While Loops in Python

A Loop That Keeps Checking

A while loop repeats a block of instructions as long as its condition remains true. In the guessing example from the source, the variable running is set to True before the loop begins. Python checks running, executes the while block when it is True, checks running again after that block finishes, and repeats the process if running is still True.

Tracing the Guessing Round

The comparison selects one of three outcomes. An equal guess produces the congratulations message and changes running to False. A lower guess produces the higher hint. A higher guess produces the lower hint. After any of these branches, Python returns to the loop condition and checks running again.

The Repeated Check

A while loop does not decide only once whether to run. The condition is checked before the loop block runs and again after the block has executed. If the condition is still true, the block runs again. If it is false, Python leaves the loop and proceeds to the optional else block, if one exists, before continuing to the next statement.

Stopping After a Correct Guess

In the source example, finding the correct number does not stop the loop in the middle of an unexplained process. The correct branch changes running to False. Python finishes that pass through the loop block, then checks the condition again. This time the condition is false, so the loop stops.

Comparing Higher and Lower Guesses

The source uses an if, elif, and else structure inside the while block. First it checks whether guess equals number. If not, it checks whether guess is less than number. When that second comparison is true, the program says the answer is a little higher. If neither comparison is true, the guess is greater than the number, so the program says it is a little lower.

The Optional Else Block

A while loop may have an else block after it. The source states that this else block executes when the while loop condition becomes False. It can execute when the condition is false on the first check, and it executes unless the loop is left with a break statement.

Mistakes Beginners Make

• Forgetting that the condition is checked again after the loop block.

  The source explains that the condition is checked again after the block executes. The next check is what finds running to be False and ends the loop.

  Fix: Trace the order as condition check, loop block, condition check again, and then either another pass or loop termination.

• Reversing the higher and lower messages.

  When guess is less than number, the source prints that the answer is a little higher. A greater guess produces the little lower message.

  Fix: Compare the guess with number before choosing the message: less means higher; greater means lower.

• Expecting the loop to stop simply because a correct guess was printed.

  In the source example, the correct branch also makes running False. That state change causes the next condition check to stop the loop.

  Fix: Ensure that the condition used by the while loop becomes False when the intended stopping event occurs.

• Assuming the while else block runs after every loop exit.

  The source states that the else block executes unless the loop is exited with a break statement.

  Fix: Distinguish normal termination through a false condition from termination caused by break.

Practice the State Trace

Key Takeaways

1. A while loop checks its condition before each pass through the loop block.
2. After the loop block finishes, Python checks the condition again.
3. In the guessing example, lower and higher guesses produce different messages, while a correct guess changes running to False.
4. The while loop's else block runs when the condition becomes False, including when it is false on the first check, unless break exits the loop.
5. To trace a while loop, follow the condition, the current branch, the condition-changing state, and the next condition check.