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    Lab 5
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# Lab 5: Loops

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## [Solutions](/content/labs/lab05/solutions/)
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{{-- ## [Worksheet solns](https://docs.google.com/document/d/1th9fUY_8kXomeyns4M0gPfdJDPCS1Z5y9DTmSgUHgRw/edit?usp=sharing) --}}


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<td> A simple diagram of the flow of a loop </td><td> <img src="loop.jpg" style='vertical-align: middle; max-width:200px' alt="A flowchart showing that when entering a loop, the loop condition is evaluated, and if it is false, the program continues past the loop. However, if the condition is true, the loop executes the loop body (conditional code) and then returns to the start of the loop where it proceeds to check the condition again."> </td>
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&nbsp;
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<td>A more engaging example of a loop in action </td><td> <img src="bao.gif" style='vertical-align: middle; max-width:400px' alt="A short animation of two hands holding an anthropomorphic bao (a kind of Chinese bun); the hands bring the bao closer to the viewpoint as the bao smiles and waves its arms."><br> <em>[From the Pixar short: Bao]</em> </td>
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## Big Questions {.bigqs}

- *If you want a while loop to stop when one of several conditions is
  met, how should you write the continuation condition??*
  <details class="answer">
    <summary>Show Answer</summary>
    In this case, you'd want to continue the loop as long as ALL of the
    conditions are NOT met, and stop as soon as ANY of them is met. So
    you can reverse each individual condition using <code>not</code>, and
    then combine them using <code>and</code>. For example, if you want to
    stop a loop when either 5 turns have passed or the user guesses
    correct, you could write: <code>while turns &lt; 5 and guess !=
    answer: ...</code> That condition would become False (stopping the
    loop) as soon as <code>turns</code> became 5, or as soon as the guess
    was equal to the answer, whichever happened first.
  </details>

- *What happens when you use <code>return</code> inside a loop, and why might
  you want to do that?*
  <details class="answer">
    <summary>Show Answer</summary>
    As always, if you use <code>return</code> inside a loop, the current
    function ends, which also exits the loop. You can use this to your
    advantage to end the loop early if you are looking for something and you
    find it (for example, does a string contain a certain letter: after you see
    that letter, you don't need to continue the loop; you can immediately
    return True). However, you have to be careful: in some cases, you need to
    process or check each item in your sequence, and an early return would
    prevent this (for example, finding how many copies of a certain letter
    a string contains; in this case, we need to check each letter, and cannot
    return early).
  </details>

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- *What is a loop variable, and how does it get its values?*
  <details class="answer">
    <summary>Show Answer</summary>
    A loop variable is the variable created when you create a loop, so if you
    write <code>for x in 'abc':</code>, <code>x</code> is your loop variable.
    In each iteration of the loop, the loop variable takes the next value from
    the sequence that the loop is iterating over (in this example, the letters
    of the string <code>'abc'</code>).
  </details>
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