title: What kind of useful function will return a char taking a char and its index in a string?
body: <p>I cant explain any better so here's this prototype:</p>
  
  <pre><code>char    *my_function(char *str, char (*f)(int, char))
  </code></pre>
  
  <p>What I'm trying to figure out is what f would stand for in here, given that the int is an index and char is str[index]. For even more clarity:</p>
  
  <pre><code>f(index, str[index]);
  </code></pre>
  
  <p>What could this be apart from my crappy test f that returns str[index] + index? Is there any USEFUL function with such prototype out there?</p>