i will try
suppose you want to travel by train, you want to go delhi junction from some location assume bareilly junction, but you purchased a ticket for pilibhit junction.
when you enter the train there is one machine which checks your ticket
machine has some rules:
1) ticket is valid means it checks itz time and date////it should not be of old date
2) ticket is of train /////it should not be platform ticket
....etc
now you inserted your ticket in machine the machine verified it and found correct..and let you enter inside the train..machine doesn't know your intention i.e you want go to different location but you purchased a ticket for different location .
let it refer to our question
consider you a wrote function that calculates the number of hours you study daily for (assume for 1 week analysis only ) gate exam:
your formula was
gate17=(m*t*we*th*fri*sat*sun)///m , t ...sun represents daily hours
after running program u got wrong output becoz ur formula was wrong instead of adding daily hours u just multiplied the hours. but compiler wont produce error becoz it does'nt voilate any semantic rules...and it don't know ur intention ..but still the meaning of program is wrong then who will detect error.
now consider by purchasing wrong ticket u are going to right destination that actually u want to go..here machine cannot check than who will check ...the ministry of railways has posted a ticket checker who will check your ticket and can catch u and even insert u inside the jail.
like wise here also programmer has to himself detect error as the ticket checker did it explicitly.
now what is need of having it ....if it does not detect errors.
answer is the line "The compiler does not detect semantic errors, because they don't violate C rules" does not mean that it will not detect all the semantic errors....there are various semantic errors which are detected..becoz they violate various rules...- say using float numbers for % operator produces semantic error in C.
as a machine checks the ticket and if it finds that a ticket is of platform it won't enter u inside the train.....as we cannot remove machine we cannot also remove semantic phase..
