lab3

New topics: file reading, string data extraction

Exercises in this lab:

  • mean(xs) and seq_sqrt exercise the processing of lists

  • line_averages(filename) is an exercise in reading data files, and processing the data.

  • noaa_temperature is an exercise in extracting data from a web page (string processing).

Relevant Socratica video:

mean(xs)

A function mean(xs) that takes a sequence xs of numbers, and returns the (arithmetic) mean \(m\) (i.e. the average value).

With xs \(= [ x_1, x_2, ..., x_n ]\), the equation for the mean \(m\) reads

\[m = \frac{1}{n} \left ( x_1 + x_2 + \ldots + x_n \right) = \frac{1}{n} \sum\limits_{i=1}^n x_i\]

Example:

In [ ]: mean([0, 1, 2])
Out[ ]: 1.0

seq_sqrt(xs)

A function seq_sqrt(xs) which takes a list of non-negative numbers xs with elements \([x_0, x_1, x_2, \ldots, x_n]\), and returns the list \([\sqrt{x_0}, \sqrt{x_1}, \sqrt{x_2}, \ldots, \sqrt{x_n}]\). In other words, the function takes a list of numbers, and returns a list of the same length that contains the square root for each number in the list.

Examples:

In [ ]: seq_sqrt([1, 2, 3])
Out[ ]: [1.0, 1.4142135623730951, 1.7320508075688772]

In [ ]: seq_sqrt([1, 4, 9])
Out[ ]: [1.0, 2.0, 3.0]

line_averages(filename)

A function line_averages(filename) that takes a string filename which contains the name of a file to be processed. The function should open and read that file. The file is expected to contain numbers that are separated by commas (the format is known as a comma-separated-value file, short csv). The function should compute the average value for every line, and return the average values in a list. For example, if we call line_averages("data.csv") and file data.csv reads:

1,2
1,1,1,1
-1,0,1
42,17

then the function should return the list [1.5, 1.0, 0.0, 29.5].

noaa_temperature(s)

Introduction

The U.S. National Oceanic and Atmospheric Administration (NOAA) provides observations of current weather conditions for airports around the globe. Using the special 4-letter “International Civil Aviation Organization (ICAO) airport code —which is EDDH for Hamburg—we can find quantitative information on current weather conditions for Southampton by pointing a web browser to: http://tgftp.nws.noaa.gov/data/observations/metar/decoded/EGHI.TXT

noaa_string()

We provide the following noaa_string() function which downloads the data from http://tgftp.nws.noaa.gov/data/observations/metar/decoded/EGHI.TXT and returns it as a string:

import urllib.request


def noaa_string(location="EDDH"):
    """Fetch from the Internet and return the current NOAA METAR
    weather observation data for `location` as a string.

    `location` needs to be the 4-letter ICAO airport code.

    If no `location` is provided, EDDH for Hamburg is chosen.

    Examples are:
    - "EDDH" for Hamburg airport (Germany)
    - "KJFK" for John F Kennedy airport (US)
    - "EGHI" for Southampton airport (UK)
    """

    # compose correct URL
    url = (
        "http://tgftp.nws.noaa.gov/data/observations/metar/decoded/"
        + location.upper()
        + ".TXT"
    )

    # read web page from URL as one long string
    noaa_data_string = urllib.request.urlopen(url).read().decode("utf-8")

    # and return
    return noaa_data_string

You should copy the code snippet above into the file in which you implement the noaa_temperature() function (including the import urllib.request line).

The library urllib.request allows to access a webpage like a file through its urlopen() function.

You do not need to understand the python code in noaa_string(): for now it is sufficient to use the function noaa_string() for this exercise to get the weather data string.

Exercise

  1. Call the function noaa_string() from the Python prompt and inspect the return value.

    Consider to send the output of the function to the print function, for example print(noaa_string()).

  2. Your task is to write a function noaa_temperature(s) which should take a string s as obtained by s = noaa_string() as the input argument. The function noaa_temperature should extract the temperature in degree Celsius from the string, and return this temperature as a floating point number.

Example

In [ ]: noaa_temperature(noaa_string())
Out[ ]: 10

NOAA may at times change the number of total lines and order of lines in the data, but you can assume that the format of the line containing the temperature data does not change.

Testing suggestions

You can test your noaa_temperature() function by extracting the temperature for other airports. For example, to extract the temperature for the JFK airport use the ICAO airport code KJFK as an argument to noaa_string:

In [ ]: noaa_temperature(noaa_string('KJFK'))
Out[ ]: 7

You could also invent your own noaa-compatible string, and pass this string to your function for additional testing.

Please submit your file lab3.py for this assignment.

Additional (voluntary) tasks are available in lab3-extra.

End of lab3.