Question about --

Question

int a=20, b = 20;
printf("%d\t%d\t%d\n", ++a, a++, --a);//why --a is 21
printf("%d\t%d\t%d\n", ++b, b++, b--);//but b-- is 20?

Answer 1

Avoid mixing data access and modification in a single execution point. Output of that code depends on compiler implementation. Output in one compiler may differ from that of another.

Answer 2

You need to familiar about Pre and Post increment/decrement in C

++X = It means it increment the initialised value of x by 1 then use in any other expression

X++ = It means it use value of x in expression then increment it by 1

--X = It means it decrement the value of x by 1 then uses it in any equation.

X-- = It means it uses the value of x by in any equation then decrement the value of X

I hope it helps, and you get the output of your code.