Why does the increment/decrement operators break the recursion functionality

Question

func(int n)
// the function should return the sum of the first n term of the 
// harmonic series (1/1 + 1/2 + 1/3 + ... + 1/n )

double sumover(int n)
{
    if (n == 0)
        return 0;
    else
    {
        return (1. / n) + sumover(--n);  // here is the bug
    }
}

When the function is called with n = 1, I am expecting it to compute 1. / 1 + sumover(0) = 1 / 1 + 0

However, it's computing 1./0 + sumover(0) , why?

This is also undefined behaviour (the assignment in `--n` is unsequenced relative to the read in `1. / n`) — M.M, Apr 03 '20 at 04:46

asmmo · Accepted Answer · 2020-04-03T04:18:45.097

3

return (1. / n) + sumover(--n);

There is no guarantee that the term (1. / n) will be calculated before sumover(--n).

The standard doesn't specify that.

Hence the second term may be calculated firstly, then (1. / n) becomes (1. /(n - 1)) and then you get an unexpected output.

Replace it by

return (1. / n) + sumover(n - 1);

edited Apr 03 '20 at 04:18

answered Apr 03 '20 at 04:11

asmmo

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Thanks. Your reply was helpful ! – Adnan Mohamed Apr 04 '20 at 14:51
I did. It's my first time asking questions in stackoverFlow, thanks for letting me know that I could do this. – Adnan Mohamed Apr 07 '20 at 03:52
@AdnanMohamed U R welcome – asmmo Apr 07 '20 at 06:56

Why does the increment/decrement operators break the recursion functionality

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