LESSON 03 – STRING MANIPULATION

 

 

LESSON NOTE/GUIDE

PROGRAM 1 – INDIVIDUAL CHARACTERS

·       Write a program that gets individual characters from a string.


THE CODE

 

alpha = "abcdefghijklmnopqrstuvwxyz"

 

s01 = alpha[3]    #c

s02 = alpha[25]   #z

s03 = alpha[-1]   #z

s04 = alpha[-3]   #x

 

print(s01)

print(s02)

print(s03)

print(s04)

 


EXPLANATION

 

We can access a specific character in a string by using the index of that character.  Indexes start at zero (just like they do for lists). 

 

We can also use negative indexes where -1 is the last characters.  So, -5 would refer to the 5th last character.

 

 

PROGRAM 2 – GETTING A SUBSTRING

·       Write a program that gets part of another string based on a specified range.  In computing, part of a string is called a substring.


THE CODE

 

alpha = "abcdefghijklmnopqrstuvwxyz"

 

s01 = alpha[1:4]     #bcd

s02 = alpha[10:15]   #klmno

s03 = alpha[7:5]     #nothing

 

print(s01)

print(s02)

print(s03)

 


EXPLANATION

 

We can get a part of a string by specifying the range we want by using the starting index and the end-before index.  

 

For example, let’s assume we wanted the substring cde from the string containing the entire alphabet (like in the program above).  The indexes of the characters c, d and e are 2, 3 and 4.  So, the range we need to use is [2:5].

 

In regards the program above, the string s01 gets the value bcd and the string s02 gets the substring klmno.  As for string s03, it gets no characters because the range has a starting point that is after the ending point.

 

 

PROGRAM 3 – MORE ON SUBSTRING

·       Write a program that uses more advanced substring ranges.


THE CODE

 

alpha = "abcdefghijklmnopqrstuvwxyz"

 

s01 = alpha[13:-9]     #nopq

s02 = alpha[-20:-18]   #gh

s03 = alpha[10:]       #klmnopqrstuvwxyz

s04 = alpha[-2:]       #yz

s05 = alpha[:5]        #abcde

 

print(s01)

print(s02)

print(s03)

print(s04)

print(s05)

 


EXPLANATION

 

Ranges work with the negative index values.  Of course, the first index has to refer to a character that comes before the second index.

 

We can also leave part of the range blank.  If we leave the number before the colon blank, it means start at the beginning.  On the other hand, if we leave the number after the colon blank, it means that the range goes to the end of the string.

 

 

PROGRAM 4 – STRING METHODS RELATED TO CASE

  • Write a program that gets a word from the user and makes use of common string methods related to the case of the characters in the string.

 


THE CODE

 

word = input("Gimme a word!")

 

s01 = word.capitalize()  # Cap followed by lower case

s02 = word.lower()       # All lower case

s03 = word.upper()       # All upper case

s04 = word.swapcase()    # Reverse the case of each letter

 

print(s01)

print(s02)

print(s03)

print(s04)

 


EXPLANATION

 

The example above contains four different methods related to strings.  In each case, when we call the method to do a transformation on the string, the method returns an entirely new string.  The original string always remains unchanged.

 

The effect of each method is given in the comments above.  But you are encouraged to try this out and explore!

 

 

PROGRAM 5 – FIND AND COUNT

 

  • Write a program that makes use of the find and count methods related to strings.

 


THE CODE

 

w = input("Can I have a word please?")

 

tot1 = w.count("b")  # number of times 'b' is in w

tot2 = w.count("B")  # number of times 'B' is in w

loc = w.find("a")    # index of first 'a' in w

 

print(tot1)

print(tot2)

print(loc)

 


EXPLANATION

 

The above methods all calculate a value based on the content of the string w and the argument that is passed to them.

 

The method count looks at the string w and counts the amount of times that the provided string (b or B in the statements above) appear in the string.

