Thread: a little puzzle

  1. #1
    Registered User
    Join Date
    Mar 2002

    a little puzzle

    here's an interesting little puzzle I found on another board:

    given the following sequence:

    int a=0, b=0, c=0, d=0;
    b = ( a++, a++, 6+a );
    d = ( c++, 6+c, c++ );

    cout << "a = " << a << endl;
    cout << "b = " << b << endl;
    cout << "c = " << c << endl;
    cout << "d = " << d << endl;

    fill in the expected output for each variable below:
    a =
    b =
    c =
    d =

    and explain your reasoning. Indicate any possible non-defined behaviors but assume the behaviors occur as expected.

  2. #2
    &TH of undefined behavior Fordy's Avatar
    Join Date
    Aug 2001
    a = 2
    b = 8
    c = 2
    d = 1

    It's due to the comma operator....this operator evaluates the left, but then discards the value....only the furthest right value is assigned in this case.

    For "b = ( a++, a++, 6+a );" nothing is assigned from the 2 "a++" unary operations, but a is incremented each time. So the assignment is "b = 6+2" which is 8 and at that point, a is 2

    For "d = ( c++, 6+c, c++ ); " , c is first incremented, but the middle evaluation is discarded (and it doesnt change the value of c). So all that is assigned is "d = c++" and at that point, c has been incremented once (postfix increment operator does not take effect until after the assignment). So d is 1 and c is 2.

    Nice little puzzle....but if you understand the comma operator it's fairly easy to understand

  3. #3
    Registered User
    Join Date
    Mar 2002
    Excellent analysis and explanation, maybe even better than on the other board!

    I thought the little details in terms of the effect of the comma operator (I'd never seen a comma used like that before) and the difference in effect of the increment operator versus the addition operator on the variable it was associated with (all of which you explained wonderfully) was interesting.

  4. #4
    Senior Member joshdick's Avatar
    Join Date
    Nov 2002
    Phildelphia, PA
    Thanks for clearing that up for me, Fordy.

    "The computer programmer is a creator of universes for which he alone is responsible. Universes of virtually unlimited complexity can be created in the form of computer programs." -- Joseph Weizenbaum.

    "If you cannot grok the overall structure of a program while taking a shower, you are not ready to code it." -- Richard Pattis.

  5. #5
    Registered User major_small's Avatar
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    May 2003
    that's interesting... i've never seen that done before...
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