Working with Strings

Strings consist of a sequence of characters. Each character is accessed using an index value. For example,

word = “chicken”
print( word[0] )
print( word[4] )

produces the output,

c
k

Here is how the string stored with the variable / identifier word is stored

Notice the index values begin with the value 0, and indicate the position of each character in the string.

The Function len()

The function len() calculates the length of the string (how many characters are in the sequence). In the example above,

len( word ) has a value of 7. For example,

print( len( word ) )

produces the output …

7

Using a for-loop with Strings

There are two ways a loop can be used with strings in Python.

for ch in word:
   print( ch, end = “ “)

produces the output …

c h i c k e n

where the variable ch stores each character of the string during each iteration of the loop.

for i in range(len(word) ):
    print( word[i] , end = “ “)

produces the output …

c h i c k e n

where the variable i stores the index values 0, 1, 2, 3, 4, 5, 6 during each iteration of the loop.

Constructing a String Character-by-Character

You can construct a string by concatenation(adding) using a for-loop and an assignment statement. For example,

word = “”
# called an empty string 
for i in range(3):
    letter =input(“Enter a letter: “)
    word = word + letter
print( word )

This python program could be run as follows:

Enter a letter: a
Enter a letter: b
Enter a letter: c
abc

This type of program is necessary in the game called Hangman ( a.k.a. Secret Word), so that the program can store each version of the secret word being guessed. For example,

secretWord = “chicken”
oldWord = “_______” 
for j in range(2):
    letter =input( “Guess a letter: “)
    newWord = “”
    for i in range(len(secretWord) ):
        if letter == secretWord[i]:
            newWord = newWord + letter
        else:
            newWord = newWord + oldWord[i]
    oldWord = newWord
    for ch in newWord:
        print( ch, end = “ “)
        print()

This python program could be run as follows:

Guess a letter: c
c _ _ c _ _ _
Guess a letter: n
c _ _ c _ _ n

Slices of a String

word = “chicken”

Sometimes you may want to select portions of a string called a slice. For example:

print( word[0:3] )

produces the output

chi

The slice of the string consists of the characters from index 0 to 3 (not including 3).

print( word[:3] )

produces the same output

chi

If the first index is missing, it is assumed the slice begins at the first character in the string. Similarly,

print( word[3:6] )

produces the output

cke

The slice of the string consists of the characters from index 3 to 6 (not including 6). Whereas,

print( word[3:] )

produces the output

cken

If the second index is missing, it is assumed the slice ends after the last character in the string. Adding a third number, allows you to step through the string with any size step. For example,

print( word[::2] )

produces the output

cikn

The slice starts at the beginning of the string and passes through in steps of 2 to just after the end of the string. This can be used to easily reverse a string as follows,

print( word[::-1] )

produces the output

nekcihc

In this case the first number is assumed to be the final position in the string, and the second just before the first position. Similarly,

print( word[5:2:-1] )

produces the output

ekc

The slice of the string consists of the characters from index 5 to 2 (not including 2).