Using Oraclize to ask a question with input variables

Question

I wish to use Oraclize to get prices for different ticker symbols. From the example

contract KrakenPriceTicker is usingOraclize {

    string public ETHXBT;

    event newOraclizeQuery(string description);
    event newKrakenPriceTicker(string price);


    function KrakenPriceTicker() {
        oraclize_setProof(proofType_TLSNotary | proofStorage_IPFS);
        update();
    }

    function __callback(bytes32 myid, string result, bytes proof) {
    if (msg.sender != oraclize_cbAddress()) throw;
        ETHXBT = result;
        newKrakenPriceTicker(ETHXBT);
        update();
    }

    function update() payable {
        if (oraclize_getPrice("URL") > this.balance) {
            newOraclizeQuery("Oraclize query was NOT sent, please add some ETH to cover for the query fee");
    } else {
            newOraclizeQuery("Oraclize query was sent, standing by for the answer..");
            oraclize_query(60, "URL", "json(https://api.kraken.com/0/public/Ticker?pair=ETHXBT).result.XETHXXBT.c.0");
        }
    }

}

I wish to modify this contract to take a symbol as an input, not ETHXBT. I see the oraclize_query to have a fixed string, how can the query be changed to allow this?

oraclize_query(60, "URL", "json(https://api.kraken.com/0/public/Ticker?pair=ETHXBT).result.XETHXXBT.c.0");

I also wish to extent this with multiple arguments if requested in input, as an example

https://api.kraken.com/0/public/Ticker?pair=ETHXBT&result=1

How to add this?

Answer 1

To use a symbol as an input, you can use the utils present in the oraclizeAPI, such as strConcat, which helps you concatenate strings:

oraclize_query(60, "URL", strConcat("json(https://api.kraken.com/0/public/Ticker?pair=", symbol,").result.", symbol,".c.0"));

If you want to receive multiple pairs prices at the same time via the Krakn API you can use the JSONPath. For example,

json(https://api.kraken.com/0/public/Ticker?pair=ETHXBT,ETHUSD&result=1).result.["XETHXXBT","XETHZUSD"].c.0

will return the current prices of ETH/BTC and ETH/USD as an array (test here).

To parse the result string and convert it to an array you should use the string-utils solidity library.