C : For two different functions with do-while loops, why does x =y give the same result as x=x y in-CodePudding

For function A below, I get a different result when I use est = XXX as compared to using est = est XXX. The former gives a result of 1.33227e-15 while the latter gives a result of 8.88178e-16.

On the other hand, for function B below, I get the same result regardless of whether I use est = XXX or est = est XXX.

Would anyone be able to explain why x =y is equivalent to x=x y in function B but not in A?

function A

double A(int nTerm){
    const double PI = 3.141592653589793238463;    
    double est = 0;
    double counter = 0;

    do {
        est  = ( 16 * pow(-1,counter) )/ (2*counter 1) * pow((double)1/5, 2*counter 1) 
             - ( 4 * pow(-1,counter) )/ (2*counter 1) * pow((double)1/239, 2*counter 1);
        counter  ;  
    } while (counter<=nTerm);
    
    return est-PI;
}

function B

double B(int nTerm){
    double est = 0;
    double counter = 0;

    do {
        est  = counter;
        counter  ;  
    } while (counter<=nTerm);

    return est;
}

CodePudding user response：

x = y - z is the equivalent to x = x ( y - z ). You likely wrote x = x y - z. You need to enforce precedence. See here that with the brackets, the return values are the same.

In your case, you want:

est = est   ( ( 16 * pow(-1,counter) )/ (2*counter 1) * pow((double)1/5, 2*counter 1) 
             - ( 4 * pow(-1,counter) )/ (2*counter 1) * pow((double)1/239, 2*counter 1) );

Note: when dealing with double values, A B - C can be very different than A ( B - C ) even though they should be the same. See Is floating point math broken?