How to check if a string is a number?
Solution 1:
Forget about ASCII code checks, use isdigit or isnumber (see man isnumber). The first function checks whether the character is 0–9, the second one also accepts various other number characters depending on the current locale.
There may even be better functions to do the check – the important lesson is that this is a bit more complex than it looks, because the precise definition of a “number string” depends on the particular locale and the string encoding.
Solution 2:
if(tmp[j] >= '0' && tmp[j] <= '9') // should do the trick