c++ swap programe

[iGAMDjzh]

i need c++ swaping programe without using temp. variable.
please i need help.

[maikel]

What type of variable can you use? Did you do something already upon we can build something together? Do you know how to define functions?

Maikel

[googyi]

If you have to swap numbers then this will do the job.

int a = 5;
int b = 7;

//swapping
a^=b;
b^=a;
a^=b;

cout << "a: " << a << endl;//7
cout << "b: " << b << endl;//5

[tajendra]

You can also use simple addition and subtraction method.

int a = 5;
int b = 7;

//swapping
a=a+b;
b=a-b;
a=a-b;

cout << "a: " << a << endl;//7
cout << "b: " << b << endl;//5 

[imi]

tajendra wrote:

You can also use simple addition and subtraction method.

If a or b is too big, you get an integer overflow, which is "undefined behaviour" in pure C++ terminology. (But practically, it should work with almost all environments nowadays).

Also, both versions may cause havoc if a and b are of different integral types.

[googyi]

"Also, both versions may cause havoc if a and b are of different integral types."
It is not wise to swap numbers of different types.

[Blitz Coder]

Don't you need a third variable to store them as they move?

int x;
x = a;
a = b;
b = x;

where x is the third "box" which starts empty.

[1yR4jE8b]

Not if you use XOR

a^=b;
b^=a;
a^=b;

let's say that a = 4 and b = 10

4 in binary is 0100
10 in binary is 1010

a ^= b assigns a to a XOR b so 0100 XOR 1010 = 1110 is 14
b ^= a assign b to a XOR b so 1110 XOR 10101 = 0100 is 4

finally, a ^= b assigns a to a XOR b so 1110 XOR 0100 = 1010 is 10

so now a = 10 and b = 4

Doing swaps this way does not require the use of a temporary variable.

Another cool little bitshifting trick is if you need to zero a variable, just XOR it with itself

int a = 9999;
a ^= a
cout << boolalpha << (a == 0);

[imi]

Another cool little bitshifting trick is if you need to zero a variable, just XOR it with itself

Except of the cocktail-party-bragging ability.. what is this usefull for? :-D

I'd rather turn it around: If you happen to recognize, that someone XOR's a variable with itself, just set it to 0 for better readability instead.

Ciao, Imi.
PS: Don't even think the word "performance" here, please. ;-)

[1yR4jE8b]

Well, I wouldn't say it's particularly usefull. A compiler would probably do that anyway.

It's just something that I picked up during my Assembly Language Programming course in University and the prof (who is like 1000 years old and still programs for MS DOS), was obsessed with efficiency. So we HAD to do things like this in our assignments or we'd lose marks.

so instead of writing

mov ax, 0

we HAD to write

xor ax, ax

or we'd lose marks.

So after 4 monthes of anal retentive obsession over this, quite a few of his optimization quirks have just stuck

I would NEVER write a ^= a in a higher level language code.

I actually did the XOR swap in a Java course once, and the marker had no idea why it worked lol
:P

[imi]

A compiler would probably do that anyway.

Or turn it around, if "mov ax,0" is faster *g*.


heh.. yea, well.. reminds me.. Some processors have hard-coded zero registers so there is "nothing faster than setting something to 0" anyway. (But I'm not so sure about this. maybe that's something for another cocktail party)

Ciao, Imi.