CODEWITHSUNDEEP
python-3.xoperators
user3896917
user3896917

Reputation:

How to have multiple "sub strings" when using the in operator with a string

I have a hopefully simple question that I can't seem to find an answer to.

In the following code, I am creating an input validation feature to make sure the user inputs any of four valid "operations" (A.K.A the single letters 'a', 's', 'm', or 'd' and nothing else), so, I'm checking if the "operation" the user inputs is NOT equal to the any of the strings 'a', 's', 'm', or 'd'. If the condition is met, an error message will be printed and the program will be restarted as the code (mostly) shows.

print("Select an operation:")
print("Add  (a)"), print("Sub  (s)")
print("Mul  (m)"), print("Div  (d)")

operation = input()
if "a" not in operation: 
    print("Invalid operation.")
    continue

Please tell me a way I can check if the operation matches ANY of the four letters. And, just to clear up, I don't mean that I need to OR (the logic function) 'a', 's', 'm', and 'd'.

All correspondence and help is much appreciated, thanks! :)

Upvotes: 0

Views: 40

Answers (3)

Joran Beasley
Joran Beasley

Reputation: 114048

ummm perhaps 

operation = input()
assert operation in  ('a', 's', 'm', 'd'), "Error you must use one of  'a', 's', 'm', or 'd'"

I guess

basically you want to check that your operation in list of options

of coarse if your list of options is huge, then using a set is much more appropriate ... or a dictionary that provides a mapping from operations to methods ie

operations = { 
    "a":add,
    "s":sub,
    "m":mul,
    "d":div
} #these are methods defined elsewhere
operation = input()
if operation in operations:
   operations[operation]() #call the method

Upvotes: 0

user2555451
user2555451

Reputation:

You can use the not in operator with a tuple of values to test for:

if operation not in ('a', 's', 'm', 'd'):

The condition of the above if-statement will evaluate to True if operation does not equal any of the values in the tuple ('a', 's', 'm', 'd').

Note too that you are using continue incorrectly: it may only be used inside a loop.

If you want to loop until the user enters a proper value, you can use something like this:

while True:                                # Loop continuously
    operation = input("Enter a value: ")   # Get the input
    if operation in ('a', 's', 'm', 'd'):  # See if it can be found in the tuple
        break                              # If so, break the loop

Upvotes: 2

Jim Dennis
Jim Dennis

Reputation: 17510

You probably want something like:

allowed_operations = set(('a', 's', 'm', 'd'))
while True:
    operation = input() # raw_input() for Python earlier than 3
    if operation not in allowed_operations:
        print ('Error: Must choose one of a, s, m, d\nPlease try again')
    else:
        break

Upvotes: 0

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