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I want to send requests to 3 Weather APIs and compare their results like if the rainfall at a specific day were the same London to avoid the single point of failure.

I already learned how I can send multiple requests to APIs. What is the best way to compare their results in Solidity? Is there already a mechanism from oraclize itself?

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Comparing the results of an API call will be the same as comparing any other data in Solidity.

In your example, you are comparing weather, which is likely a uint256. You will likely also need the date you are comparing, which would be a timestamp (also a uint256). Each of these data sources would need a mapping of timestamp to temperature. The mappings would look as follows:

mapping (uint256 => uint256) public sourceOneWeather;
mapping (uint256 => uint256) public sourceTwoWeather;
mapping (uint256 => uint256) public sourceThreeWeather;

Comparing them would simply be to put them into a conditional statement and returning the output of whatever comparison you desire.

Add function example, per comments

An example comparison function would be:

function compareResults(uint256 _timestamp) public returns (uint256){
    if (sourceOneWeather[_timestamp] > sourceTwoWeather[_timestamp]):
        if (sourceOneWeather[_timestamp] > sourceThreeWeather[_timestamp]):
            return sourceOneWeather[_timestamp];
        else:
            return sourceThreeWeather[_timestamp];
    else if (sourceTwoWeather[_timestamp] > sourceThreeWeather[_timestamp]):
        return sourceTwoWeather[_timestamp];
    return sourceThreeWeather[_timestamp];
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    Thanks for your fast answer. Saving them is no problem. But how i do now, that all 3 Requests gave me an answer so i can call a method to compare the results? – Serdar Nurgün Nov 16 '18 at 22:36
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    That simply depends on what you are comparing. You would have to create a function that compares the results, yes. I will add a small example to the code. – shane Nov 16 '18 at 22:39
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    Thanks for the example of code. But i think you did not understand my question. As example i want to know, if it really rained in London at the 10th of November 2018. To avoid single point of failure, i want to request at least 3 APIs. Requesting these three APIs is no problem. And comparing their results is no problem too. My question is: How can i know, that all 3 Responses are received so i can compare their results? – Serdar Nurgün Nov 16 '18 at 22:45
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    Set an isReceived flag after each Oracle call return. – shane Nov 16 '18 at 22:47
  • Ok had that solution in my head. Thought there would be another possiblity. Thank you @shane ! – Serdar Nurgün Nov 16 '18 at 22:50

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