CODEWITHSUNDEEP
python
Jonathan Musso
Jonathan Musso

Reputation: 1374

How does Python interpret this conditional?

The problem I am working on is explained below:

2.1) Write a program that asks a user to input a color. If the color is black or white, output "The color was black or white". If it starts with a letter that comes after "k" in the alphabet, output "The color starts with a letter that comes after "k" in the alphabet". (Optional: consider both capitalized and non-capitalized words. Note: the order of the alphabet in Unix and Python is: symbols, numbers, upper case letters, lower case letters.)

Here is the authors solution:

#!/usr/bin/env python
#
# guess a color
#
answer = raw_input ("Please enter a color: ")

if (answer == "black") or (answer == "white"):
 print "The color was black or white."
elif answer >= "k":
  print "The color starts with a letter that comes after \"k\" in the alphabet."

This is my answer to the problem:

#!usr/bin/env python
#
#This program asks the user to input a color

color = raw_input("Please, enter a color, any color.")

if (color == "black") or (color == "white"):
 print "The color was black or white."

elif color[0] != "a" or "b" or "c" or "d" or "e" or "f" or "g" or "h" or "i" or "j" or "k":
 print "The color starts with a letter that comes after 'k' in the alphabet."

else:
 print "The color was niether black nor white."

I am having trouble understanding how the authors solution works, specifically for identifying if, "The color starts with a letter that comes after "k" in the alphabet".

How is does Python make this work?

elif answer >= "k":

How is Python identifying the first character, such as color[0] and the range of letters beyond k?

Upvotes: 1

Views: 226

Answers (4)

Cat Plus Plus
Cat Plus Plus

Reputation: 130004

Because in general, Python sequences (strings included) implement lexicographical ordering for their elements. So first element 0 is compared, if the same then element 1, etc.

Note, though, that your solution is wrong. It's parsed as (color[0] != "a") or "b" or "c" or "d" or "e" or "f" or "g" or "h" or "i" or "j" or "k", which is false only when color[0] == 'a'. You're looking for color[0] not in ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j') (you shouldn't exclude 'k', either), but using >= is just a much, much cleaner thing to do.

Upvotes: 2

Shraddha
Shraddha

Reputation: 2335

In Python, you can compare two strings using ==, !=, <, >, <=, and >=.operators.

for answer >= "k":

the strings are compared letter wise.

answer[0] >= "k"[0] and so for all indices until conclusion can be derived

Upvotes: 0

U-DON
U-DON

Reputation: 2140

Have you ever noticed the result of sorting files by name? If you sort strings, "k" comes before anything that starts with "k" and has other letters ("k[a-z]+"), and therefore, any string >= "k" is something that starts with "k" or later.

Upvotes: 0

Winston Ewert
Winston Ewert

Reputation: 45089

Think of the way words are sorted in a dictionary. Every word in a dictionary that begins with K or later will be after "k" in the dictionary. That's the basis of python's definition

answer >= "k" is true if answer would be after "k" in the dictionary, which is the same as starting with "k" or a later letter.

Now it looks like according to the description you don't want to include words starting with K. This suggests that the author's solution is subtly wrong as it will include words that start with K.

Upvotes: 0

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