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It returns True if the character is a letter and False otherwise. When used with an entire string, it will only return **True if every character of the string is a letter.
The values True and False are called booleans** and are covered in Section 10.2. For now, though, just remember that you can use isalpha in if conditions.
Here is a simple example: |ue if every character of the string is a letter. The values True and False are called booleans d are covered in Section 10.2.
For now, though, just remember that you can use isalpha in if ditions. Here is a simple example:|Col2| |---|---| ||| |s = input('Enter a string')|| ----- 48 _CHAPTER 6.
STRINGS_ **if s[0].isalpha():** **print('Your string starts with a letter')** **if not s.isalpha():** **print('Your string contains a non-letter.')** **A note about index** If you try to find the index of something that is not in a string, Python will raise an error.
For instance, if s='abc' and you try s.index('z'), you will get an error.
One way around this is to check first, like below: **if 'z' in s:** location = s.index('z') **Other string methods** There are many more string methods.
For instance, there are methods isdigit and isalnum, which are analogous to isalpha. Some other useful methods we will learn about later are join and split.
To see a list of all the string methods, type dir(str) into the Python shell. If you do this, you will see a bunch of names that start with __. You can ignore them.
To read Python’s documentation for one of the methods, say the isdigit method, type help(str.isdigit). ###### 6.9 Escape characters The backslash, \, is used to get certain special characters, called escape characters, into your string. There are a variety of escape characters, and here are the most useful ones: - ...
It is used to advance to the next line. Here is an example: **print('Hi\n\nthere!')** - \' for inserting apostrophes into strings.
Say you have the following string: s = 'I can't go' This will produce an error because the apostrophe will actually end the string.
You can use \' to get around this: s = 'I can\'t go' Another option is to use double quotes for the string: "s = I can't go" - \" analogous to \'. ----- _6.10.
EXAMPLES_ 49 - \\ This is used to get the backslash itself.
For example: filename = 'c:\\programs\\file.py' - \t the tab character ###### 6.10 Examples **Example 1** An easy way to print a blank line is print().
However, if we want to print ten blank lines, a quick way to do that is the following: **print('\n'*9)** Note that we get one of the ten lines from the print function itself. **Example 2** Write a program that asks the user for a string and prints out the location of each 'a' in the string. s = input('Enter some t...
The loop variable i keeps track of our location in the string, and s[i] gives the character at that location.
Thus, the third line checks each character to see if it is an 'a', and if so, it will print out i, the location of that 'a'. **Example 3** Write a program that asks the user for a string and creates a new string that doubles each character of the original string.
For instance, if the user enters Hello, the output should be HHeelllloo. s = input('Enter some text: ') doubled_s = '' **for c in s:** doubled_s = doubled_s + c*2 Here we can use the second type of loop from Section 6.7.
The variable c will run through the characters of s. We use the repetition operator, *, to double each character.
We build up the string s in the way described at the end of Section 6.2. **Example 4** Write a program that asks a user for their name and prints it in the following funny pattern: |ample 2 Write a program that asks the user for a string and prints out the location of each 'a' he string.|Col2| |---|---| ||| |s = inpu...
For instance, if the user enters Hello, the output should be eelllloo.|Col2| |---|---| ||| |s = input('Enter some text: ') doubled_s = '' for c in s: doubled_s = doubled_s + c*2|| ----- 50 _CHAPTER 6.
STRINGS_ We will require a loop because we have to repeatedly print sections of the string, and to print the sections of the string, we will use a slice: name = input('Enter your name: ') **for i in range(len(name)):** **print(name[:i+1], end=' ')** The one trick is to use the loop variable i in the slice.
Since the number of characters we need to print is changing, we need a variable amount in the slice. This is reminiscent of the triangle program from Section 2.4.
We want to print one character of the name the first time through the loop, two characters the second time, etc.
The loop variable, i, starts at 0 the first time through the loop, then increases to 1 the second time through the loop, etc. Thus we use name[:i+1] to print the first i+1 characters of the name.
Finally, to get all the slices to print on the same line, we use the print function’s optional argument end=''. **Example 5** Write a program that removes all capitalization and common punctuation from a string s. s = s.lower() **for c in ',.;:-?!()\'"':** s = s.replace(c, '') The way this works is for every charac...
One technical note here: We need the ' character in a string.
As described in the previous section, we get it into the string by using the escape character \'. **Example 6** Write a program that, given a string that contains a decimal number, prints out the decimal part of the number.
For instance, if given 3.14159, the program should print out .14159. s = input('Enter your decimal number: ') **print(s[s.index('.'):])** The key here is the index method will find where the decimal point is.
The decimal part of the number starts there and runs to the end of the string, so we use a slice that starts at s.index('.'). Here is another, more mathematical way, to do this: **from math import floor** num = eval(input('Enter your decimal number: ') **print(num - floor(num))** One difference between the two metho...
