Problem with two functions in single cout

This is a discussion on Problem with two functions in single cout within the C++ Programming forums, part of the General Programming Boards category; Code: #include<iostream> #include<conio.h> using namespace std; typedef unsigned long ulong; long eeuclid(long m, long b, long *inverse){ /// eeuclid( modulus, ...

  1. #1
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    Problem with two functions in single cout

    Code:
    #include<iostream>
    #include<conio.h>
    using namespace std;
    typedef unsigned long ulong;
    
    
    long eeuclid(long m, long b, long *inverse){        /// eeuclid( modulus, num whose inv is to be found, variable to put inverse )
        /// Algorithm used from Stallings book
        long A1 = 1, A2 = 0, A3 = m,
             B1 = 0, B2 = 1, B3 = b,
             T1, T2, T3, Q;
    
             cout<<endl<<"eeuclid() started"<<endl;
    
            while(1){
                cout<<"in loop\n";
                if(B3 == 0){
                    *inverse = 0;
                    return A3;      // A3 = gcd(m,b)
                }
    
                if(B3 == 1){
                    *inverse = B2; // B2 = b^-1 mod m
                    return B3;      // A3 = gcd(m,b)
                }
    
                Q = A3/B3;
    
                T1 = A1 - Q*B1;
                T2 = A2 - Q*B2;
                T3 = A3 - Q*B3;
    
                A1 = B1; A2 = B2; A3 = B3;
                B1 = T1; B2 = T2; B3 = T3;
     /*
                cout<<"here"<<endl;
                if( getch() =='q' ) exit(0);
    //  */
           }
        cout<<endl<<"ending eeuclid() "<<endl;
    }
    
    int main(){
        long a, b, c, d=0, e, inverse = 0;
        int ch;
            cout<<"Preparing extended Euclid ---> "<<endl;
            cout<<"m >> ";
            cin >> a;
            cout<<"b >> ";
            cin >> b;
            cout<<"gcd("<<a<<","<<b<<") = "<<eeuclid(a, b, &inverse);
            cout<<" and inverse  = "<<inverse<<endl;
            cout<<endl<<"Press q to quit ... "<<endl;
        return 0;
    }
    In the above code, in the main() function, if
    Code:
    cout<<"gcd("<<a<<","<<b<<") = "<<eeuclid(a, b, &inverse);
            cout<<" and inverse  = "<<inverse<<endl;
    is replaced by
    Code:
    cout<<"gcd("<<a<<","<<b<<") = "<<eeuclid(a, b, &inverse)<<" and inverse  = "<<inverse<<endl;
    the value of inverse prints out as 0, even though single stepping through the code and examining the value in memory shows that the variable inverse has the correct value
    Why is that??

    FYC: also posted here
    Problem with two functions in single cout - Dev Shed

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by mahaju
    the value of inverse prints out as 0, even though single stepping through the code and examining the value in memory shows that the variable inverse has the correct value
    Why is that??
    The order of evaluation of a function's arguments, and also of expressions in general, is implementation defined. This allows the compiler more leeway for optimisation, but it also means that if you are not careful when you want something to be sequenced before something else, you could get unexpected results.

    In this case, your unexpected result was that the value of inverse was used before the change to inverse from calling eeuclid. Using two statements instead ensures that the call to eeuclid is sequenced before the use of the updated value of inverse.
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    What I actually don't understand is, this is code that I had completed and tested months ago
    Yesterday when I as running it again it was working fine for the first few runs
    Since this is not a very long code I usually do a rebuild and rerun of this projct every time I wasnt to run it again
    For the first few rebuilds and reruns it worked as expected
    Then suddenly this happened
    I didn't expect something like that was possible

  4. #4
    C++ Witch laserlight's Avatar
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    If you did not rebuild, the behaviour should be consistent between runs. Once you compile, the compiler may determine that the evaluation should take place differently, hence it is possible that you end up with a different result.

    By the way, SimonB2's answer in DevShed is not correct: right to left evaluation of function arguments is not guaranteed. It may be easier if you examined a statement like this:
    Code:
    a << b() << c();
    rewriting it to:
    Code:
    operator<<(operator<<(a, b()), c());
    So, the compiler can make it such that either operator<<(a, b()) or c() is evaluated first. If the former is evaluated first, then b() will be called before c(), otherwise c() will be called before b().
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  5. #5
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    I did do a rebuild
    I do a rebuild everytime I want to run it
    My problem is that, it was working all right the first 5 or so times
    Then suddenly this problem appeared
    It was solved by breaking the cout into two lines
    but I still don't understand why this happened in the first place

  6. #6
    C++ Witch laserlight's Avatar
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    I am telling you that what happened is that the compiler chose a different order of evaluation. That is all. Breaking up the single statement into two was the correct approach to avoid this problem.
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