Understanding for Loops

The Loop Behind the Guessing Game

The source example is presented under the topic of for loops, but the mechanism it demonstrates is a while loop. The program repeatedly asks for an integer, compares the guess with a target number, and gives feedback. It keeps repeating while the variable running is True. When the guess equals the target, the program prints a congratulatory message and changes running to False.

One Complete Loop Cycle

Before the loop block runs, the program checks whether running is True. If it is, the block executes. The block obtains a guess and uses conditional statements to compare that guess with number. After the block finishes, the loop condition is checked again. If running is still True, another iteration begins. If running has become False, the loop ends and its optional else block can run.

Following the Guess Sequence

Enter an integer : 50
No, it is a little lower than that.
Enter an integer : 22
No, it is a little higher than that.
Enter an integer : 23
Congratulations, you guessed it.
The while loop is over.
Done

Choosing the Feedback Branch

The first test asks whether guess == number. If that comparison is true, the program prints the congratulations message and makes the loop stop. If the first test is false, the elif test asks whether guess < number. A true result produces the message that the target is a little higher. If neither comparison is true, the final else produces the message that the target is a little lower.

What Happens After Repetition

The while loop has an optional else block. In this example, that else block prints "The while loop is over." It runs when the while-loop condition becomes False. The source also explains that the else block is always executed unless the loop is exited with a break statement. In the demonstrated path, the condition becomes False because the successful guess changes running to False, so the else block runs before the next statement prints "Done."

Mistakes in Loop Tracing

• Assuming the loop ends immediately after a comparison.

  A comparison selects a branch, but repetition is controlled by the running condition.

  Fix: After each loop block, check the value of running again. The loop repeats while it is True.

• Forgetting that the successful branch changes the loop state.

  If running stays True, the loop condition remains true and another iteration can begin.

  Fix: On the successful comparison, track the assignment that changes running to False.

• Reversing the higher and lower feedback.

  The source sequence uses 22 as a lower guess and therefore prints that the target is a little higher.

  Fix: For guess < number, select the message saying the target is a little higher. Otherwise, after the equality test fails, select the message saying the target is a little lower.

• Expecting the while-loop else block after every exit.

  The source states that the else block is skipped when break exits the loop.

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

A Reliable Tracing Method

1. Write down the current value of running before the loop iteration begins.
2. Record the current guess and compare it with number.
3. Identify which of if, elif, or else supplies the message.
4. Check whether the successful branch changes running to False.
5. If running is still True, trace the next guess; otherwise, trace the while-loop else block and then the following statement.

Key Takeaways

1. The source example uses a while loop controlled by the variable running.
2. The loop checks its condition before each iteration and checks it again after the loop block finishes.
3. The if, elif, and else branches choose feedback based on the relationship between guess and number.
4. A correct guess changes running to False, which ends repetition.
5. The while-loop else block runs when the condition becomes False, but not when break exits the loop.