libpq SELECT loop and UPDATE within the loop

Question

I have a C++ program that does something like the following:

PGresult* res = PQexec(conn, "SELECT id, foo FROM tbl WHERE some_condition");
for (int r = 0; r < PQntuples(res); ++r) {
    auto id = PQgetvalue(res, r, 0);
    auto foo = PQgetvalue(res, r, 1);
    cout << "Processing: " << foo << endl;
    // process foo ...
    res = PQexecParams(conn, "UPDATE tbl SET bar = $1 WHERE id = $2", args...);
}

If I comment out the PQexecParams line, and say, there are three rows that match the some_condition, then I see three "Processing" lines on stdout. However, with the PQexecParams in there, only the first row gets processed and updated and the loop is then exited.

I believe this is because there is some interaction between UPDATE within a SELECT loop that basically cuts off the loop after the first UPDATE.

[As pointed out by @jjanes, the problem was my stoopid mistake of using the same variable name for the result of the SELECT and the UPDATE].