if (a= something) means 'something' assigned to 'a' and it will return 'a' value (i.e. something) to if condition.
For i=0, if (a = cos pi*0/2) = if (a = 1) = if (a) = if (1) i.e. true
For i=1, if (a = cos pi*1/2) = if (a = 0) = if (a) = if (0) i.e false
For i=2, if (a = cos pi*2/2) = if (a = -1) = if (a) = if (-1) i.e. true
It will print 101.