--- title: Floors & Ceilings TARGET DECK: Obsidian::STEM FILE TAGS: algebra algorithm tags: - algebra --- ## Overview The **floor** of $x$ is the greatest integer less than $x$. The **ceiling** of $x$ is the least integer greater than $x$. These values are denoted $\lfloor x \rfloor$ and $\lceil x \rceil$ respectively. %%ANKI Basic How is the floor of $x$ denoted? Back: $\lfloor x \rfloor$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What is the floor of $x$? Back: The greatest integer less than or equal to $x$. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic How is the ceiling of $x$ denoted? Back: $\lceil x \rceil$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What is the ceiling of $x$? Back: The least integer greater than or equal to $x$. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given integer $x$, when does $\lfloor x / 2 \rfloor = \lceil x / 2 \rceil$? Back: When $x$ is even. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given integer $x$, when does $\lfloor x / 2 \rfloor \neq \lceil x / 2 \rceil$? Back: When $x$ is odd. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What does $\lceil x \rceil - \lfloor x \rfloor$ equal? Back: Either $0$ or $1$. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What can be said about $x$ if $\lceil x \rceil - \lfloor x \rfloor = 0$? Back: $x$ is an integer. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What can be said about $x$ if $\lceil x \rceil - \lfloor x \rfloor = 1$? Back: $x$ is not an integer. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What values of $x$ satisfy $\lfloor x \rfloor = \lceil x \rceil - 1$? Back: Non-integral values. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What values of $x$ satisfy $\lceil x \rceil = \lfloor x \rfloor + 1$? Back: Non-integral values. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What values of $x$ satisfy $\lfloor x \rfloor = \lceil x \rceil + 1$? Back: None. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What values of $x$ satisfy $\lceil x \rceil = \lfloor x \rfloor - 1$? Back: None. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic For what values $k$ is horizontal and vertical shifting of $\lfloor x \rfloor$ the same? Back: Integral values. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What C operator corresponds to floor division? Back: N/A. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What C operator corresponds to integer division? Back: `/` Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic When does C operator `/` behave like floor division? Back: When the result is a nonnegative value. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic When does C operator `/` behave like ceiling division? Back: When the result is a nonpositive value. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic What C operator corresponds to ceiling division? Back: N/A. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic How does C evaluate `10 / 3`? Back: `3` Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). Tags: c17 END%% %%ANKI Basic How does C evaluate `floor(10.f / 3)`? Back: `3` Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). Tags: c17 END%% %%ANKI Basic How does C evaluate `ceil(10.f / 3)`? Back: `4` Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). Tags: c17 END%% %%ANKI Basic How does C evaluate `-10 / 3`? Back: `-3` Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). Tags: c17 END%% %%ANKI Basic How does C evaluate `floor(-10.f / 3)`? Back: `-4` Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). Tags: c17 END%% %%ANKI Basic How does C evaluate `ceil(-10.f / 3)`? Back: `-3` Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). Tags: c17 END%% %%ANKI Basic Given $r = \lfloor (p + q) / 2 \rfloor$, fair partitioning requires `A[r]` to be included in which of `A[p:r-1]` or `A[r+1:q]`? Back: `A[p:r-1]` Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given $r = \lfloor (p + q) / 2 \rfloor$, when is `A[p:r]` or `A[r+1:q]` equally sized? Back: When `A[p:q]` has even size. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given $r = \lceil (p + q) / 2 \rceil$, fair partitioning requires `A[r]` to be included in which of `A[p:r-1]` or `A[r+1:q]`? Back: `A[r+1:q]` Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic If `A[p:q]` has odd size, what `r` most fairly allows partitions `A[p:r]` and `A[r+1:q]`? Back: $r = (p + q) / 2$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic If `A[p:q]` has odd size, what `r` most fairly allows partitions `A[p:r-1]` and `A[r:q]`? Back: $r = (p + q) / 2$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic If `A[p:q]` has odd size, what `r` ensures `A[p:r-1]` has same size as `A[r+1:q]`? Back: $r = (p + q) / 2$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic If `A[p:q]` has even size, what `r` most fairly allows partitions `A[p:r]` and `A[r+1:q]`? Back: $r = \lfloor (p + q) / 2 \rfloor$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic If `A[p:q]` has even size, what `r` most fairly allows partitions `A[p:r-1]` and `A[r:q]`? Back: $r = \lceil (p + q) / 2 \rceil$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given `A[p:q]` and $r = \lfloor (p + q) / 2 \rfloor$, how does the size of `A[p:r]` compare to `A[r+1:q]`? Back: It either has zero or one more members. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given `A[p:q]` and $r = \lfloor (p + q) / 2 \rfloor$, what is the size of `A[p:r]` in terms of $n = q - p + 1$? Back: $\lceil n / 2 \rceil$ Reference: Thomas H. Cormen et al., *Introduction to Algorithms*, 3rd ed (Cambridge, Mass: MIT Press, 2009). END%% %%ANKI Basic Given `A[p:q]` and $r = \lfloor (p + q) / 2 \rfloor$, what is the size of `A[r+1:q]` in terms of $n = q - p + 1$? Back: $\lfloor n / 2 \rfloor$ Reference: Thomas H. Cormen et al., *Introduction to Algorithms*, 3rd ed (Cambridge, Mass: MIT Press, 2009). END%% %%ANKI Basic Given `A[p:q]` and $r = \lceil (p + q) / 2 \rceil$, how does the size of `A[p:r-1]` compare to `A[r:q]`? Back: It either has zero or one fewer members. