Use a list instead of an array in c++

Question

I just started to study c++ and I did not get to understand the list. I have some code using the array.

#include <iostream>
#include <string>

using namespace std;

class test {
public:
string a,b;
};

int main(){
test param [3]; 
param[0].a = "asdf";
param[0].b = "ghjk";
param[1].a = "qwer";
param[1].b = "tyui";
param[2].a = "zxcv";
param[2].b = "vbnm";
cout << "param\ta\tb\n";
for(int i = 0; i<3; i++){
cout << i << "\t" << param[i].a <<
"\t" << param[i].b << endl;
}
}

How to realize same code using the list? Below are my attempts to do this:

#include <list>
...
list <test> param;
param.push_back();
param.back().a = "asdf";
param.back().b = "ghjk";
...

second try:

...
test o; 
o.a = "asdf"; o.b = "ghjk";
param.push_back(o);
...

As you know, none of them works...

As for your problem, just saying "none of them works" isn't much of a description. Please elaborate on what's wrong, including showing expected and actual output for example. — Some programmer dude
– Some programmer dude, Commented Feb 15, 2014 at 10:13

user2897690 · Accepted Answer · 2014-02-15 10:16:30Z

2

First make a object of the class test by

test temp

Then initialize temp properly

temp.a="alpha";
temp.b="beta";

Now push the temp in the list

param.push_back(temp);

answered Feb 15, 2014 at 10:16

user2897690

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2 Comments

Orange Fox Over a year ago

Thank you for your answer. Can you show same code using a vector?

user2897690 Over a year ago

@OrangeFox not too much changes required for vector. Just change the declaration of param as vector<test> param; and rest of code is same. And also don't forget to include vectors by #include<vector>

juanchopanza · Accepted Answer · 2014-02-15 10:49:31Z

2

You need to push test objects into the list. For example:

param.push_back(test());
param.back().a = "asdf";
param.back().b = "ghjk";

or

param.push_back({"asdf", "ghjk"});
param.push_back({ "qwer", "tyui" });
param.push_back({ "zxcv", "vbnm" });

Note that an std::list is a doubly-linked list, and does not support random access. As such, it is not necessarily a good replacement for an array. You might want to have a look at std::vector.

edited Feb 15, 2014 at 10:49

answered Feb 15, 2014 at 10:11

juanchopanza

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4 Comments

Manu343726 Over a year ago

Why not emplace_back() instead of the second way?

juanchopanza Over a year ago

@Manu343726 I am not sure there is anything to gain.

Manu343726 Over a year ago

Well, emplace_back() avoid one copy, isn't? It just initialices the object allocated on the list, instead of passing an object and copying it on the list.

juanchopanza Over a year ago

@Manu343726 Not really, because there isn't a to parameter constructor. So we need to pass a temporary test object anyway, which gets moved with push_back.

Michał Banasiak · Accepted Answer · 2014-02-15 10:12:24Z

0

test test1;
test test2;
test test3;

test1.a = "asdf";
test1.b = "ghjk";
test2.a = "qwer";
test2.b = "tyui";
test3.a = "zxcv";
test3.b = "vbnm";

param.push_back(test1);
param.push_back(test2);
param.push_back(test3);

answered Feb 15, 2014 at 10:12

Michał Banasiak

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Comments

Vlad from Moscow · Accepted Answer · 2014-02-15 10:29:24Z

For example

#include <iostream>
#include <string>
#include <list>

class test 
{
public:
    std::string a, b;
};

int main()
{
    std::list<test> l;
    l.push_back( { "asdf", "ghjk" } );
    l.push_back( { "qwer", "tyui" } );
    l.push_back( { "zxcv", "vbnm" } );

    std::cout << "param\ta\tb\n";
    int i = 0;
    for ( const test &t : l )
    {
        std::cout << i++ << '\t' << t.a 
                         << '\t' << t.b << std::endl;
    }
}

Or

#include <iostream>
#include <string>
#include <list>

class test 
{
public:
    std::string a, b;
};

int main()
{
    std::list<test> l;

    l.push_back( test() );
    l.back() = { "asdf", "ghjk" };
    l.push_back( test() );
    l.back() =  { "qwer", "tyui" };
    l.push_back( test() );
    l.back() = { "zxcv", "vbnm" };

    std::cout << "param\ta\tb\n";
    int i = 0;
    for ( const test &t : l )
    {
        std::cout << i++ << '\t' << t.a 
                       << '\t' << t.b << std::endl;
    }
}

Or

#include <iostream>
#include <string>
#include <list>

class test 
{
public:
    std::string a, b;
};

int main()
{
    std::list<test> l;

    l.push_back( test() );
    l.back().a = "asdf"; l.back().b = "ghjk";
    l.push_back( test() );
    l.back().a = "qwer"; l.back().b = "tyui";
    l.push_back( test() );
    l.back().a = "zxcv"; l.back().b = "vbnm";

    std::cout << "param\ta\tb\n";
    int i = 0;
    for ( const test &t : l )
    {
        std::cout << i++ << '\t' << t.a 
                       << '\t' << t.b << std::endl;
    }
}

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