change value of element in list c++

Question

this is what im doing, im getting some info from a .txt file those are numbers of course when im getting them i get them as strings in a list of strings...(i know it could be chars but in this example is strings) so when im inserting them on another list it should be something like this

list<int> l;

//i is the iterator for the 1st list that already has all the strings
l.push_back(atoi(*i.c_str()));

i cant use c_str() because i is trying to access the elements of the list is there anyway i can do this?

basically what im trying to do is just getting my strings from one list and change them to integers while keeping the list format

any help would be really appreciated thank you

I don't know if I understood your question correctly, but if I did, isn't it just `i->c_str()`? — Daniel Fleischman, Oct 04 '12 at 01:04

score 5 · Accepted Answer · edited Oct 04 '12 at 16:04

5

You need (*i).c_str() or i->c_str().

You probably want to do the job as a whole a bit differently, specifically in a way that doesn't require you to deal directly with iterators at all. For example, let's assume what you have is:

list<string> strings;
list<int> ints;

Then I'd almost certainly do the job with std::transform, probably something like this:

std::transform(strings.begin(), strings.end(), std::back_inserter(ints),
    [](std::string const &in) { return atoi(in.c_str()); });

or:

transform(begin(strings), end(strings), std::back_inserter(ints),
          [](string const &in) { return stoi(in); });

In case you're not familiar with it, that last line is a lambda (new with C++11). Assuming you're using a fairly new compiler (e.g., gcc 4.7.x or newer, VC++ 2010 or newer) it shouldn't be a problem.

I'd also seriously consider using std::vector instead of std::list. I've virtually never found std::list to be a particularly good choice. Even situations that should theoretically favor a linked list usually don't, at least in my experience.

edited Oct 04 '12 at 16:04

Zan Lynx

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answered Oct 04 '12 at 01:04

Jerry Coffin

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If you're going to use C++11, you may as well use `std::stoi` too. :-] – ildjarn Oct 04 '12 at 01:05
With `std::vector`, you'll also benefit from calling `ints.reserve(strings.size())` beforehand. – Marcelo Cantos Oct 04 '12 at 01:06
wow awesome answer yeah im using c++11 but im still not used to it thank you very much – Makenshi Oct 04 '12 at 01:08
@Makenshi: As you can see from ildjarn's comment, I'm not yet accustomed to everything it's added either. – Jerry Coffin Oct 04 '12 at 01:10
Isn't a lambda overkill on the stoi example? Anything wrong with `transform(begin(strings), end(strings), stoi)` ? – Zan Lynx Oct 04 '12 at 01:11
1

@ZanLynx : `stoi` is overloaded for `std::string` and `std::wstring`, so I think you would need an explicit cast in order for that to compile. – ildjarn Oct 04 '12 at 01:25
@ildjarn: This was a great excuse to download VS Express 2012. You were correct. A lambda appears necessary. – Zan Lynx Oct 04 '12 at 02:40

change value of element in list c++

1 Answers1