Understanding this python code better

Question

import urllib
from xml.etree.ElementTree import parse
candidates = ['4198', '4168']
daves_latitude = 41.98062

def distance(lat1, lat2):
    'Return distance in miles between two lats'
    return 69*abs(lat1 - lat2)

def monitor():
    u = urllib.urlopen('http://ctabustracker.com/bustime/map/getBusesForRoute.jsp?route=22')
    doc = parse(u)
    for bus in doc.findall('bus'):
        busid = bus.findtext('id')
        if busid in candidates:
            lat = float(bus.findtext('lat'))
            dis = distance(lat, daves_latitude)
            print busid, dis, 'miles'

    print '-'*10

import time
while True:
    monitor()
    time.sleep(60)

I did this exercise based on a real life problem. Dave forgets his case in the bus, and he wants to find out wich one of the canditate buses is carrying the lost case. I understand the code, but I can't find out the relationship between the first function and the second one, as

def distance(lat1, lat2): 'Return distance in miles between two lats' return 69*abs(lat1 - lat2)

I understand what monitor() does but not the relationship between distance() and monitor() and how they interact to show the results, can you please enlighten me ? I am a n00b.

Thanks

Ivan Klass · Accepted Answer · 2014-04-06 11:30:06Z

1

Each degree of latitude is approximately 69 miles (111 kilometers) apart. So, lat1 is a latitude of bus and the second lat2 is a Daves latitude. By requesting this url in monitor you got latitude of bus, and then, using distance function, you got the distance in miles.

answered Apr 6, 2014 at 11:30

Ivan Klass

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2 Comments

Chroma Funk Over a year ago

Ok, got it now. Things I don't understand yet : what is "abs" ? and why when we create distance() we assign parameters lat1 and lat2 ? Wouldn't it be the same writing lat and daves_latitude instead ?

Ivan Klass Over a year ago

Yes, you are right about second parameter - in this case it's always same value. For such a basic function as "abs" I would recommend using built-in help function before asking any questions - just type «help('abs')» in python console

TeachMeEverything · Accepted Answer · 2015-05-15 06:15:41Z

0

abs() is the absolute value of a number.

^ little link there for python 3 works the same for python 2.

if your looking for the meaning of absolute value <- that link there shows you a video.

answered May 15, 2015 at 6:15

TeachMeEverything

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