Exercise: Rudimentary Calculator

Inside the directory you created for this course on Python, create a new folder called Exercise-4-Rudimentary-Calculator and put the .py solution files for this exercise within it.

We start off by laying out three different calls to input() — first two for the integers x and y; while the third for the arithmetic operation.

x = int(input('x: '))
y = int(input('y: '))
op = input('Operation: ')

Next up, we check which letter did the user enter in the last input prompt i.e. what is the value of op. This is done using if, elif and else statements.

x = int(input('x: '))
y = int(input('y: '))
op = input('Operation: ')

# Blank line before output.
print()

if op == 'a':
   print(x, '+', y, '=', x + y)
elif op == 's':
   print(x, '-', y, '=', x - y)
elif op == 'm':
   print(x, '*', y, '=', x * y)
elif op == 'd':
   print(x, '/', y, '=', x / y)
elif op == 'e':
   print(x, '**', y, '=', x ** y)
elif op == 'r':
   print(x, '%', y, '=', x % y)
else:
   print('Unknown operation.');

With this, our exercise has its solution.

When programming, there is one principle to always keep in mind. The DRY principle.

DRY stands for Don't Repeat Yourself, and is fairly self explanatory. It says that you should not repeat yourself — or better to say, you should not repeat code.

In the last code snippet above, notice that we are merely repeating the print() statement over and over again, with just changing the symbol for the operation and the actual expression to be computed.

Imagine if we had to change the output value made in any case. For example, let's suppose, we just had to print:

where <result> denotes the result of performing the respective operation on x and y.

Shown below is an example:

To account for this change in the output value, we'd need to change each print() statement in our original code.

Doesn't this sound quite inefficient? Well, it sure does. That's because we are repeating a lot in our code.

Following DRY, we must keep from repeating unwanted statements in our code, and rather replace them with statements that could ultimately enable us to make small changes (such as changing the output in the case above) in the code very easily.

So coming back to our code, we could do one very basic thing to make it much much more flexible. Let's see that:

x = int(input('x: '))
y = int(input('y: '))
op = input('Operation: ')

is_op_known = True

if op == 'a':
   result = x + y
   op_sym = '+'

elif op == 's':
   result = x - y
   op_sym = '-'

elif op == 'm':
   result = x * y
   op_sym = '*'

elif op == 'd':
   result = x / y
   op_sym = '/'

elif op == 'e':
   result = x ** y
   op_sym = '**'

elif op == 'r':
   result = x % y
   op_sym = '%'

else:
   is_op_known = False


# Blank line before output.
print()

if is_op_known:
   print(x, op_sym, y, '=', result)
else:
   print('Unknown operation.')

Instead of directly making an output in each conditional's body, we now create two variables: result holding the result of the operation, and op_sym holding the symbol for the operator.

At the end of the script, we make the desired output using result and op_sym.

Time to clarify one thing...

Applying DRY doesn't always lead to shorter code. The principle just states to not repeat statements that shouldn't be repeated.

In our case, when we were repeating print(), making a change to the output meant that we'd have to make a change to each print() statement. Here, print() was the thing that shouldn't be repeated.

But now, with only one print() statement, if we need to make a change to the output, we just ought to make a change to this single print() statement.

Without any doubts, this new code is way more flexible than the previous one.

Knowing how, and when to apply DRY is another paramount skill for programmers to have.

Yes, sometimes, we could move on with inflexible, repetitive pieces of code, as with the print() statements in the solution above. This usually happens in personal projects, or in competitive programming, where we are just concerned with the output of the program.

But, when we work in large teams where multiple programmers collaborate on the same pieces of code, having repetitive stuff can not only make your work unpresentable, but also unprofessional.