extract an integer from a string

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1.00/5 (1 vote)

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VS2010

hi !

i want to extract an integer from a string but i dont know how, for example if i have the folowing string :

C++

"100Mbps"

i just want to get as a result : 100

thank you for your help :)

Posted 18-Aug-13 15:33pm

Manel1989

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Volynsky Alex · Accepted Answer · 2013-08-19T08:41:00

Solution 2

You can use this code:

C++

std::string num ("");
std::string str ("100Mbps");

for (int i=0; i < str.length(); i++) {
    char c = str.at(i);
    if (c >= '0' && c <= '9')
    {
       num += c; // or num.push_back(c) or num.append(c);
    }
}

Good luck.
Alex.

Posted 19-Aug-13 8:41am

Volynsky Alex

Ron Beyer · Accepted Answer · 2013-08-18T15:35:00

Solution 1

strtol[^]

I thought there was also a strtoi, but I can't seem to find it, maybe its not part of the standard library.

Posted 18-Aug-13 15:35pm

Ron Beyer

Updated 18-Aug-13 15:39pm

v2

Comments

Ron Beyer 18-Aug-13 21:42pm

Was atoi, but seems to be depreciated and replaced by strtol.

Sergey Alexandrovich Kryukov 18-Aug-13 21:57pm

By the way, all those functions is total and complete lame.

Documentation says that return value is 0 in case of invalid input format, and, specifically "No-throw guarantee: this function never throws exceptions". The real translation of this politically correct specification means: "developers of this function did not have a clue what the exception is".

This way, the input "0" and "unknown trash" will give identical result.
"#include" forever! :-)

Your answer is 5ed. :-)

—SA

Ron Beyer 18-Aug-13 22:12pm

I certainly don't agree with the "never throw guarantee" with most code, if I give it a bad value, it needs to tell me, not just return a valid value for invalid input. Thanks!

Klaus-Werner Konrad 19-Aug-13 7:29am

"0" will give you an other value in *endptr (*endptr == (ptr+1)) than "trash" (*endptr==ptr)

Sergey Alexandrovich Kryukov 19-Aug-13 10:05am

Good point, thank you, Claus-Werner. However, this is still lame.
—SA

Klaus-Werner Konrad 21-Aug-13 17:39pm

Hi Sergey, I don't know exactly why you think it's lame, 'cause it is from the standard C library, and C don't know exceptions.
The only other passibility would be a additional pointer to the result so the function could return TRUE or FALSE

Sergey Alexandrovich Kryukov 21-Aug-13 18:46pm

Exceptions or not, it does not matter. Yes, additional pointer or whatever. Why do you think I should respect lame if it is "standard"? It's defaulting to 0 on failure to parse. How else should I called if not lame? If a job candidate makes such mistake, we consider it as a serious fault...
—SA

Klaus-Werner Konrad 22-Aug-13 17:52pm

Well - it returns an integer value, so to not exclude any possible value in the range there is no possibility to return an error value.
You haven't said WHY you think it's lame - THAT was my question ...

Sergey Alexandrovich Kryukov 22-Aug-13 17:59pm

I though it's obvious: the problems with telling the difference between 0 and invalid string. As you explained, it could be done by the position from endptr, good point, but it is not even clearly documented, if you look at the link...
—SA

Klaus-Werner Konrad 22-Aug-13 18:32pm

Well - for me it wasn't obvious, 'cause you never mentioned the link ...
If I come accross such a question, I Google first and follow multiple links. In this case, the third reference led to http://linux.die.net/man/3/strtol
and there it states
"If endptr is not NULL, strtol() stores the address of the first invalid character in *endptr. If there were no digits at all, strtol() stores the original value of nptr in *endptr (and returns 0)."
So, IMHO it's not the function that's lame, but the docuzmentation from the original link's site.

Sergey Alexandrovich Kryukov 22-Aug-13 18:46pm

Yes, that's reasonable, come to think about. And "no digits at all", logically, should be interpreted as failure to parse. But it still leaves not 100% what to do it there are some digits, but parsing still fails due to invalid format. By the way, if you can explain it as you know it, I would appreciate it.

You see, I don't question your knowledge and understanding and think they are very good; we are arguing only on relative values of styles and requirements to APIs and code. This API is still much better than, say, atoi:
http://www.cplusplus.com/reference/cstdlib/atoi/

In my opinion, strtol could be considered at least questionable, by the reasons I explained (I can admit that it is not completely useless :-), but would you argue that atoi isn't a lame? (Sorry for shifting the subject. :-)

Anyway, thank you for all your comments.
—SA

Klaus-Werner Konrad 22-Aug-13 19:03pm

"Anyway, thank you for all your comments" - I always enjoy a Logical conversion - thanks to you !
"I don't question your knowledge" - Very Kind; with 30+ years of C experience from my side everything else would make me really wonder :-)

About itoa:
- first, it's deprecated
- second, check the link (yes, Linux again) http://linux.die.net/man/3/atoi
where is clearly stated: "The behavior is the same as
strtol(nptr, NULL, 10); except that atoi() does not detect Errors"

Ok, it's a Little bit lame, but you should consider that all this functions are NOT C++ library but C library, and that is optimized for Speed. The standard library functions just assumes that you checked for pitfalls beforehand, and you should remember in this context, that C by itself is nothing more than the lowest possible level procedural language (aka high level ASM), aside from FORTH ...

Sergey Alexandrovich Kryukov 22-Aug-13 19:43pm

I understand that. Anyway, I think it was useful conversation.
—SA

Klaus-Werner Konrad 22-Aug-13 19:16pm

"But it still leaves not 100% what to do it there are some digits, but parsing still fails due to invalid Format"
Well - the link I submitted states:
"If an underflow occurs, strtol() returns LONG_MIN. If an overflow occurs, strtol() returns LONG_MAX. In both cases, errno is set to ERANGE"
If the format is invalid, there is nothing to convert - dot

Sergey Alexandrovich Kryukov 22-Aug-13 19:44pm

Setting error approach is also a questionable technique, I think.
—SA

Klaus-Werner Konrad 22-Aug-13 20:11pm

Umm ... now I'm stunned

- C has no exception mechanism
- all possible values MUST be allowed, 'cause they are valid

what are the opportunities in your mind ?
(Remember, we're talking about pure C...