Are you smart enough?

Most highly trained and skilled professors, programmers, text books, etc. claim that to swap two variables, A and B, so that A contains B's value and B contains A's value, you need a third temporary variable, C.

Code:

---

Dim a As Integer
Dim b As Integer
Dim c As Integer

Let a = 10
Let b = 20

'
' Swap them...
'
Let c = a  ' c is 10
Let a = b  ' a is 20
Let b = c  ' b is 10

---

Now, if you guys are so smart, then let me say this: There is a way to swap the two variables WITHOUT a third variable. Can anyone figure out this little riddle?

If you want a hint, wait for a few days, and I will post it.

Let's see how clever you all are.

Probably not a very elegant solution, but here goes.

dim a as string
dim b as string

a="Hello"
b="There"

open "c:\test.txt" for random as #1
put #1, 1, a
close #1

a = b

open "c:\test.txt" for random as #1
get #1, 1, b
close #1

et voila!!!

I GOT IT AFTER LIKE 1 HOUR:

Code:

---

'I give credit to oetje for helping me with the positive and negative stuff
Private Sub Form_Load()
Dim a As Integer
Dim b As Integer

Let a = -753
Let b = 233

'Let b = a  ' 10 is 10
'Let a = b  ' b is 20

If b > a Then
b = b - a
a = a + b
b = b - a
'****help from oetje
b = b - (2 * b)
'*******************
Else
b = b - a
a = a + b
b = b - a
'****help from oetje
b = Abs(b)
'*******************
End If


Label1.Caption = a
Label2.Caption = b

End Sub

---

add 2 labels and run it to see for your self

now do I get a nobel prize award for it? or a million dollars? or even better... A Lolli POP

For some pairs of variables, Xor can be used to swap values.

Code:

---

A = A Xor B
B = A Xor B '(A Xor B) Xor B = A
A = B Xor A 'A Xor (A Xor B) = B

---

I would not be sure that the above would work if A, B are different length or different types of variables.

Code:

---

'Label1,Label2,Text1,Text2,Command!,Form1
'the only sticky part was using double to avoid overflow
'of course if you are moving words that is another story
'for another time

Private Sub Command1_Click()

    Dim a As Double
    Dim b As Double
   
   
    a = Text1.Text
    b = Text2.Text
    a = CDbl(a)
    b = CDbl(b)
    Label1.Caption = "a = " & a & "  " & "b =  " & b
    b = b * a
    a = b / a
    b = b / a
   
    Label2.Caption = "a = " & a & "  " & "b =  " & b
End Sub

Private Sub Form_Load()
    Text1.Text = ""
    Text2.Text = ""
End Sub

---

thats aalmost the same thing that I did except I didn't add textboxes

Yeah man.

GUV, you've got it right. The correct answer is use XOR.

Code:

---

Dim a As Integer
Dim b As Integer

Let a = 'anything
Let b = 'anything

Let a = a Xor b
Let b = a Xor b
Let a = a Xor b

---

I really like dimawa's solution and also the one that came from CyberSurfer.

However, although this problem has been solved: What would you do if you could NOT use Xor and no external storages, including but not limited to (heard that somewhere), clipboard, files, variables, web servers, haha.

You can still use logical operators such as AND, OR, but you CAN'T use XOR.

I didn't use any other stuff like that, no Xor and no external storages, including but not limited to (heard that somewhere), clipboard, files, variables, or web servers

I am not gonna judge you...

I haven't tried out your solution, but does it work with ANY value for a and b???

I know this was not explicitly an initial requirement, but is now, haha

Please don't hate me too much :(

1. I didn't get the inslut
2. yes it does woerk with any numerical value

OK, I admit I have still not tried it out, mainly because of time pressure, but I will.

If it turns out your solution works, then poor Guv will have to share the victory with you. (The first one), the second victory would go to you. But, I gotta check it out first. So, tomorrow, is the big day.

All you guys, keep those suggestions coming in. Surely there must be more than one simple solution to such a simple problem.

It occurred to me that for numeric variables, the following would work if roundoff/truncation does not throw a monkey wrench into the works.

Code:

---

A = A + B
B = A - B '(A+B)-B = A
A = A - B '(A+B)-A = B

---

what do you mean?

If construction of Xor from And, Or, & Not is allowed, you can do just that.

(A Or B) And Not(A And B) is equivalent to Xor.

well what you are saying is that what I made is the same thing as xor?

what is the fastest way to reverse a 30,000 character string?

this should keep you occupied.

hint, it can be done in 0.07 of a second on a P200MMX PC.

Have fun

wait wait wait, let hax soft say that I'm right first

dimava, You're right.

You are just fabulous and smart, haha :)

thanks, I KNEW I WAS CORRECT

That string reversing question is very interesting ... are we working with or without Windows API calls?

I think the following might work (I did not test it).

Code:

---

Private Sub Reverser(LongStr As String)
Dim N As Integer
Dim M As Integer
Dim A As Integer
Dim Z As Integer

N = Len(LongStr)
M = N / 2

Z = N
For A = 1 to M
        Mid(LongStr, A, 1) = Mid(LongStr,  A, 1) _
                                Mid(LongStr, Z, 1)
        Mid(LongStr, Z, 1) = Mid(LongStr,  A, 1) _
                                Mid(LongStr, Z, 1)
        Mid(LongStr, A, 1) = Mid(LongStr,  A, 1) _
                                Mid(LongStr, Z, 1)
        Z = Z - 1
    Next A

End Sub

---

I think the easyest way would be:

StringB = StrReverse(StringA)

How do you store the structure of a whole hard drive in a treeview without using a recursive loop?
No controls, type libraries, dos commands are allowed.
BTW and FYI, I don't have the answer :D

I dont think its possible without using a recursive loop, unless the FileSystemObject has a method to do it. Why would you constrain yourself to not using recursion? that would only become a problem if you had thousands of nested directories, and then call stack space would fill up fast.



[Edited by wossname on 08-27-2000 at 06:31 AM]