Multi-conditional if statements.

This is a discussion on Multi-conditional if statements. within the C++ Programming forums, part of the General Programming Boards category; Is there a way to do them? For example: Code: if (x=1 or y=2 or z=3) { do something } ...

  1. #1
    Devil's Advocate SlyMaelstrom's Avatar
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    Multi-conditional if statements.

    Is there a way to do them?

    For example:

    Code:
    if (x=1 or y=2 or z=3)
    {
    do something
    }
    else
    {
    return=0;
    }
    Thanks for any help you can give.

  2. #2
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    Code:
    if (x==1 || y==2 ||  z==3)
    {
    do something
    }
    else
    {
    return 0;
    }
    You use || as or, && is and.

  3. #3
    Registered User major_small's Avatar
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    also, know that && (AND) is figured before || (OR)

    for example: if(true && false || true) evaluates to true

    there are also other operators, like the ! (NOT) operator. it simply negates the value. for example, if(!true) evaluates to false.

    you can also use parenthesis. I assume you know how parenthesis mess with the order of operations in arithmetic, so all I'm going to say is it's essentially the same for logic.
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  4. #4
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    be careful though

    c++ uses short circuit evaluation

    take for example
    Code:
    bool thisIsTrue = true;
    if(thisIsTrue || ImportantFunction())
    {
        .........
    }

    when it sees the || it knows that either the left operand (thisIsTrue) or the right operand (the return value of ImportantFunction()), must be true in order to execute the code within the if{} block. since "thisIsTrue" is in fact true, there is no need to evaluate the rest of the expression and does not call "ImportantFunction()".



    don't get cute with that piece of information either and do
    Code:
    //bad
    if(x || func())
    in place of
    Code:
    //good
    if(!x)
       func();
    Last edited by misplaced; 03-23-2005 at 03:30 AM.
    i seem to have GCC 3.3.4
    But how do i start it?
    I dont have a menu for it or anything.

  5. #5
    Devil's Advocate SlyMaelstrom's Avatar
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    Thanks for this info guys.

    I saw info exactly like this on the internet, except it was refering to PHP. The statements looked exactly like C++, but I wanted to ask just to be sure.

  6. #6
    Devil's Advocate SlyMaelstrom's Avatar
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    Also... since I'm thinking about things I've seen in other languages.

    In strings, can you make an if statement that looks to see if the user's input is contained in your criteria but isn't exactly your criteria?

    For instance, if my statement is looking for the user to say "North" but they choose to say "Nor" or "N" it would accept that so long as it doesn't interfere with other criteria.

  7. #7
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    Quote Originally Posted by major_small
    also, know that && (AND) is figured before || (OR)

    for example: if(true && false || true) evaluates to true
    That example evaluates to true regardless of which operator has precedence.

    ( (true && false) || true ) == true
    ( true && (false || true) ) == true

  8. #8
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    If you use -Wall (as you really should) when compiling the compiler will warn you if you forgot the second = in the confrontation.
    Also, if you are afraid to make mistakes, just use the constant in front:
    if (0 == p)
    so, if you forgot a equal the compiler will tell that the assignement is not valid.

  9. #9
    and the hat of wrongness Salem's Avatar
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    > if (0 == p)
    Oddly, it seems easier to remember to use == than remember to swap the variables round.

    Not only is p == 0 the natural way of reading it, because you've remembered that it should be == and not =, you DON'T have a problem when you try and do something like
    if ( p == q )

    Rearranging it to
    if ( q == p )
    doesn't help you because both can be assigned and your syntactic crutch to stop you falling over no longer works as it should.
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  10. #10
    Carnivore ('-'v) Hunter2's Avatar
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    In strings, can you make an if statement that looks to see if the user's input is contained in your criteria but isn't exactly your criteria?
    Assuming you're talking about std::string and not char[]:
    Code:
    if(str.find("No") != std::string::npos)
       //it's contained.
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  11. #11
    & the hat of GPL slaying Thantos's Avatar
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    also, know that && (AND) is figured before || (OR)
    NOT true. The order of evaluation is always left to right in a boolean statement. && binds the results tighter then ||.

    So if you have:
    Code:
    if ( x==5 || y==2 && z==7)
    the x==5 is evaluated before the rest of it.

  12. #12
    Registered User major_small's Avatar
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    Quote Originally Posted by Thantos
    NOT true. The order of evaluation is always left to right in a boolean statement. && binds the results tighter then ||.

    So if you have:
    Code:
    if ( x==5 || y==2 && z==7)
    the x==5 is evaluated before the rest of it.
    I think you misunderstood what I meant. I meant that && has a higher precedence than ||. bad wording on my part. and a bad example as well. (thanks to skiingwiz for pointing that out). with that in mind, here's a better example:

    if(false && true || true) evalutates to true
    if((false && true) || true) evaluates to true
    if(false && (true || false)) evaluates to false


    C operator precedence:
    http://msdn.microsoft.com/library/de...m/expre_13.asp
    Last edited by major_small; 03-23-2005 at 11:06 PM. Reason: linkage
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  13. #13
    Carnivore ('-'v) Hunter2's Avatar
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    Still a bad example This also demonstrates the left-to-right evaluation order (evaluates the && first, because it is the leftmost operator).

    **EDIT**
    Hmm, I can't seem to come up with an example that will make a distinction between the two. Could it be that the two will always just give you the same result? In which case, the only real way to find out the order of evaluation would be to create bool functions that give output when they are called.
    Last edited by Hunter2; 03-23-2005 at 11:16 PM.
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  14. #14
    Registered User major_small's Avatar
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    Quote Originally Posted by Hunter2
    Still a bad example This also demonstrates the left-to-right evaluation order (evaluates the && first, because it is the leftmost operator).
    fine, how's this:

    if(!(!A && B || A && !B))

    jeebus... and 10 points to anybody whow knows what that gate's called.

    edit: and another 20 points to the person who brings me the head of the person that gives me bad rep for trying to be helpful.
    Last edited by major_small; 03-23-2005 at 11:48 PM.
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  15. #15
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    just a little note to circuit evaluation:
    it makes it possible to write code like:

    Code:
    int *p;
    /* code */
    if(NULL != p && 1 == *p) {
    
    }
    without circuit evaluation (1 == *p) would be evaluated even though p could be NULL.
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