Iterating Over Strings with for Loops

One Instruction, Many Characters

Suppose you want to display every character in a word. You could handle each character separately, but a for loop lets you write the processing instruction once and run it for every character. When Python traverses a string with a for loop, it automatically assigns one character at a time to a loop variable.

fruit = "banana"
for char in fruit:
    print(char)

b
a
n
a
n
a

Positions Become Iterations

The string banana contains six characters. Their positions correspond to the order in which the loop receives them: b first, then a, then n, then a, then n, then a. The loop variable receives the character itself rather than displaying the position number. You do not manually write an index, increment a counter, or check whether the end of the string has been reached.

Following the Loop Variable

To trace a loop, record the loop variable's value at every iteration and then record what the loop body does with that value. In the example below, char is reassigned as control moves through fruit. It does not keep all the characters at once; it holds the current character for the current iteration.

word = "code"
for char in word:
    print("Character:", char)
print("Finished")

Character: c
Character: o
Character: d
Character: e
Finished

Inside and After the Loop

Indentation determines which statements belong to the loop body. A statement indented under the for statement runs once for every character. A statement at the original indentation level runs after the loop has processed all characters. This difference controls whether an action repeats or happens only once.

Naming the Current Character

The loop variable name is arbitrary. In a string loop, names such as char or letter can indicate that the variable holds one character. A descriptive name makes the loop self-documenting. A short name may work, but it gives the reader less information about the value being processed.

fruit = "pear"
for letter in fruit:
    print("Letter:", letter)

Mistakes During Traversal

• Forgetting to indent the loop body.

  The statement intended to run for each character is not indented under the for statement.

  Fix: Indent the loop body so Python can identify it as part of the loop.

• Indenting code that should run after the loop.

  Finished is inside the loop, so it runs once per character rather than once after all characters.

  Fix: Place the final print statement at the original indentation level.

• Using the wrong variable name inside the loop.

  The loop assigns characters to char, not letter.

  Fix: Use the same loop variable name in the body, such as print(char).

• Expecting the loop variable to contain an index number.

  A string loop gives the loop variable each character directly. It does not display the position number.

  Fix: Treat char as the current character and trace the characters in their string order.

Practice the Trace

A reliable trace follows the same pattern every time: identify the next character, assign it mentally to the loop variable, execute the indented body, and then move to the next character. After the final character, stop repeating the indented body and continue with the unindented code.

Key Takeaways

1. A for loop over a string assigns each character to its loop variable in sequence.
2. The loop body runs once for every character, while unindented code runs after the loop completes.
3. Tracing the loop variable step by step helps you predict output and locate unexpected behavior.
4. The loop variable name can be chosen freely, but a descriptive name makes the code easier to understand.
5. A for loop handles the traversal automatically, so manual index tracking is not required.