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SPEAKER 1: This is the
iconic CS50 phone book tear.

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David J. Malan does it every year,
but what does it actually mean?

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He does it to illustrate an
efficient algorithm, a concept that's

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very important to computer science.

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Let's say you need to find a name
in a phone book, an inefficient way

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to do this would be to check every
page until you find the right one.

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That would take ages.

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An efficient way, however, would be
going to the middle of the phone book

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and seeing if the name
you're looking for

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is contained in one or the other half.

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Once you know which half
you need, you can literally

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throw away the other half.

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You do that until you get to
the name you need and ultimately

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end up saving tons of time, or in
the case of CS, computing power.

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Check out CS50 Lecture 3 on YouTube
to learn more about algorithms.