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given `A[p:q]` and $r = \lceil (p + q) / 2 \rceil$, what is the size of `A[r:q]` in terms of $n = q - p + 1$? Back: $\lceil n / 2 \rceil$. Reference: Thomas H. Cormen et al., *Introduction to Algorithms*, 3rd ed (Cambridge, Mass: MIT Press, 2009). END%% %%ANKI Basic Given `A[p:q]` and $r = \lceil (p + q) / 2 \rceil$, what is the size of `A[p:r-1]` in terms of $n = q - p + 1$? Back: $\lfloor n / 2 \rfloor$. Reference: Thomas H. Cormen et al., *Introduction to Algorithms*, 3rd ed (Cambridge, Mass: MIT Press, 2009). END%% %%ANKI Basic Given `A[p:q]` and $r = \lfloor (p + q) / 2 \rfloor$, how does the size of `A[p:r-1]` compare to `A[r:q]`? Back: It either has one or two fewer members. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given `A[p:q]` and $r = \lceil (p + q) / 2 \rceil$, how does the size of `A[p:r]` compare to `A[r+1:q]`? Back: It either has one or two more members. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given `A[p:q]` and $r = \lfloor (p + q) / 2 \rfloor$, *why* is the size of `A[p:r]` potentially larger than `A[r+1:q]`? Back: If `A[p:q]` has odd size, `A[p:r]` contains the midpoint. Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% ## Identities For integers $x$ and $y > 0$, $$\begin{align*} \left\lfloor \frac{x}{y} \right\rfloor & = \left\lceil \frac{x}{y} - \frac{y - 1}{y} \right\rceil \\ \left\lceil \frac{x}{y} \right\rceil & = \left\lfloor \frac{x}{y} + \frac{y - 1}{y} \right\rfloor \end{align*}$$ %%ANKI Basic If $n$ is even, what integer value does $\lfloor n / 2 \rfloor$ evaluate to? Back: $n / 2$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic If $n$ is odd, what integer value does $\lfloor n / 2 \rfloor$ evaluate to? Back: $(n - 1) / 2$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic If $n$ is even, what integer value does $\lceil n / 2 \rceil$ evaluate to? Back: $n / 2$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic If $n$ is odd, what integer value does $\lceil n / 2 \rceil$ evaluate to? Back: $(n + 1) / 2$ Reference: Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). END%% %%ANKI Basic Given integers $x$ and $y > 0$, what value of $Bias$ satisfies the following identity? $$\left\lceil \frac{x}{y} \right\rceil = \left\lfloor \frac{x}{y} + Bias \right\rfloor$$ Back: $(y - 1) / y$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Given integers $x$ and $y > 0$, what value of $Bias$ satisfies the following identity? $$\left\lceil \frac{x}{y} \right\rceil = \left\lfloor \frac{x + Bias}{y} \right\rfloor$$ Back: $(y - 1)$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Given integers $x$ and $y > 0$, what operator satisfies the following identity? $$\left\lceil \frac{x}{y} \right\rceil = \left\lfloor \frac{x}{y} \;\square\; \frac{y - 1}{y} \right\rfloor$$ Back: $+$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What intuition explains why the following identity holds for integers $x$ and $y > 0$? $$\left\lceil \frac{x}{y} \right\rceil = \left\lfloor \frac{x}{y} + \frac{y - 1}{y} \right\rfloor$$ Back: $(y - 1) / y$ only affects the RHS if and only if $x / y$ is not an integer. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Given integers $x$ and $y > 0$, what value of $Bias$ satisfies the following identity? $$\left\lfloor \frac{x}{y} \right\rfloor = \left\lceil \frac{x}{y} - Bias \right\rceil$$ Back: $(y - 1) / y$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Given integers $x$ and $y > 0$, what value of $Bias$ satisfies the following identity? $$\left\lfloor \frac{x}{y} \right\rfloor = \left\lceil \frac{x - Bias}{y} \right\rceil$$ Back: $(y - 1)$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic Given integers $x$ and $y > 0$, what operator satisfies the following identity? $$\left\lfloor \frac{x}{y} \right\rfloor = \left\lceil \frac{x}{y} \;\square\; \frac{y - 1}{y} \right\rceil$$ Back: $-$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What intuition explains why the following identity holds for integers $x$ and $y > 0$? $$\left\lfloor \frac{x}{y} \right\rfloor = \left\lceil \frac{x}{y} - \frac{y - 1}{y} \right\rceil$$ Back: $(y - 1) / y$ only affects the RHS if and only if $x / y$ is not an integer. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Cloze For any integer $n$, floor expression {$\lfloor n / 2 \rfloor$} is equal to ceiling expression {$\lceil (n - 1) / 2 \rceil$}. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Cloze For any integer $n$, ceiling expression {$\lceil n / 2 \rceil$} is equal to floor expression {$\lfloor (n + 1) / 2 \rfloor$}. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What identity generalizes the following? $$\left\lfloor \frac{n}{2} \right\rfloor = \left\lceil \frac{n - 1}{2} \right\rceil$$ Back: $$\left\lfloor \frac{n}{d} \right\rfloor = \left\lceil \frac{n - (d - 1)}{d} \right\rceil$$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI Basic What identity generalizes the following? $$\left\lceil \frac{n}{2} \right\rceil = \left\lfloor \frac{n + 1}{2} \right\rfloor$$ Back: $$\left\lceil \frac{n}{d} \right\rceil = \left\lfloor \frac{n + (d - 1)}{d} \right\rfloor$$ Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% ## Bibliography * Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. * Ronald L. Graham, Donald Ervin Knuth, and Oren Patashnik, *Concrete Mathematics: A Foundation for Computer Science*, 2nd ed (Reading, Mass: Addison-Wesley, 1994). * Thomas H. Cormen et al., *Introduction to Algorithms*, 3rd ed (Cambridge, Mass: MIT Press, 2009).