NOISE AND DATA

Suggestion needed

I have a set of noiseless data , x= [1 2 3 4 5 6 ].

I want to add noise to it using 

g= x+ 0.5* rand(1,6)

How do i calculate the signal to noise ratio of this signal if the
variance of the noise is 0.5.I know the relationship between SNR and
variance, mathematicallly and can implement that in MATLAB. Is there any
inbuilt function that can calculate that easily ?

The second scenario is that i have a signal

d= [ 5 6 7 8 9], no other information than the signal contains noise. How
do i get the SNR of this signal.