String to long double conversion in c++98

Question

Anyone know how to convert a string to long double? For example lets say we have

string S = "3.236568949"

I want to take what inside the string and convert it to a long double variable x.

Note: In c++11 their exist stold function do so but I'm using c++98 not c++11. Also I don't need to cout the string with certain precision because I know setprecision function already but what I want is just to convert to long double. In other wards,I need x to equal the exact number inside the string and not an approximated version of the number. Finally I want to tell you that my string contains a valid number.

there's no way to get the exact value unless you're using a decimal floating-point type — phuclv, May 05 '17 at 17:15

score 0 · Answer 1 · answered May 05 '17 at 16:57

0

you can use the C function :

#include <stdlib.h>
int main(){
   char* convertme="3.236568949";
   double converted=atof(convertme);
   return 0;
}

answered May 05 '17 at 16:57

GhaziBenDahmane

548
3
15

OP wants `long double`, `atof()` returns `double` – Slava May 05 '17 at 16:59
The OP wants a `long double` – NathanOliver May 05 '17 at 16:59

score 0 · Answer 2 · edited May 23 '17 at 12:02

0

You can use a std::stringstream to convert the string into a long double. The std::stringstream acts just like cin but instead of getting the input from the user you load it into the stream with a std::string. That looks like

std::string data = "3.236568949"
std::stringstream ss(data);
long double ld;
ss >> ld;

Do note that when dealing with floating point numbers you are going to have to deal with imprecision. For more on this see Is floating point math broken?

edited May 23 '17 at 12:02

Community

1
1

answered May 05 '17 at 17:02

NathanOliver

171,901
28
288
402

your code wokrs very well , can i select certain percision for ld ? – user288086 May 05 '17 at 17:25
@user288086 You cannot set the precision of the output but you can specify precision for things you input into the `stringstream`. – NathanOliver May 05 '17 at 17:27
@user288086 Just like you would using `cout`. – NathanOliver May 05 '17 at 17:37
tell me how please. – user288086 May 05 '17 at 17:43
@user288086 Well, you could extract a substring from the original string with only the desired figures or do something like [this](https://ideone.com/9w8srR), but given the nature of floating point representation (please, read the Q&A linked by NathanOliver and the mentioned [doc](http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html)) it seems quite an exercise in futility. – Bob__ May 05 '17 at 18:53

score -1 · Answer 3 · answered May 05 '17 at 17:00

-1

You can use strtold() function from C99:

long double d = strtold( str.c_str(), nullptr );

to convert std::string to long double. Though floating point representation is approximate so nobody would guarantee that value after conversion is exactly the same is in your string.

answered May 05 '17 at 17:00

Slava

43,454
1
47
90

1

This function is only available in C++ in C++11. The OP needs a C++98 solution. – NathanOliver May 05 '17 at 17:03
This function is avaiilable in C99 – Slava May 05 '17 at 17:04
@NathanOliver Actually I think it's C. – May 05 '17 at 17:04
3

Being in C99 does not mean it is part of C++98 – NathanOliver May 05 '17 at 17:04
It does not mean that C99 is unavailable either – Slava May 05 '17 at 17:05
The OP clearly wants a C++98 answer. I fail to see why giving them a C99 answer helps them. – NathanOliver May 05 '17 at 17:06
C++98 is orthogonal to C99 and the fact that this solution requires C99 I mentioned in my answer. OP may use old compiler which only supports C98 or required by company policy or whatever not to enable C++11. – Slava May 05 '17 at 17:07

String to long double conversion in c++98

3 Answers3