How can I execute a statement AFTER a loop finishe

问题:

I'm querying a mongo database to retrieve the tiles for the display in rougelike game. This is the function I use:

function get_display(){
    var collections = ['austinsroom'];
    var db  = mongojs(uri, collections);
    var temphtml = '';

    for(var j = 0; j < 3; j++) {
        console.log("y=" + String(j));
        db.austinsroom.find({"y": j}, {}).sort({"x": 1}, function(err, records) {
            if(err) {
                console.log("There was an error executing the database query.");
                return;
            } 
            var i = records.length;
            while(i--) {
                temphtml += records[i].display;
            }   
            temphtml += '<br>';
            //console.log(temphtml);
            //return temphtml;
            //THE ONLY WAY I CAN GET ANYTHING TO PRINT IN THE CONSOLE IS IF I PUT IT INSIDE THE LOOP HERE
            });
        //console.log(temphtml);
        //return temphtml;
        //THIS DOES NOTHING
    }
    //console.log(temphtml);
    //return temphtml;
    //THIS DOES NOTHING
}
get_display();

If I put the console.log(temphtml) inside the loop, it prints out three times which isn't what I want. I only want the final string (i.e. ...<br>...<br>...<br>. Also I can't ultimately return the temphtml string, which is actually the important thing. Is this some quirk of javascript? Why would it not execute statements after the loop?

Also: is there a better way to retrieve every element of a grid that's stored in a mongo database, in order, so it can be displayed properly? Here's what the mongo documents look like:

{
    "_id": {"$oid": "570a8ab0e4b050965a586957"},
    "x": 0,
    "y": 0,
    "display": "."
}

Right now, the game is supposed to display a "." in all empty spaces using the x and y values for the coordinates. The database is indexed by "x" values.

回答1:

See async.whilst. You want flow control of the for loop, for which this provides a callback to control each loop iteration.

var temphtml = "",
    j = 0;

async.whilst(
  function() { return j < 3 },
  function(callback) {
    db.austinsroom.find({"y": j }, {}).sort({"x": 1}, function(err, records) 
      temphtml += records.map(function(el) {
          return el.display;
      }).join("") + '<br>';
      j++;
      callback(err);
    });
  },
  function(err) {
     if (err) throw err;
     console.log(temphtml);
  }
)

Either that or use Promise.all() on collected promises to return "one big result". But you would also need to switch to promised-mongo from mongojs, as the nearest equivalent, since there are more mongodb drivers that actually support promises. That one is just the direct fork from mongojs:

var temphtml = "",
    j = 0,
    promises = [];

for ( var j=0; j < 3; j++ ) {
   promises.push(db.austinsroom.find({"y": j }, {}).sort({"x": 1}).toArray());
   promises.push('<br>');   // this will just join in the output
)

Promise.all(promises).then(function(records) {
    temphtml += records.map(function(el) {
        return el.display;
    }).join("");
})

Not exactly the same thing, since it's one list output and not three, but the point is that the Promise objects defer until actually called to resolve, so you can feed the paramters in the loop, but execute later.

回答2:

I do not use MongoDB but from what I am reading it is asynchronous. So what is happening is your db.austinsroom.find call fires another "thread" and returns to the for loop to continue the next iteration.

One way to do what you want is have a check at the end of your db.austinsroom.find function to see if you're done with the for loop. Something like:

function get_display()
{
    var collections = ['austinsroom'];
    var db  = mongojs(uri, collections);
    var temphtml = '';
    var doneCounter = 0;

    for(var j = 0; j < 3; j++)
    {
        console.log("y = " + String(j));
        db.austinsroom.find({"y": j}, {}).sort({"x": 1}, function(err, records)
        {
            if(err)
            {
                console.log("There was an error executing the database query.");
                return;
            } 
            var i = records.length;
            while(i--)
            {
                temphtml += records[i].display;
            }   
            temphtml += '<br>';

            // we're done with this find so add to the done counter
            doneCounter++;

            // at the end, check if the for loop is done
            if(doneCounter == 3)
            {
                // done with all three DB calls
                console.log(temphtml);
            }
       });
    }
}

This is probably not the best way as I know nothing about MongoDB but it should put you on the right path.