hi. 
i have a vector a that is [1:5]' and also i have a string 'hi'. (i just use a of length of 5 for simplicity, but can be large)
(1) how can i generate a \_matrix\_ b in such a way

 hi 1
 hi 2
 hi 3
 hi 4
 hi 5

(2) how can i convert matrix b to a 'single' \_string\_ c as it is (i.e. after each hi there is a space and after each number there is an enter (mew line character). in other words string c is

 hi 1
 hi 2
 hi 3
 hi 4
 hi 5

note that bi is a matrix or cell array, but c is only a string.
generate a single string from a matrix (or cell)
with r2017a or later,

 strjoin( "hi " + (1:5).', newline )

for r2016b you can use

 strjoin( string('hi ') + (1:5).', newline )