Cleaning the "n.a" in my dataset using Stata

[Mo Hos]

To replace the missing values "n.a" to "", I used the following code:

ds, has(type string)
quietly foreach v in `r(varlist)'{
replace `v' = "" if inlist(`v', "n.a")
}

However, after using such code, the numbers are still in red/string format. How can I change all string variables to numeric variables so that Stata can understand them?
Note: I have a lot of variables with different names (e.g. age, number of qualifications).

[Nick Cox]

help destring

[Leonardo Guizzetti]

In addition to Nick's helpful advice, if you simply wish to covert strings of numeric values to numbers, and convert anything with text into a missing value, then you can do the following.

gen double new_var = real(var)

[Mo Hos]

Thanks.