Why is this python while loop missing the logical operator?

Question

I'm working through learn python the hard way, and in exercise 33 extra credit 2 I'm trying to utilize either raw_input or argv to set a variable that will be used in a while loop:

# from sys import argv
# script, my_num = argv

def all_the_numbers(n):
   """increment by 1 up to limit n"""
   i = 0
   numbers = []
   while i < n:
      print "At the top i is %d" % i
      numbers.append(i)

      i = i + 1
      print "Numbers now: ", numbers
      print "At the bottom i is %d" % i

# print "Please enter an integer: "
# n = raw_input("#")
# n = my_num
n = 10
all_the_numbers(n)

The hard coded n = 10 works as expected; printing the lines up to 10. But passing in a value as my_num from argv and/or setting the variable from raw_input results in an endless integer incrementation upwards. What is different about the latter two forms of setting variables that they do not behave exactly like the hard coded setting of the same variable?

Greg Hewgill · Accepted Answer · 2011-03-28 23:58:12Z

5

The raw_input() function returns a string, not an integer. Try:

n = int(raw_input("#"))

or

n = int(my_num)

This converts the string returned by raw_input() into an integer, which your all_the_numbers() function expects.

Here's the relevant passage from the Python docs (emphasis mine):

The operators <, >, ==, >=, <=, and != compare the values of two objects. The objects need not have the same type. If both are numbers, they are converted to a common type. Otherwise, objects of different types always compare unequal, and are ordered consistently but arbitrarily.

In your case, a number and a string are ordered arbitrarily, and in your case the < comparison is always evaluates to True. It is the programmer's responsibility to ensure that the types of such a comparison are compatible.

edited Mar 28, 2011 at 23:58

answered Mar 28, 2011 at 23:51

Greg Hewgill

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1 Comment

gonzo Over a year ago

Thanks, Greg. That makes a lot of sense; though I did not suspect it as the result was an increasing chain of integers.

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