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I am attempting to plot the most populous cities on a world map.

Currently, I have the line of code:

numberofpeople = 
  Map[{#, Length[CityData[#, "Population"]]} &, CityData[All]];

which should give me the City name, and the population. However, when I type in the line of code

Reverse[SortBy[numberofpeople, Last]][[1 ;; 20]]

I am getting odd data, which is that the most populous cities have 1 person.

Could somebody debug my code, and answer any future questions I have on the matter? Much appreciated.

EDIT: How do I plot these points on a World Map? Currently I have

Graphics[{EdgeForm[Black], , CountryData[#, "SchematicPolygon"]} & /@ CountryData[]]

but I have no idea what goes between the points.

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2 Answers

up vote 2 down vote accepted

Leave out Length in Length[CityData[#, "Population"]], and delete cases of Missing data:

numberofpeople = Map[{#, CityData[#, "Population"]} &, CityData[All]];

Reverse[SortBy[DeleteCases[numberofpeople, {_, _Missing}], Last]] ~Take~ 20 // Grid

enter image description here

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Thank you very much for your help. I have one more question: how do I plot these points on a world map? My edit should have the information required for assistance. –  user9876 Oct 21 '13 at 3:02
    
@user9876 You're welcome. Rather than extending your question please consider posting a new one; also, I don't understand what you want to do with the "world map" -- have you seen this question? –  Mr.Wizard Oct 21 '13 at 3:06
    
I figured that since my new question is related to this one, I would just have an add-on to this one, instead of filling up the site with another question. I will make a new question regarding this topic. –  user9876 Oct 21 '13 at 3:10
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I was gonna post this as a comment but it'll be too long. As Mr. Wizard has shown you how to obtain the most popluar cities, here is how to plot them:

 Graphics[{{White, EdgeForm[Black],  CountryData[#, "FullPolygon"] & /@ CountryData[]}, 
{PointSize[Large], Red,    Tooltip[Point[Reverse[CityData[#, "Coordinates"]]], 
          CityData[#, "Name"]] & /@ largeCities}}]

Mathematica graphics

Here, largeCities are the names of the cities without their populations. So you can do something like

largeCities = Take[SortBy[numberofpeople, Last] // Reverse, 20][[All, 1]]
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FWIW the OP already asked this as a new question, which was subsequently marked as a duplicate. –  Mr.Wizard Oct 21 '13 at 15:38
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