For instance, if given 3.14159, the program should print out .14159.|Col2| |---|---| ||| |s = input('Enter your decimal number: ') print(s[s.index('.'):])|| |re is another, more mathematical way, to do this:|Col2| |---|---| ||| |from math import floor num = eval(input('Enter your decimal number: ') print(num - floor(n...
EXERCISES_ 51 **Example 7** A simple and very old method of sending secret messages is the substitution cipher. Basically, each letter of the alphabet gets replaced by another letter of the alphabet, say every a gets replaced with an x, and every b gets replaced by a z, etc.
Write a program to implement this. alphabet = 'abcdefghijklmnopqrstuvwxyz' key = 'xznlwebgjhqdyvtkfuompciasr' secret_message = input('Enter your message: ') secret_message = secret_message.lower() **for c in secret_message:** **if c.isalpha():** **print(key[alphabet.index(c)],end='')** **else:** **print(c, end='...
What it does is go through the message one character at a time, and, for every letter it finds, it replaces it with the corresponding letter from the key.
This is accomplished by using the index method to find the position in the alphabet of the current letter and replacing that letter with the letter from the key at that position.
All non-letter characters are copied as is. The program uses the isalpha method to tell whether the current character is a letter or not. The code to decipher a message is nearly the same.
Just change key[alphabet.index(c)] to alphabet[key.index(c)]. Section 19.11 provides a different approach to the substitution cipher. ###### 6.11 Exercises 1.
Write a program that asks the user to enter a string.
The program should then print the following: (a) The total number of characters in the string (b) The string repeated 10 times (c) The first character of the string (remember that string indices start at 0) (d) The first three characters of the string (e) The last three characters of the string (f) The string bac...
ically, each letter of the alphabet gets replaced by another letter of the alphabet, say every a gets laced with an x, and every b gets replaced by a z, etc.
Write a program to implement this.|Col2| |---|---| ||| |alphabet = 'abcdefghijklmnopqrstuvwxyz' key = 'xznlwebgjhqdyvtkfuompciasr' secret_message = input('Enter your message: ') secret_message = secret_message.lower() for c in secret_message: if c.isalpha(): print(key[alphabet.index(c)],end='') else: print(c, end='')||...
STRINGS_ (k) The string with every letter replaced by a space 2. A simple way to estimate the number of words in a string is to count the number of spaces in the string.
Write a program that asks the user for a string and returns an estimate of how many words are in the string. 3. People often forget closing parentheses when entering formulas.
Write a program that asks the user to enter a formula and prints out whether the formula has the same number of opening and closing parentheses. 4.
Write a program that asks the user to enter a word and prints out whether that word contains any vowels. 5. Write a program that asks the user to enter a string.
The program should create a new string called new_string from the user’s string such that the second character is changed to an asterisk and three exclamation points are attached to the end of the string.
Finally, print new_string. Typical output is shown below: Enter your string: Qbert Q*ert!!! 6.
Write a program that asks the user to enter a string s and then converts s to lowercase, removes all the periods and commas from s, and prints the resulting string. 7.
Write a program that asks the user to enter a word and determines whether the word is a palindrome or not. A palindrome is a word that reads the same backwards as forwards. 8.
At a certain school, student email addresses end with @student.college.edu, while professor email addresses end with @prof.college.edu.
Write a program that first asks the user how many email addresses they will be entering, and then has the user enter those addresses.
After all the email addresses are entered, the program should print out a message indicating either that all the addresses are student addresses or that there were some professor addresses entered. 9.
Ask the user for a number and then print the following, where the pattern ends at the number that the user enters. 1 2 3 4 10.
Write a program that asks the user to enter a string, then prints out each letter of the string doubled and on a separate line.
For instance, if the user entered HEY, the output would be HH EE YY ----- _6.11. EXERCISES_ 53 11. Write a program that asks the user to enter a word that contains the letter a.
The program should then print the following two lines: On the first line should be the part of the string up to and including the first a, and on the second line should be the rest of the string.
Sample output is shown below: Enter a word: buffalo buffa lo 12. Write a program that asks the user to enter a word and then capitalizes every other letter of that word.
So if the user enters rhinoceros, the program should print rHiNoCeRoS. 13. Write a program that asks the user to enter two strings of the same length.
The program should then check to see if the strings are of the same length. If they are not, the program should print an appropriate message and exit.
If they are of the same length, the program should alternate the characters of the two strings. For example, if the user enters abcde and ABCDE the program should print out AaBbCcDdEe. 14.
Write a program that asks the user to enter their name in lowercase and then capitalizes the first letter of each word of their name. 15. When I was a kid, we used to play this game called Mad Libs.
The way it worked was a friend would ask me for some words and then insert those words into a story at specific places and read the story.
The story would often turn out to be pretty funny with the words I had given since I had no idea what the story was about.
The words were usually from a specific category, like a place, an animal, etc. For this problem you will write a Mad Libs program.
