Creating a calculator

[Chris Warrick]

On 1 July 2016 at 05:08, Elizabeth Weiss <cake240 at gmail.com> wrote:
> while True:
>         print("Options:")
>         print("Enter 'add' to add two numbers")
>         print("Enter 'subtract' to subtract two numbers")
>         print("Enter 'multiply' to multiply two numbers")
>         print("Enter 'divide' to divide two numbers")
>         print("Enter 'quit' to end the program")
>         user_input=input(":")
>         if user_input=="quit":
>                 break
>         elif user_input=="add":
>                 num1=float(input("Enter a number"))
>                 num2=float(input("Enter another number"))
>                 result=str(num1+num2)
>                 print("The answer is"+ result)
>         elif user_input=="subtract":
>                 num1=float(input("Enter a number"))
>                 num2=float(input("Enter another number"))
>                 result=str(num1-num2)
>                 print("The answer is"+result)
>
> Two questions:
> 1. Why do I need to put ' ' around the words add, subtract, multiply, quit, etc. when it is already in quotes in print()? When the calculator asks me which option I would like to choose I do not write 'add'- I only write add.

This is used for display. The single quotes will be displayed as part
of the string. This is so that people notice the commands, for
example.

>>> print("Enter 'add' to add two numbers")
Enter 'add' to add two numbers
>>> print("Enter add to add two numbers")
Enter add to add two numbers
>>> print('Enter "add" to add two numbers')
Enter "add" to add two numbers

> 2. The program I am using to help me learn python mentions that the output line could be put outside the if statements to omit repetition of code. What does this mean and how would I write the code differently according to this?

Look at your current code. The following three lines appear twice (and
will appear 4 times if you add multiplication and division):

> num1=float(input("Enter a number"))
> num2=float(input("Enter another number"))
> print("The answer is"+ result)

This is code repetition. It’s discouraged, because if you have 4
copies of a longer segment, and you want to change something, you
would have to remember to change it in 4 places. In this case, you can
avoid code reuse like this:

1. Check if user said 'quit', and if yes, break from the loop. (Ignore
invalid input for now)
2. Ask the user for two numbers.
3. Make an if/elif/else structure to calculate the result.
4. Print out the result outside of `if`.

Example for 3. and 4.:
if user_input == 'add':
    result = num1 + num2  # no need to call str() if you use commas in print()
elif user_input == 'subtract':
    result = num1 - num2
# other elif clauses go here
print("The result is", result)

-- 
Chris Warrick <https://chriswarrick.com/>
PGP: 5EAAEA16