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contract Shopping {

    struct Article {
        uint id; 
        address seller;
        address buyer;
        string name;
        string description;
        uint256 price;
    }

    // state variable 
    mapping(uint256 => Article) public articles; 
    uint articleCounter;

    function buyArticle(uint _id ) public payable {
        Article  storage article = articles[_id];
        require( article.buyer == 0x0);                                      
        require(msg.sender != article .seller]);
    }
}

1) I have got this code in online

2) i am wondering why storage is declared rather than memory

3) For getting values from mapping articles[] i used slicing technique which is similar to array slicing

4) But Now in storage type we are using article.buyer rather than article[buyer]

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1) I have got this code in online

Just in case it isn't clear, you can run your contracts on your own chain or a simulator like ganache-cli or even Remix to start so dev doesn't cost money with each iteration.

2) i am wondering why storage is declared rather than memory

They mean different things. storage will create a pointer to values in a storage, known as a reference variable. memory will copy the values from storage to a new memory variable. It would make quite a difference if the contract was updating storage values. In this case, the difference is this way is slight cheaper on gas.

For more on the difference and a warning, have a look over here: https://vessenes.com/solidity-frustrations-references-and-mapping/

4) But now in storage type we are using article.buyer rather than article[buyer]

Storage and memory types don't change this syntax.

You use mapping(uint256 => Article) public articles; which means there is a struct at each index. The struct contains members. So, you can say

Article storage myArticle = articles[myArticleId]
string n = myArticle.name;

or you can say

string n = articles[myArticleId].name;

Hope it helps.

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