Good points. What about multi-threading? Isn't it possible that another thread entirely reassigns "chr" to something different, while the for loop is running?

Good points. What about multi-threading? Isn't it possible that another thread entirely reassigns "chr" to something different, while the for loop is running? Obviously this would only happen in a pathological program written purely for perpetrating evil on unsuspecting programmers :) But seriously, it's possible, and it's the kind of thing optimizing compilers have to take into consideration.