What does a comma mean inside an 'if' statement? [duplicate]

Question

Consider:

for (auto i = 0; i < g.size(); ++i)     for (auto j = 0; j < g.size(); ++j) if (g[i][j] == 0) dfs(g, i, j), ++regions; return regions; 

I don't like one line code. What does the code execute in the if()?

I am confused by the "," sign.

Usually I would write it as:

  for (auto i = 0; i < g.size(); ++i)   {       for (auto j = 0; j < g.size(); ++j)       {           if (g[i][j] == 0)           {              dfs(g, i, j)           }           ,++regions; // I am not sure what to do here. Inside the "if" scope??       } }    return regions; 

Answer 1

The programmer has used the comma operator to provide two unrelated expressions in a single statement. Because it's a single statement, both expressions are "inside" the if condition.

It's a poor hack, which would be better done with actual {} braces surrounding two statements.

Your example is not equivalent; it should be:

if (g[i][j] == 0)  {    dfs(g, i, j);    ++regions; }