Input variables in Python 3

Question

Is it possible to have a user input equal to a variable for tasks that involve chemical elements.

For example, Carbon has the molecular mass 12, but i do not want the use to input 12, They should input 'C'. but as the input turns this into a string, it is not possible to lik this to the variable C = 12.

Is there any way to input a variable istead of a string?

If not, could i set a string as a variable.

example:

C = 12

element = input('element symbol:')
multiplier = input('how many?')

print(element*multiplier)

This just returns an error stating that you can't multiply by a string.

(off-topic) the molecular mass of carbon is 12.0107 g/mol, not 12. — kennytm
– kennytm, Commented Nov 2, 2010 at 18:49

SilentGhost · Accepted Answer · 2010-11-02 18:42:06Z

9

You could change your code like this:

>>> masses = {'C': 12}
>>> element = input('element symbol:')
element symbol:C
>>> masses[element]
12
>>> multiplier = input('how many?')
how many?5
>>> multiplier
'5'                                          # string
>>> masses[element] * int(multiplier)
60

answered Nov 2, 2010 at 18:42

SilentGhost

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Comments

kennytm · Accepted Answer · 2010-11-02 18:59:28Z

3

input in Python 3.x is equivalent to raw_input in Python 2.x, i.e. it returns a string.

To evaluate that expression like Python 2.x's input, use eval, as shown in the doc for changes from 2.x to 3.0.

element = eval(input("element symbol: "))
....

However, eval allows execution of any Python code, so this could be very dangerous (and slow). Most of the time you don't need the power of eval, including this. Since you are just getting a global symbol, you could use the globals() dictionary, and to convert a string into an integer, use the int function.

element = globals()[input("element symbol: ")]
multiplier = int(input("how many? "))

but when a dictionary is needed anyway, why not restructure the program and store everything in a dictionary?

ELEMENTS = {'C': 12.0107, 'H': 1.00794, 'He': 4.002602, ...}

try:
  element_symbol = input("element symbol: ")
  element_mass = ELEMENTS[element_symbol]

  multiplier_string = input("how many? ")
  multiplier = int(multiplier_string)

  print(element_mass * multiplier)

# optional error handling
except KeyError:
  print("Unrecognized element: ", element_symbol)
except ValueError:
  print("Not a number: ", multiplier_string)

edited Nov 2, 2010 at 18:59

answered Nov 2, 2010 at 18:40

kennytm

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2 Comments

jfs Over a year ago

Do not use eval(), globals() here. Use if element_symbol not in MASSES: ....

kennytm Over a year ago

@JF: I agree on eval and globals which is why the final answer doesn't have these, but I disagree on if element_symbol not in MASSES: over exception handling. "It is Easier to Ask for Forgiveness than Permission."

Rostyslav Dzinko · Accepted Answer · 2012-08-16 09:20:14Z

1

Since input always return string type. Multiplication to the string is not allowed. So after taking the input, you need to type cast if using int type in python .

Try this:

multiply_string = input("how many? ")
multiplier = int(multiplier_string) #type cast here as int

edited Aug 16, 2012 at 9:20

Rostyslav Dzinko

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answered Aug 16, 2012 at 8:03

Ashutosh Makharia

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Comments

jon_darkstar · Accepted Answer · 2010-11-02 18:44:48Z

0

element = eval(input("element symbol: "))

would be the simplest, but not necessarily the safest. Plus, your symbol needs to be in the local scope

you might prefer to have a dictionary object

answered Nov 2, 2010 at 18:44

jon_darkstar

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