How to convert a string to a signed float?

Question

like :

string num = "-0.25";

how can I convert it to a signed float?

Mike Seymour · Answer 1 · 2014-03-14T16:35:16.487

2

C++11: std::stof(num) to convert to float; also stod for double and stold for long double.

Historically: std::strtod (or std::atof if you don't need to check for errors); or string streams.

edited Mar 14 '14 at 16:35

answered Mar 14 '14 at 16:27

Mike Seymour

score 1 · Answer 2 · answered Mar 14 '14 at 16:24

1

strtod() is a good bet for C.

I have no idea if C++ has other bets.

answered Mar 14 '14 at 16:24

pmg

score 1 · Answer 3 · answered Mar 14 '14 at 16:25

1

The std::stof function should work fine.

answered Mar 14 '14 at 16:25

score 1 · Answer 4 · answered Mar 14 '14 at 16:26

1

You can use istringstream:

std::string num = "-0.25";

std::istringstream iss ( num);

float f_val = 0;

iss >> f_val;

answered Mar 14 '14 at 16:26

4pie0

edtheprogrammerguy · Answer 5 · 2014-03-14T16:33:35.030

0

edited Mar 14 '14 at 16:33

answered Mar 14 '14 at 16:24

`atof()` isn't really idiomatic *C++*, and even in *C* isn't usually recommended because it lacks any facility for error-handling. – Emmet Mar 14 '14 at 16:28
@Emmet - the question was 'how to convert' and my answer will do the trick. Boo on you for downvote. – edtheprogrammerguy Mar 14 '14 at 16:29
1

The downvote was when your answer just mentioned `atof()`. What should we do when someone makes a recommendation that might work, but is the worst available option? – Emmet Mar 14 '14 at 16:36

score 0 · Answer 6 · answered Mar 14 '14 at 16:28

You can convert the string to a signed float by using the function atof. Like :

float myValue = (float)atof("0.75");

Note that you should also checked if the passed value is a valid numerical value otherwise the behaviour could be unpredictable.

There is also an other solution :

    string mystr ("1204");
    int myint;
    stringstream(mystr) >> myint;

6 Answers6