Python: Problem with Raw_Input Reading a Number

Python: Problem with raw_input reading a number

Do

totPrimes = int(totPrimes)
while count < totPrimes:
    # code

raw_input gives you a string you must convert to an integer or float before making any numeric comparison.

How can I read inputs as numbers?

Solution

Since Python 3, input returns a string which you have to explicitly convert to ints, with int, like this

x = int(input("Enter a number: "))
y = int(input("Enter a number: "))

You can accept numbers of any base and convert them directly to base-10 with the int function, like this

>>> data = int(input("Enter a number: "), 8)
Enter a number: 777
>>> data
511
>>> data = int(input("Enter a number: "), 16)
Enter a number: FFFF
>>> data
65535
>>> data = int(input("Enter a number: "), 2)
Enter a number: 10101010101
>>> data
1365

The second parameter tells what is the base of the numbers entered and then internally it understands and converts it. If the entered data is wrong it will throw a ValueError.

>>> data = int(input("Enter a number: "), 2)
Enter a number: 1234
Traceback (most recent call last):
  File "<input>", line 1, in <module>
ValueError: invalid literal for int() with base 2: '1234'

For values that can have a fractional component, the type would be float rather than int:

x = float(input("Enter a number:"))

Differences between Python 2 and 3

Summary

• Python 2's input function evaluated the received data, converting it to an integer implicitly (read the next section to understand the implication), but Python 3's input function does not do that anymore.
• Python 2's equivalent of Python 3's input is the raw_input function.

Python 2.x

There were two functions to get user input, called input and raw_input. The difference between them is, raw_input doesn't evaluate the data and returns as it is, in string form. But, input will evaluate whatever you entered and the result of evaluation will be returned. For example,

>>> import sys
>>> sys.version
'2.7.6 (default, Mar 22 2014, 22:59:56) \n[GCC 4.8.2]'
>>> data = input("Enter a number: ")
Enter a number: 5 + 17
>>> data, type(data)
(22, <type 'int'>)

The data 5 + 17 is evaluated and the result is 22. When it evaluates the expression 5 + 17, it detects that you are adding two numbers and so the result will also be of the same int type. So, the type conversion is done for free and 22 is returned as the result of input and stored in data variable. You can think of input as the raw_input composed with an eval call.

>>> data = eval(raw_input("Enter a number: "))
Enter a number: 5 + 17
>>> data, type(data)
(22, <type 'int'>)

Note: you should be careful when you are using input in Python 2.x. I explained why one should be careful when using it, in this answer.

But, raw_input doesn't evaluate the input and returns as it is, as a string.

>>> import sys
>>> sys.version
'2.7.6 (default, Mar 22 2014, 22:59:56) \n[GCC 4.8.2]'
>>> data = raw_input("Enter a number: ")
Enter a number: 5 + 17
>>> data, type(data)
('5 + 17', <type 'str'>)

Python 3.x

Python 3.x's input and Python 2.x's raw_input are similar and raw_input is not available in Python 3.x.

>>> import sys
>>> sys.version
'3.4.0 (default, Apr 11 2014, 13:05:11) \n[GCC 4.8.2]'
>>> data = input("Enter a number: ")
Enter a number: 5 + 17
>>> data, type(data)
('5 + 17', <class 'str'>)

raw_input function in Python

It presents a prompt to the user (the optional arg of raw_input([arg])), gets input from the user and returns the data input by the user in a string. See the docs for raw_input().

Example:

name = raw_input("What is your name? ")
print "Hello, %s." % name

This differs from input() in that the latter tries to interpret the input given by the user; it is usually best to avoid input() and to stick with raw_input() and custom parsing/conversion code.

Note: This is for Python 2.x

how to check whether raw_input is integer, string and date in python

1. datetime.datetime.strptime() will raise ValueError if the string does not match the format.

2. raw_input() always return string object, so isinstance(data1, int) always is False if you don't covert the data1 to int object before check.

3. I'd suggest use try...except to catch the ValueError like the following code:

   import datetime
   
   data1 = raw_input("data = ")
   print(data1)
   
   try:
       datetime.datetime.strptime(data1, '%Y-%m-%d')
   
   except ValueError:
   
       try:
           int(data1)
   
       except ValueError:
           print("str it is")
   
       else:
           print("int it is")
   else:
       print("date it is")
   

   Demo:

   kevin@Arch ~> python2 input_check.py 
   data = 2014-01-02
   2014-01-02
   date it is
   
   kevin@Arch ~> python2 input_check.py 
   data = 12
   12
   int it is
   
   kevin@Arch ~> python2 input_check.py 
   data = foobar
   foobar
   str it is
   

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