Lesson 3 - Problem solving techniques

In the last lesson, we learned control flow statements such as if-else and for.

In this lesson, let us try to write a program for this problem:

“Given a day, the program should print if it is a weekday or weekend.”

$ python3 day_of_the_week.py monday
weekday

$ python3 day_of_the_week.py sunday
weekend

First, we need to accept the input from the command line.:

import sys

day = sys.argv[1]

Now, we need to find what type of day it is. The most straightforward way of doing it is checking the input against each day, like so:

if day == "monday":
    print("weekday")
elif day == "tuesday":
    print("weekday")
elif day == "wednesday":
    print("weekday")
elif day == "thursday":
    print("weekday")
elif day == "friday":
    print("weekday")
elif day == "saturday":
    print("weekend")
elif day == "sunday":
    print("weekend")

Notice, how multiple if conditions can be combined using elif.

The program works but there are some issues with it. First, code looks a bit cluttered. Second, what if you need to write a similar program but with a bigger list? You can’t keep adding elif statements.

What we are trying to do here is to see if a given input (name of the day in this case) belongs to a group (such as “weekday” or “weekend”). Such problems can be solved by using lists. Since there are two types of days here for the purpose of this problem, let us define two lists.:

weekdays = ["monday", "tuesday", "wednesday", "thursday", "friday"]
weekend = ["saturday", "sunday"]

We can simply check if the input is in the first list or second list:

if day in weekdays:
    print("weekday")
elif day in weekend:
    print("weekend")
else:
    print("not a day")

Notice how the second version is much smaller and much easier to read. In general, you should try to use types such as lists (and others provided by Python which we will see later) to solve problems.

Also note how we added else clause at the very end. Now our program gives a proper error message if the input is not valid. This is another thing you need to keep in mind when you accept inputs. Your code needs to be prepared to handle both valid and invalid inputs. In the case of latter, the program should provide a good error message.

Problem solving techniques

• Divide the problem into smaller ones and solve them. Once smaller problems are solved, you just need to figure out how to combine them. This technique is called divide and conquer.
• Don’t try to write a perfect working program in the first go. Instead, write something that works and slowly improve it.
• Don’t try to write the program directly. First solve the problem for some example inputs and see how you are solving it (you can use paper and pencil for this). Then, try to convert the solution into a program.

Assignment

Given a word, your program should print number of vowels in the word, like so:

$ python3 count_vowels.py "test"
Number of vowels: 1

$ python3 count_vowels.py "it is snowing"
Number of vowels: 4

$ python3 count_vowels.py "xyz"
Number of vowels: 0

You will be using most of the concepts you learned in this lesson and the previous ones to solve this problem, so go over the notes if required.