How do you read in a double from a file in C++?
For ints I know you can use the getline() and then atoi, but I am not finding an array to double function. What is available for reading in doubles, or converting a char array to a double?
+4
A:
You can use stream extraction:
std::ifstream ifs(...);
double d;
ifs >> d;
This work provided that other then whitespace, the next data in the stream should be a double in textual representation.
After the extraction, you can check the state of the stream to see if there were errors:
ifs >> d;
if (!ifs)
{
// the double extraction failed
}
R Samuel Klatchko
2010-04-10 21:36:47
This is wrong. Test the stream for failure (`.fail()`, which is subtly different than `.good()`, which is equivalent to `.rdstate() == 0`). Use `if (!(ifs >> d)) {/*failed*/}` or `ifs >> d; if (!ifs) {/*..*/}` or (the generally more useful) `if (ifs >> d) {/*use d*/} else {/*failed*/}`.
Roger Pate
2010-04-10 21:47:08
(Using `!stream` is the same as `stream.fail()`, using `bool(stream)` is the same as `!stream.fail()`.)
Roger Pate
2010-04-10 21:53:40
@RogerPate - thank you.
R Samuel Klatchko
2010-04-11 02:15:06
@wilhelmtell: It's much more effective to stick to the technical merits, because it is perfectly standard C++. "Proper" depends on *many* subjective and situational factors.
Roger Pate
2010-04-10 22:06:28
@Roger by improper i mean it's nasty. you practically need to do some parsing of the string yourself if you want to check `atof()` worked correctly. The question asked about reading a double from a file in C++, and this just isn't the right answer. it isn't a good advice even if the question would be about extracting a string from a stream.
wilhelmtell
2010-04-10 22:21:53
@wilhelmtell: It's not good advice for this question, but that doesn't make atof "improper C++" at all.
Roger Pate
2010-04-10 22:52:53
+2
A:
Do not consider using atof(), or any of the ato.. functions, as they do not allow you to diagnose errors. Take a look at strtod and strtol. Or use the stream extraction operators.
anon
2010-04-10 21:44:15
@sblom Is it the grammar you object to? If so, please suggest improvements.
anon
2010-04-10 22:47:37
wilhelmtell
2010-04-11 02:40:33
A:
You can leverage istringstream For example, here are toDouble and toInt:
double toDouble(string s) {
double r = 0;
istringstream ss(s);
ss >> r;
return r;
}
int toInt(string s) {
int r=0;
istringstream ss(s);
ss >> r;
return r;
}
Mohamed Mansour
2010-04-10 21:49:10
This already exists in boost, in a much better form including error checking, as lexical_cast.
Roger Pate
2010-04-10 21:51:55
@Roger Pate: Yes, it does. This is useful though for cases where you can't use boost.
Billy ONeal
2010-04-10 21:56:14
@Billy: It is no more useful than atof, which Neil correctly [points out](http://stackoverflow.com/questions/2615078/how-to-read-in-a-double-from-a-file-in-c/2615119#2615119) should be avoided (unless you've already validated the input, but that's not true here).
Roger Pate
2010-04-10 21:59:50
(@Billy: Since you got a mysterious downvote on your now-deleted answer, I feel compelled to say that wasn't me, nor have I downvoted any answer here for merely suggesting atof or omitting error handling. I've been out of votes for the past 18 hours and have my next 30 already bookmarked for 0:01 GMT too. :P)
Roger Pate
2010-04-10 22:03:59
@Roger Pate: No problem. Reason I deleted it is because I considered your comment and I agree ;)
Billy ONeal
2010-04-10 22:07:59
+1
A:
I'm wondering, does one need to be careful about locale settings (e.g. a locale could use comma instead of dot to separate the decimal part) or do stringstreams always default to some standard "C locale" notation?
bohan
2010-04-11 03:19:08