First, you should make up a story and leave out some words of the story. Your program should ask the user to enter some words and tell them what types of words to enter.
Then print the full story along with the inserted words.
Here is a small example, but you should use your own (longer) example: Enter a college class: CALCULUS Enter an adjective: HAPPY Enter an activity: PLAY BASKETBALL CALCULUS class was really HAPPY today.
We learned how to PLAY BASKETBALL today in class. I can't wait for tomorrow's class! 16. Companies often try to personalize their offers to make them more attractive.
One simple way to do this is just to insert the person’s name at various places in the offer.
Of course, companies don’t manually type in every person’s name; everything is computer-generated. Write a program that asks the user for their name and then generates an offer like the one below.
For simplicity’s sake, you may assume that the person’s first and last names are one word each. Enter name: George Washington ----- 54 _CHAPTER 6.
STRINGS_ Dear George Washington, I am pleased to offer you our new Platinum Plus Rewards card at a special introductory APR of 47.99%.
George, an offer like this does not come along every day, so I urge you to call now toll-free at 1-800-314-1592. We cannot offer such a low rate for long, George, so call right away. 17.
Write a program that generates the 26-line block of letters partially shown below.
Use a loop containing one or two print statements. abcdefghijklmnopqrstuvwxyz bcdefghijklmnopqrstuvwxyza cdefghijklmnopqrstuvwxyzab ... yzabcdefghijklmnopqrstuvwx zabcdefghijklmnopqrstuvwxy 18.
The goal of this exercise is to see if you can mimic the behavior of the in operator and the count and index methods using only variables, for loops, and if statements. (a) Without using the in operator, write a program that asks the user for a string and a letter and prints out whether or not the letter appears in th...
If the letter is not in the string, the program should say so. 19.
Write a program that asks the user for a large integer and inserts commas into it according to the standard American convention for commas in large numbers.
For instance, if the user enters 1000000, the output should be 1,000,000. 20. Write a program that converts a time from one time zone to another.
The user enters the time in the usual American way, such as 3:48pm or 11:26am. The first time zone the user enters is that of the original time and the second is the desired time zone.
The possible time zones are Eastern, Central, Mountain, or Pacific. Time: 11:48pm Starting zone: Pacific Ending zone: Eastern 2:48am 21.
An anagram of a word is a word that is created by rearranging the letters of the original. For instance, two anagrams of idle are deli and lied.
Finding anagrams that are real words is beyond our reach until Chapter 12.
Instead, write a program that asks the user for a string and returns a random anagram of the string—in other words, a random rearrangement of the letters of that string. ----- _6.11.
EXERCISES_ 55 22. A simple way of encrypting a message is to rearrange its characters.
One way to rearrange the characters is to pick out the characters at even indices, put them first in the encrypted string, and follow them by the odd characters.
For example, the string message would be encrypted as msaeesg because the even characters are m, s, a, e (at indices 0, 2, 4, and 6) and the odd characters are e, s, g (at indices 1, 3, and 5). (a) Write a program that asks the user for a string and uses this method to encrypt the string. (b) Write a program that dec...
A more general version of the above technique is the rail fence cipher, where instead of breaking things into evens and odds, they are broken up by threes, fours or something larger.
For instance, in the case of threes, the string secret message would be broken into three groups. The first group is sr sg, the characters at indices 0, 3, 6, 9 and 12.
The second group is eemse, the characters at indices 1, 4, 7, 10, and 13. The last group is ctea, the characters at indices 2, 5, 8, and 11.
The encrypted message is sr sgeemsectea. (a) Write a program the asks the user for a string and uses the rail fence cipher in the threes case to encrypt the string. (b) Write a decryption program for the threes case. (c) Write a program that asks the user for a string, and an integer determining whether to break thi...
Encrypt the string using the rail-fence cipher. (d) Write a decryption program for the general case. 24. In calculus, the derivative of x [4] is 4x [3]. The derivative of x [5] is 5x [4].
The derivative of x [6] is 6x [5]. This pattern continues. Write a program that asks the user for input like x^3 or x^25 and prints the derivative.
For example, if the user enters x^3, the program should print out 3x^2. 25.
In algebraic expressions, the symbol for multiplication is often left out, as in 3x +4 _y or 3(x_ +5). Computers prefer those expressions to include the multiplication symbol, like 3*x+4*y or 3*(x+5).
Write a program that asks the user for an algebraic expression and then inserts multiplication symbols where appropriate. ----- 56 _CHAPTER 6.
STRINGS_ ----- ### Chapter 7 ## Lists Say we need to get thirty test scores from a user and do something with them, like put them in order.
We could create thirty variables, score1, score2, ..., score30, but that would be very tedious. To then put the scores in order would be extremely difficult.
The solution is to use lists. ###### 7.1 Basics **Creating lists** Here is a simple list: L = [1,2,3] Use square brackets to indicate the start and end of the list, and separate the items by commas. **The empty list** The empty list is [].