 

The method find looks at the string w and returns the index of the provided string if it appears inside w.  If it doesn’t then it return -1.

 

 

 

PROGRAM 6 – THE FUNCTIONS ORD & CHR

 

  • Write a program that uses the functions ord and chr to demonstrate how to convert from ASCII character to ASCII number and vice versa.

 


THE CODE

 

print(ord("a"))     # 97

print(chr(97))      # a

 

print(ord("A"))     # 65

print(ord("Z"))     # 90

print(ord("a"))     # 97

print(ord("z"))     # 122

 

print(chr(120))     # x

print(chr(88))      # X


EXPLANATION

 

The ord function gives us the ASCII number for a character.  The chr function does the opposite, it gives us the ASCII character for a provided number.

 

Note that the ord function will cause an error if it is provided with a string that contains more than one character.

 

Got time?  De a web search for “ASCII chart” to see a full listing of ASCII characters and their corresponding numbers.

 

 

PROGRAM 7 – COMBINING ORD & CHR

 

  • Write a program that uses the functions ord and chr to demonstrate how to convert from ASCII character to ASCII number and vice versa.

 


THE CODE

 

print(ord("m"))        # 109

 

print(chr(ord("m")))   # m

print(ord(chr(109)))   # 109

 

val = ord("m") + 1

print(chr(val))        # n (the next letter after m)


EXPLANATION

 

The ord and chr functions can be used together to manipulate characters.  Notice in lines 2 and 3 of the code, both functions are used to cancel each other out. 

 

In the last two lines, we use the functions to convert the letter m into the next letter, the letter n.  This is often referred to as shifting.

 

 

PROGRAM 8 – SHIFTING CHARACTER BY 1

 

  • Write a program that asks the user for a character and outputs the next ASCII character.

 


THE CODE – OPTION 1

 

letter = input("Enter a character")

value = ord(letter) + 1

print(chr(value))

 

THE CODE – OPTION 2

 

letter = input("Enter a character")

print(chr(ord(letter) + 1))

 


EXPLANATION

 

To get the next character of a character, we simply find that character’s ASCII value, then add one, then convert that number back to ASCII.

 

 

PROGRAM 9 – BUILDING A STRING IN A LOOP

 

  • Write a program that gets a five letters from the user one at a time.  The program should add all of these to a string inside a loop.

 


THE CODE

 

word = ""

for x in range(1, 6):

    word = word + input("Enter a letter:")

print(word)

 


EXPLANATION

 

In the first line, we create a string object named word.  We set it to the empty string.

 

The for loop is set to loop 5 times.

 

Inside the loop, for each pass, we append the inputted letter to the string word. 

 

Finally, we print the created word at the end.

 

 

PROGRAM 10 – SHIFTING ENTIRE WORD BY 1

 

  • Write a program that gets a word from the user and shifts every letter in the word over by one and then outputs that word.

 


THE CODE – OPTION 1

 

word = input("Enter a word")

word2 = ""

 

for c in word:

    word2 = word2 + chr(ord(c) + 1)

 

print(word2)

 

THE CODE – OPTION 2

 

word = input("Enter a word")

for c in word:

    print(chr(ord(c) + 1), end="")  # no next line

 


EXPLANATION (OPTION 1)

 

Note that this program is considerably more complex than other programs in this lesson because we are gradually building a new string each pass in the loop.

 

In the first line of code, the user provides a word.

 

In the second line of code, we create the string object named word2 and set it to the empty string.

 

The for loop goes through the string word with c getting each character in word one at a time for each loop pass.

 

In each pass, we append the shifted letter at the end of word2.

 

Finally, after the loop, we output word2.


EXPLANATION (OPTION 2)

 

This program is considerably easier to understand than option 1.  However, option 1 does allow for more flexibility and could be used in more scenarios.

 

The only new concepts in this program is the use of end=”” in the print statement.  This forces the cursor to remain on the same line instead of moving to the next line.