What's the real use of using n[c-'0']?

Question

I\'m a novice in C and I came across the code like this :

int n[10];
if(c>=\'0\' && c<=\'9\')
++n[c-\'0\']

In if

Answer 1

It's a way to convert the ascii value of something to its number. In C the ascii value of the char '0' is 48. So subtracting:

'0' - '0' = 0
'1' - '0' = 1
 ...
 c - '0' = 

Conveniently the ASCII decimal number increments are consecutive otherwise this trick won't work. In other words c has to be one of '0'..'9' for this to work. This explains the restriction:

if(c>='0' && c<='9')