c++assignment-operatorcompound-assignment

Compound assignment in C++

I would like to know the execution flow of compound assignments in C++. I came across a CodeChef question, where I am calculating NCR mod p values and adding them together to get the final answer:

// correct
for(int i=min1; i<=max1; i+=2){
     ans = (ans+ncr_mod_p(n,i))%mod;
}

// incorrect
for(int i=min1; i<=max1; i+=2){
     ans+=ncr_mod_p(n,i)%mod;
}

This is happening because of integer overflow.

So, what is the execution sequence of compound assignment?

Let's say, if we have an equation a+=b%c then what would be the execution sequence:

a = (a+b)%c
// OR
a = a+(b)%c;
Solution

  • This statement

    ans+=ncr_mod_p(n,i)%mod;

    is equivalent to the statement

    ans = ans + ( ncr_mod_p(n,i)%mod );

    As you can see it differs from the statement

    ans = (ans+ncr_mod_p(n,i))%mod;

    From the C++ 14 Standard (5.18 Assignment and compound assignment operators)

    7 The behavior of an expression of the form E1 op = E2 is equivalent to E1 = E1 op E2 except that E1 is evaluated only once. In += and -=, E1 shall either have arithmetic type or be a pointer to a possibly cv-qualified completely-defined object type. In all other cases, E1 shall have arithmetic type.