It is not entirely clear what this question means. I am going to assume that 'value' of a string refers to a numerical value, i.e. the strings represent numbers. The question also does not mention the programming language to use. There is a function in Delphi:
function isGreater(number1, number2: string): boolean;
begin
Return StrToFloat(number1)>StrToFloat(number2);
end;