c++ trouble using sort for struct

Question

I got a struct that a user can define what string values go in where. I've tried ordering it alphabetically but had no luck with what research I found online. I was hoping if one of use can see where im going wrong

libraries being used : iostream, string, fstream and algorithm

struct House
{
    int room_num;               
    string person;      
};

struct compare_by_word
{
    bool operator()(const House& lhs, const House& rhs)
    {
        return lhs.person < rhs.person;
    }
};

I get errors on this line, by the way im using visual studios 2010

  void asc_order()
    {
        sort(data.begin(), data.end(), compare_by_word());
//for loop will be displayed here to show table
    }

Errors I get:

Error: Identifier data is undefined

struct compare_by_word Error: type name is not allowed

data should be declared. We assumed this code is just a snippet. for demonstration. You need to have something like vector<House> data; in your code somewhere in side asc_order before calling sort. Hopefully filling it with some Houses. — user995502
– user995502, Commented Mar 31, 2013 at 21:50

SomeWittyUsername · Accepted Answer · 2013-03-31 21:42:24Z

2

You're passing the type as comparator. You need to pass object of compare_by_word as comparator to sort.

answered Mar 31, 2013 at 21:42

SomeWittyUsername

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Comments

Community · Accepted Answer · 2017-05-23 12:28:52Z

1

You need to pass an instance of compare_by_word. This is done by calling its constructor:

std::sort(data.begin(), data.end(), compare_by_word());
//                                  ^^^^^^^^^^^^^^^^^

Live Demo

I also see that you're not compiling with any headers that introduce an object with a begin or end method. These are commonly used in vectors and other dynamic containers. So I think you should try passing the address range instead as feasible alternative:

std::size_t len = sizeof(data)/sizeof(*data);

std::sort(data, data + len, compare_by_word());

Live Demo

Or if you're compiling in C++11 you can pass a lambda callback in place of an explicit functor and use the begin and end library functions instead of the address range:

using std::begin;
using std::end;

std::sort(begin(data), end(data), [] (House const& lhs, House const& rhs)
{
    return lhs.person < rhs.person;
});

Live Demo

edited May 23, 2017 at 12:28

CommunityBot

11 silver badge

answered Mar 31, 2013 at 21:43

David G

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6 Comments

Xymostech Over a year ago

You should... reorder the elements in your example. It works great, but it doesn't really show that the sort is doing anything.

David G Over a year ago

@Xymostech Yeah, maybe I should mix them up a bit. :)

lecardo Over a year ago

thanks for that david! Never used OO programming before so sorry about newbish stuff, Just wondering if you could show me a live example with std::size_t len = sizeof(data)/sizeof(*data); std::sort(data, data + len, compare_by_word()); Thanks, when i tried it it tells me data is undefined =/

David G Over a year ago

@Bobski Sure give me a second. :)

David G Over a year ago

Here you go. Basically std::size_t is an unsigned integer type used for holding the size of things.

|

user995502 · Accepted Answer · 2013-03-31 21:45:41Z

0

An alternative would be

struct compare_by_word
{
    bool operator()(const House& lhs, const House& rhs)
    {
        return lhs.person < rhs.person;
    }
} compare_by_word;  // Here.

answered Mar 31, 2013 at 21:45

user995502

1 Comment

user995502 Over a year ago

to the answers provided before me

Ritesh Kumar Gupta · Accepted Answer · 2013-03-31 21:50:49Z

0

There is an error in sort statement.Replace it by:

sort(&data[0],&data[5],compare_by_word());

DEMO AT IDEONE

answered Mar 31, 2013 at 21:50

Ritesh Kumar Gupta

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