C++ Primer 5 Edition: Stream iterators and Sales_item

In C++ primer 5Ed the chapter talks about stream iterators he gives this example (page 512):

int main(){

    istream_iterator<Sales_item> item_iter(cin), eof;
    ostream_iterator<Sales_item> out_iter(cout, "\n");
    // store the first transaction in sum and read the next record
    Sales_item sum = *item_iter++;
    while (item_iter != eof) {
        // if the current transaction (which is stored in item_iter) has the same ISBN
        if (item_iter->isbn() == sum.isbn())
            sum += *item_iter++; // add it to sum and read the next
        transaction
        else {
            out_iter = sum; // write the current sum
            sum = *item_iter++; // read the next transaction
        }
    }
    out_iter = sum; // remember to print the last set of records

}

It works fine but I've noticed that inside the loop it does the same thing twice in if-statement:

if (item_iter->isbn() == sum.isbn())
    sum += *item_iter++; // add it to sum and read the next transaction
else {
    out_iter = sum; // write the current sum
    sum = *item_iter++; // read the next transaction
}

Why shouldn't he optimized it to only use the de-reference and increment in a single statement? So Only when records are different print and whatever the condition increment and de-reference:

while (item_iter != eof) {
    if (item_iter->isbn() != sum.isbn()) // instead of check for equality
        out_iter = sum; // write the current sum
    sum = *item_iter++; // read the next transaction
}

Is this a good idea or should I stick to the example of the book? Thank you?

Please stick to the example of the book. You did not check the statements exactly.

In

if (item_iter->isbn() == sum.isbn())
    sum += *item_iter++; // add it to sum and read the next transaction
else {
    out_iter = sum; // write the current sum
    sum = *item_iter++; // read the next transaction
}

the statements are different.

Please see: sum += and sum =

Maybe you misinterpretetd that.

No big deal.