sample code for comment in http://stackoverflow.com/questions/11738939/never-seen-before-c-for-loop

a_ original version.cpp

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

// "unrolled" original version
// init
u = b.size(), v=b.back();
// condition
u--;
// code
b[u] = v;
// increment
v = p[v];
// condition
u--;
// code
b[u] = v;
// increment
v = p[v];

//... until the condition fails

b_ modified version.cpp

// -- modified version --
for (u = b.size(); (u--) > 0; v = p[v])
{      
  v = b.back();    // back fetches the last element of vector in c++.
  b[u] = v;      
}

// "unrolled" modified version
// init
u = b.size();
// condition
(u--) > 0;
// code
v = b.back(); // <-- on the first iteration it's quite OK, because it really needed to be initialized.
b[u] = v;
// increment
v = p[v];
// condition
(u--) > 0;
// code
v = b.back(); // <-- but here, this statement overwrites whatever value the statement `v=p[v];` put on `v` (not to mention side-effects)
b[u] = v;
// increment
v = p[v];

// ..until condition fails