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Implementing a Proceedure to Work Out Distances

Now that you know how to work out distance, you can make your app only show places that are within 5km of a user.

  • Drag out a procedure block (the result one) and call it distanceBetween. Click on the wrench icon in the top left-hand corner and drag two inputs into it. Name these address1 and address2.

  • Next, from the Variables section, drag out a initialize local name to block (the one that has a side attachment instead of a top attachment).

  • Add six variables to this: lat1, long1, lat2, long2, x, and y. Use the same method you used for the procedure, by clicking on the wrench icon in the corner of the initialise local block.

  • Now you need a way of converting text addresses into latitude and longitude coordinates. Thankfully, the LocationSensor does this, so go to the Designer view and add one.

  • Get two call LocationSensor.LatitudeFromAddress blocks. Attach a get address1 block to one, and a get address2 block to the other. Put these into the lat1 and lat2 attachments.

  • Repeat the same thing with for longitude.

  • Drag in two 0 blocks and attach these to initialize local x to and initialize local y to blocks.

  • Get a do result block from Control, and put it into the in attachment of the (now very big!) initialize local block.

Great! Now you need to work out the distance.

+Get out the blocks set x to, get lat1, get lat2, x, -, and 0.

  • Place the get lat2 and get lat1 block into the - block, and place the - block into the x block.

Now you’ve got the difference in latitude!

  • Multiply this by 111 to get the distances in kilometres between the two latitudes. Then just plug that into the set x to block, and put the set x to block into the do section of the do result block.

  • Do the same thing with the set y to block, changing 111 to 89 and lat to long.

Perfect! With that you have the lengths of two of your triangle’s sides to use in the distance formula!

  • From the Math section, get the square root and + blocks along with two ^(power) blocks and two 0 blocks.

  • Put a get x into the left input of one of the ^ blocks, and put a get y into the left input of another. Put the 0 blocks into the ^ block also, with 0 changed to 2.

This will square both x and y (x squared is x times x, meaning x^2 = x * x).

  • Place both ^ blocks into the + block and attach this to the square root block. Finally, plug this into the result attachment.

Here is what your finished distanceBetween procedure should look like:

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