verify input from the user
an int between first and last
first=10
last=20

What types of errors?

-- an int outside the range first to last

-- ValueError - something that can't be translated into an int

Basic strategy (not yet code):

while ans is not valid:
   get an input and assign it to ans


get an input and assign it to ans
while ans is not valid:
   get an input and assign it to ans

---Aside:  what should the input call print?

ans = input("Enter an int between" + first + "and" + last)

There are still a couple problems with the above.

ans = input("Enter an int between " + str(first) + " and " + str(last))
  
However: this is a long line.   This is nicer:

msg = "Enter an int between " + str(first)
msg += " and " + str(last)

ans = input(msg)

now we have the message... back to the program.

ans = input(msg)
while ans is not valid:
   ans = input(msg)

ans = input(msg)
while ans cannot be converted to an int or
      (int(ans) < first or int(ans) > last)
   print error message
   ans = input(msg)

def canBeInt(s):
  '''s is a string.  Returns True if s can be converted
     into an int, and False otherwise.'''

ans = input(msg)
ans = "jjj"
while not canBeInt(ans) or
      (int(ans) < first or int(ans) > last):
   if not canBeInt(ans):
     print("The value must be an int")
   else:
     print("The number must be between",first,"and",last)
   ans = input(msg)

---
def canBeInt(s):
  '''s is a string.  Returns True if s can be converted
     into an int, and False otherwise.'''

  #eg:  543\n

  numbers = "0123456789"

  returnVal = True

  # s[-1] is the last char of s
  # s[:-1] is s without the last char

  # eg 1j2
  for c in s[:-1]:
     if not c in numbers:
        returnVal = False
  returnVal = returnVal and (s[-1] in (numbers + "\n"))

  return returnVal

===However, we don't want to write canBeInt.
we want to use int(), and catch the exceptions thrown by Python.

See the python programs on the schedule for Day 15:
verifyWithException.py.txt 
verifyWithExceptions1.py.txt