array passed as reference still doesnt modify its elements???

Discussion in 'C' started by IndiraP, Nov 19, 2012.

  1. IndiraP

    IndiraP New Member

    Joined:
    Nov 10, 2012
    Messages:
    41
    Likes Received:
    2
    Trophy Points:
    0
    void fun(int *i);
    void main( )
    {
    int gyan[] = { 10, 20, 30, 40, 50 };
    int i, *ptr ;
    ptr = gyan;
    for ( i = 0 ; i <4 ; i++ )
    {
    fun(ptr++);
    printf ( “\n%d”, *ptr ) ;
    }
    }
    void fun(int *i)
    {
    *i = *i + 1;
    }
    Output is : 20 30 40 50
    y not 11 21 31 41 since the array's reference is sent...but the modifications doesnt fall into the gyan[]..y???
     
    Last edited: Nov 19, 2012
  2. xpi0t0s

    xpi0t0s Mentor

    Joined:
    Aug 6, 2004
    Messages:
    3,009
    Likes Received:
    203
    Trophy Points:
    63
    Occupation:
    Senior Support Engineer
    Location:
    England
    Have another look at the precise location of your post-increment operator. There should be a big clue in the fact that your output doesn't include 10.
     
  3. IndiraP

    IndiraP New Member

    Joined:
    Nov 10, 2012
    Messages:
    41
    Likes Received:
    2
    Trophy Points:
    0
    ya ok sir..if i want to see the array again after the current for loop...again from 0 to 4 in another loop..y am i not getting the correct answerss??
     
  4. xpi0t0s

    xpi0t0s Mentor

    Joined:
    Aug 6, 2004
    Messages:
    3,009
    Likes Received:
    203
    Trophy Points:
    63
    Occupation:
    Senior Support Engineer
    Location:
    England
    Hmm..ok..obviously that wasn't precise enough.

    Explain to me EVERYTHING that this statement does: fun(ptr++);
     
  5. IndiraP

    IndiraP New Member

    Joined:
    Nov 10, 2012
    Messages:
    41
    Likes Received:
    2
    Trophy Points:
    0
    ok sir..here goes..
    given gyan[]={10,20,30,40,50};

    func(ptr++) when called takes ptr[0] ie., 10 to its module does 10+1=11 n returns 11 to the main but the main now points to 20 n prints it...similarly
    i=1=> 20 becomes 21 in func() n returns 21 to main() n 30 is printed by main()
    i=2=>30 becomes 31 in func() n returns 31 to main() n 40 is printed by main()
    i=3=>40 becomes 41 in func() n returns 41 to main() n 50 is printed by main()
    i=4 conditions fails n comes out of the loop n the program here terminates..

    since location of the element in the array is being sent, the modified must be present in that location of the array..??!!
    now if i did
    for(i=0;i<4;i++)
    printf("%d",*ptr++);
    i am getting some other values other than these..

    i hope wat i understood is correct about func(ptr++) .. :D
     
  6. xpi0t0s

    xpi0t0s Mentor

    Joined:
    Aug 6, 2004
    Messages:
    3,009
    Likes Received:
    203
    Trophy Points:
    63
    Occupation:
    Senior Support Engineer
    Location:
    England
    Hmm, you still can't see it. What is ptr pointing at after fun(ptr++) returns?

    When exactly do you think that post-increment takes place? If you do this as a separate statement instead of in either line, which of these do you think the existing code is equvalent to:
    Code:
    ptr++;
    fun(ptr);
    printf ( “\n%d”, *ptr ) ; 
    
    OR
    
    fun(ptr);
    ptr++;
    printf ( “\n%d”, *ptr ) ; 
    
    OR
    
    fun(ptr);
    printf ( “\n%d”, *ptr ) ; 
    ptr++;
    
     
  7. IndiraP

    IndiraP New Member

    Joined:
    Nov 10, 2012
    Messages:
    41
    Likes Received:
    2
    Trophy Points:
    0
    fun(ptr);
    printf ( “\n%d”, *ptr ) ;
    ptr++;

    this one matches it...rite sir??
     
  8. xpi0t0s

    xpi0t0s Mentor

    Joined:
    Aug 6, 2004
    Messages:
    3,009
    Likes Received:
    203
    Trophy Points:
    63
    Occupation:
    Senior Support Engineer
    Location:
    England
    Guess again.
     
  9. IndiraP

    IndiraP New Member

    Joined:
    Nov 10, 2012
    Messages:
    41
    Likes Received:
    2
    Trophy Points:
    0
    fun(ptr);
    ptr++;
    printf ( “\n%d”, *ptr ) ;


    ya..this one for sure... :| !!!!!
     
  10. xpi0t0s

    xpi0t0s Mentor

    Joined:
    Aug 6, 2004
    Messages:
    3,009
    Likes Received:
    203
    Trophy Points:
    63
    Occupation:
    Senior Support Engineer
    Location:
    England
    So have you figured out why the program doesn't do what you think?
     
  11. IndiraP

    IndiraP New Member

    Joined:
    Nov 10, 2012
    Messages:
    41
    Likes Received:
    2
    Trophy Points:
    0
  12. xpi0t0s

    xpi0t0s Mentor

    Joined:
    Aug 6, 2004
    Messages:
    3,009
    Likes Received:
    203
    Trophy Points:
    63
    Occupation:
    Senior Support Engineer
    Location:
    England
    My favourite debugging technique is to get in there with a debugger, or failing that to sprinkle printf statements liberally across the code to display variables and why we are where we are, and find out what the program is *actually* doing. If you try debugging code based on what you *think* it's doing then you'll get nowhere, because you'll always think the code is doing what you were thinking when you wrote it - hence you not realising that ++ was in the wrong place.
     
  13. IndiraP

    IndiraP New Member

    Joined:
    Nov 10, 2012
    Messages:
    41
    Likes Received:
    2
    Trophy Points:
    0
    yes sir...i will try to use printf to see wats actually happening from now on..:)
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice