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EE Cleanup Volunteer

09.09.2004 at 12:16PM PDT, ID: 12020286

jmcg:

Nothing has happened on this question in more than 3 months. It's time for cleanup!

My recommendation, which I will post in the Cleanup topic area, is to
PAQ, forfeit points.

Please leave any comments here within the next four days.

jmcg
EE Cleanup Volunteer

09.13.2004 at 06:08PM PDT, ID: 12050355

Computer101:

PAQed - no points refunded (of 20)

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