Never seen before C++ for loop

Question

I was converting a C++ algorithm to C#. I came across this for loop:

for (u = b.size(), v = b.back(); u--; v = p[v]) 
b[u] = v;

It gives no

Answer 1

for (u = b.size(), v = b.back(); u--; v = p[v]) 
   b[u] = v;

In above code, u and v are initialized with b.size() and b.back().

Every time the condition is checked, it executes decrement statement too i.e. u--.

The for loop will exit when u will become 0.