Mathematica, convert string to number

Question

I imported a table in mathematica using Import["..file","table"] now I cant use the numbers in the table as they are in string formta, can someone please explain how can i convert them from string to number again?

Answer 1

Another approach is to use Read. This is useful if you have numbers in "E" form scientific notation:

    Read[StringToStream[#], Number] &/@{ "1" ,"1.5" , "1E-20" , "2.E10" }

{1, 1.5, 1.5*10^-20, 2.*10^10}

Note ToExpression gets these wrong:

    ToExpression /@ {"1", "1.5", "1.5E-20", "2.E10"}

{1, 1.5, -15.9226, 2. E10}

"1.5E-20" is evaluated as 1.5 * 2.71828-20 in the last case the "E10" is taken as a new symbol..

ToExpression is however faster if you can use it..

Edit...per comment StringToStream leaves the streams open. This closes the streams:

 stringToNumber[s_String] := 
         Module[{str = StringToStream@s, n}, n = Read[str, Number]; Close@str; n]
 stringToNumber /@ {"1", "1.5", "1E-20", "2.E10"}

Answer 2

The intrinsic function ToExpression will convert its argument to an expression; if the argument is the string representation of a number the function will return the number.

These days (early 2023) there are other ways to convert a number string to a number. For example

Interpreter["Number"]["4.1234"]

ought to return the number 4.1234, and ought to fail when the string can't be parsed as a number. This approach makes it more difficult for arbitrary code to execute.