Is this a function?

Dear Student

Let A and B are two non-empty sets. A function ‘f’ from sets A to set B is a rule or correspondence which associates elements of sets A to elements of set B such that
(a) all elements of set A are associated to elements in set B
(b) an element of set A is associated to a unique element in set B.
 
The function f  from A to B is denoted by f: A → B. Here, A is the domain and B is the co-domain of f.
And as per the above definition we can conclude that given relation is not a function as all elements of A are not associated with B

Regards