6.5. Loop Through a String

The range command assigns the loop variable a new integer value each time the loop repeats. What if we want to assign the variable something other than whole numbers?

6.5.1. Characters Instead of Integers

Recall that a string is a series of characters enclosed in quotes, like 'Hello, World!' or "Python". Just like range assigns a new integer to the loop variable, we can set up a for loop to assign different characters from a string.

Run the following program to see how this works:

Some points to notice:

  1. In the for statement, a string (or a variable containing a string) replaces range.
  2. The first time the loop runs, the loop variable gets assigned the first character in the string (character = 'H' or char = 'T').
  3. Each time the loop repeats, the variable holds the next character in the string.
  4. Unlike range(n), which does NOT assign the value of n to the loop variable, the last character in the string IS used.
  5. Once Python reaches the end of the string, the loop ends.

6.5.2. Characters Another Way

Take a look at the following code and its output:

Example

1
2
3
4
5
6
word = 'Python'

print(word)
print(word[0])
print(word[1])
print(word[5])

Console Output

Python
P
y
n

Notice how lines 4 - 6 each print a single character from the string. print(word[0]) displays only the first letter (P). word[1] prints the second character, and word[5] prints the last.

We will explore strings in more detail in a later chapter. For now, we just need to recognize that every character in a string has an index. An index is a number that tells us the location of a character in the string.

Like range, the index values of a string start counting at 0.

Diagram showing the index values of a string.

The index values of a string.

Indexes provide a different way to loop through a string:

Example

Some items to note:

  1. By using range(6), the index variable takes values from 0 - 5.
  2. word[index] refers to the character in the string at position index.
  3. As written, the loop only displays characters up to index 5.
    • Try replacing the string with a longer one.
  4. If we replace the string with a shorter one, we get an error! For word[5] to work, there must be a character at index 5.
  5. To prevent “index out of range” errors in this loop, we should NOT use a specific number inside range.
  6. Replace range(6) with range(len(word)). len(word) always returns the length of the current string, so changing that string will not break the loop.

6.5.3. Check Your Understanding

Question

If phrase = 'Code for fun', then phrase[2] evaluates to:

  1. "o"
  2. "d"
  3. "for"
  4. "fun"

Question

What will be printed in the THIRD iteration of the loop:

1
2
for char in 'ABCDEFGHIJ':
   print(char)
  1. "ABC"
  2. "B"
  3. "C"
  4. "D"