Pointers in C vs PB

[Olav Bergesen]

Hello,

The code below is a PB version of C/C++ code found at http://www.geeksforgeeks.org/write-a-c-program-to-print-all-permutations-of-a-given-string/

The problem is that the code below produce the same string for each call to permute.
Could someone shed some light on what is wrong? Thank you!
--

Code Select Expand


#Compiler PBCC 6.04
#Compile Exe
#Dim All
#Register None

'%Debug = 1&
'#Include Once "Debug.bi"
%USEMACROS = 1
#Include Once "Win32Api.inc"

'------------------------------------------------------------------------------

Function PBMain () As Long

  Local s As StringZ * 4

  s = "ABC"
  permute(s, 0, Len(s)-1)

  ? "Press a key to end the program..."
  WaitKey$

End Function
'----------------------------------------------------------------------------
'/* Function to print permutations of string
'   This function takes three parameters:
'   1. String
'   2. Starting index of the string
'   3. Ending index of the string. */

Sub permute(a As StringZ,ByVal n As long,ByVal r As long)

  Local s1 As StringZ Ptr ' * 4
  Local s2 As StringZ Ptr  ' * 4
  Local i As Byte

  s1 = VarPtr(a)
  s2 = s1

  If n = r Then
     ? @s1
  Else
   For i = n To r
     s1 = s1+n
     s2 = s2+i
     Swap s1,s2
     Call permute(a,n+1,r)
     s1 = s2+n
     s2 = s2+i
     Swap s1,s2 '; //backtrack
   Next
  End If

End Sub


[Olav Bergesen]

Works now. Has anyone any clue to why the previous version didn't produce correct result?
--

Code Select Expand


#Compiler PBCC 6.04
#Compile Exe
#Dim All
#Register None

%Debug = 1&
'#Include Once "Debug.bi"
%USEMACROS = 1
#Include Once "Win32Api.inc"

'------------------------------------------------------------------------------

Function PBMain () As Long

  Local s As String

  s = "ABC"
  permute(s, 0, Len(s)-1)

  ? "Please, press a key to end the program..."
  WaitKey$

End Function

'/* Function to print permutations of string
'   This function takes three parameters:
'   1. String
'   2. Starting index of the string
'   3. Ending index of the string. */

Sub permute(ByVal a As String,ByVal n As Long,ByVal r As Long)

  Local i As Long
  Local s1 As Byte Ptr

s1 = StrPtr(a)

  If n = r Then
     ? a
  Else
   For i = n To r
     Swap @s1[n],@s1[i] 'a+n,a+i
     Call permute(a,n+1,r)
     Swap @s1[n],@s1[i] 'a+n,a+i) '; //backtrack
   Next
  End If
End Sub



[Frederick J. Harris]

I'll look at it in just a bit Olav.

[Frederick J. Harris]

Well, this is a bit tricky...

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void swap(char* x, char* y)    /* Function to swap values at two pointers */
{
    char temp;
    temp = *x;
    *x = *y;
    *y = temp;
}


The function parameters are two char* (character pointers), which can point to arrays of chars containing from zero to 2^31 characters.  But note the local char temp variable is not a pointer to anything, but simply is a C variable type capable of only containing one single character.  When they make this assignment...

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temp = *x;


...temp will only contain the 1st character of whatever char pointer variable x points to, e.g., if x points to this string...

Code Select Expand


"ABC"


...then temp will contain just the 'A' character and nothing more. 

I took a quick look at your 2nd version which you say works, and I see your are using pointer offset notation to extract just the single character.  The closest thing in PowerBASIC to the C/C++ char is the Byte.