C : #define usage [duplicate]

Answer 1

Consider what this expands to

MAXLEN * 10 == MAX+1 * 10

which, when you consider operator precedence gives

MAX + (1 * 10) == 10 + (1 * 10)
               == 10 + 10
               == 20

You probably wanted

#define MAXLEN (MAX+1)

instead.

Answer 2

Since MAX is 10, MAXLEN is 10+1, and you print 10+1*10 which is 20.

Macro expansion substitutes the strings verbatim and that's why you must use parenthesis when defining them. For example:

#define MAXLEN (MAX+1)

Answer 3

MAXLEN * 10 => MAX + 1 * 10 => 10 + (1 * 10) => 20