"""
This program:
- asks the user to enter a string.
- verifies that the string represents an integer, and has the format:
- [optional] white space at the start
- [optional] minus sign
- a sequence of consecutive characters of digits, from 0 to 9.
- [optional] white space at the end
"""

string = raw_input("enter a string: ")
print "checking if", string, "is an integer..."

position = 0

# we will set illegal_flag to true if we find proof that the string is
# not a valid integer
illegal_flag = False

# skip any white space at the beginning
while position < len(string):
    character = string[position]

    # if we find white space (plain space or a tab or a newline),
    # skip it and continue
    if (character in " \t\n"):
        position = position + 1
    # otherwise, we have reached the end of any possible initial white space.
    # Time to move on to the next checks
    else:
        break

# check if first non-space character is minus sign, skip it if it is
if position < len(string):
    character = string[position]
    if character == "-": #skip minus sign
        position = position + 1

# we now should get a string of consecutive digits. Let's count how many
digit_count = 0
while position < len(string):
    character = string[position]
    if character in "0123456789":
        position = position + 1
        digit_count = digit_count + 1
    else:
        break

# we should make sure we found at least one digit
if digit_count == 0:
    print "no digits found after initial whitespace"
    illegal_flag = True

# we should now skip any whitespace left at the end. Note: anything
# that is not whitespace means the string is not a valid integer.
# skip any white space at the beginning
while position < len(string):
    character = string[position]

    # if we find white space (plain space or a tab or a newline),
    # skip it and continue
    if (character in " \t\n"):
        position = position + 1
    # otherwise, we have reached the end of any possible initial white space.
    # Time to move on to the next checks
    else:
        print "illegal character found in trailing white space:", character
        illegal_flag = True
        break

if (illegal_flag == True):
    print "String", string, "is not a valid integer."
else:
    print "Verified string", string, "as a valid integer."