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Beginner Python exercises

25 December 2014

File Overlap Solutions

Exercise 23

Given two .txt files that have lists of numbers in them, find the numbers that are overlapping. One .txt file has a list of all prime numbers under 1000, and the other .txt file has a list of happy numbers up to 1000.

(If you forgot, prime numbers are numbers that can’t be divided by any other number. And yes, happy numbers are a real thing in mathematics - you can look it up on Wikipedia. The explanation is easier with an example, which I will describe below.)

Sample solution

For a simple look at the solution (without using functions and using a for loop), look no further. Read on for a solution using functions and list comprehensions, along with a detailed explanation in Python 3.

The solution without functions using a for loop (read on for the one with functions and the explanation)

The solution with functions using list comprehensions (read on for the explanation)

The explanation

The interesting thing about this problem is that it is an extension of a previous exercise asking to find the overlap of two lists. Instead of lists that are hard-coded into the file, the program will now read lists of information from files somewhere on the computer and perform the same overlap operation.

The first thing to do is open the .txt files and save them somewhere on your computer (easiest is in the same folder as the Python file you are working in). You notice that when you look at the file, each line of the file is an integer. Next, in Python we want to open one of the files and save the contents (as integers) into a list.

This code snippet, as taken from the Exercise 23 explanation about reading from files, will open the file and print out all the lines.

  with open('primenumbers.txt') as f:
  	line = f.readline()
  	while line:
  		line = f.readline()

We now want to just save each line as a separate integer into the list.

  primeslist = []
  with open('primenumbers.txt') as f:
  	line = f.readline()
  	while line:
  		line = f.readline()

All we did was change the print into an append to the list, and make an empty list at the beginning. Note that before I append the line to my list, I turn it into an int with the int() statement.

Now we have a choice. We can either do this twice (copy and paste the exact same code), or write a function to do this for us whenever we want.

Let’s make this into a function, for explanation’s sake. What we want to do here is make a function that I give the name of the file to, and it gives back to me a list of all the numbers in that file, assuming each line contains a separate integer. We need to add two lines to our code snippet from above, the function header and the return statement. The function header is just the name of the function with the list of variables to return, and the return statement is the line return list_of_ints at the end of the function.

  def filetolistofints(filename):
	  list_of_ints = []
	  with open(filename) as f:
	  	line = f.readline()
	  	while line:
	  		line = f.readline()
	  return list_of_ints

Now I can use this function to read both of my files in two simple lines:

  primeslist = filetolistofints('primenumbers.txt')
  happieslist = filetolistofints('happynumbers.txt')

What I have now in my variables primeslist and happieslist are lists where each element was a number on a separate line in each of the files.

My last step is to find the overlap between them. I can either use two for loops or a list comprehension. (If you need a refresher, Exercise 3 talks about for loops and Exercise 7 talks about list comprehensions.)

As a list comprehension, you construct a new list that takes each element from primeslist and only adds it to our new list if it is inside happieslist.

  overlaplist = [elem for elem in primeslist if elem in happieslist]

This can also be done with a for loop, as below:

  overlaplist = []
  for elem in primeslist:
    if elem in happieslist:

And when the loop is done, overlaplist will contain all the elements of the overlap.

Now we can just print our result (using Python 3 syntax) and be done.


For a full and concise solution once again, here it is:

Happy hacking!

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