explain xkcd - User contributions [en]

342: 1337: Part 2

2012-09-11T06:18:19Z

Readerboy7: /* Explanation */

{{comic
| number = 342
| date = November 13, 2007
| title = 1337: Part 2
| image = 1337_part_2.png
| imagesize =
| titletext = Trivia: Elaine is actually her middle name.
}}

==Explanation==
{{w|Donald Knuth}} is a computer science Professor Emeritus at {{w|Stanford University}} who is famous for writing {{w|The Art of Computer Programming}} and developing the TeX computerized typesetting system. He may not have a mountain hideaway, but he would be one of the best mentors a budding hacker could have.

The {{w|A* search algorithm}} and {{w|Dijkstra's algorithm}} are {{w|Graph traversal|graph search}} {{w|algorithm}}s. And what study of algorithms wouldn't be complete without a healthy study about finding complexities? {{w|Computational complexity theory|Time complexity}} is the amount of time an algorithm takes to execute. Upper and lower bounds for complexity is written in {{w|Big O notation}}. Best possible execution of an algorithm is constant time, or O(1), said in words, for any given data set no matter how large the algorithm will always return the answer in the same time. However, constant time is extremely difficult to achieve, linear time (O(n)) is also very good. For more complex algorithms, [http://www.wolframalpha.com/input/?i=plot+x*logx O( n*log(n) )] is good, but [http://www.wolframalpha.com/input/?=plot+x*log%28logx%29 O( n*log(log(n)) )] is better.

Her son Bobby, might be the Robert'); DROP TABLE students;-- in xkcd [[327]]

==Transcript==
[Man 1 standing near Man 2, who is on the floor near the armchair.]

Man 2: So the greatest hacker of our era is a cookie-baking mom?

Man 1: Second-greatest.

Man 2: Oh?

[A young girl with a ponytail is laying on the floor looking at the screen of a computer that appears to have been pieced together. A younger boy is finger painting at an easel]

Man 1 (Narrating) : Mrs. Roberts had two children. Her son, Bobby, was never much for computers, but her daughter Elaine took to them like a ring in the bell.

[The back of a car is in frame. Mrs. Roberts is waving goodbye to her daughter who is wearing a backpack and is holding a walking stick. She is about to begin climbing a staircase built into a mountain.]

Man 1 (Narrating) : When Elaine turned 11, her mother sent her to train under Donald Knuth in his mountain hideaway.

[Donald Knuth is standing with a pointing stick at a chalk board with graph traversal patterns on it.]

Man 1 (Narrating) : For four years she studied algorithms.

Knuth: Child—

[Knuth whips around slashing the stick like a sword. Elaine jumps and lands on the stick.]

Knuth: Why is A* search wrong in this situation?

<<swish>>

Elaine: Memory usage!

Knuth: What would you use?

Elaine: Dijkstra's algorithm!

[They are outside both working on a chalkboard with a separator down the middle so they cannot look at each other's work. Elaine is no longer wearing her hair in a ponytail.]

Man 1 (Narrating) : Until one day she bested her master

Knuth: So our lower bound here is 0(n log n)

Elaine: Nope. Got it in 0(n log (log n))

Man 1 (Narrating) : And left.

{{comic discussion}}
[[Category:Comics featuring Cueball]]

[[Category:Comics featuring Donald Knuth]]

342: 1337: Part 2

2012-09-11T06:15:52Z

Readerboy7: /* Explanation */

{{comic
| number = 342
| date = November 13, 2007
| title = 1337: Part 2
| image = 1337_part_2.png
| imagesize =
| titletext = Trivia: Elaine is actually her middle name.
}}

==Explanation==
{{w|Donald Knuth}} is a computer science Professor Emeritus at {{w|Stanford University}} who is famous for writing {{w|The Art of Computer Programming}} and developing the TeX computerized typesetting system. He may not have a mountain hideaway, but he would be one of the best mentors a budding hacker could have.

The {{w|A* search algorithm}} and {{w|Dijkstra's algorithm}} are {{w|Graph traversal|graph search}} {{w|algorithm}}s. And what study of algorithms wouldn't be complete without a healthy study about finding complexities? {{w|Computational complexity theory|Time complexity}} is the amount of time an algorithm takes to execute. Upper and lower bounds for complexity is written in {{w|Big O notation}}. Best possible execution of an algorithm is constant time, or O(1), said in words, for any given data set no matter how large the algorithm will always return the answer in the same time. However, constant time is extremely difficult to achieve, linear time (O(n)) is also very good. For more complex algorithms, [http://www.wolframalpha.com/input/?i=plot+x*logx O( n*log(n) )] is good, but [http://www.wolframalpha.com/input/?=plot+x*log%28logx%29 O( n*log(log(n)) )] is better.

Her son Bobby, might be the Robert'); DROP TABLE Students in xkcd [[327]]

==Transcript==
[Man 1 standing near Man 2, who is on the floor near the armchair.]

Man 2: So the greatest hacker of our era is a cookie-baking mom?

Man 1: Second-greatest.

Man 2: Oh?

[A young girl with a ponytail is laying on the floor looking at the screen of a computer that appears to have been pieced together. A younger boy is finger painting at an easel]

Man 1 (Narrating) : Mrs. Roberts had two children. Her son, Bobby, was never much for computers, but her daughter Elaine took to them like a ring in the bell.

[The back of a car is in frame. Mrs. Roberts is waving goodbye to her daughter who is wearing a backpack and is holding a walking stick. She is about to begin climbing a staircase built into a mountain.]

Man 1 (Narrating) : When Elaine turned 11, her mother sent her to train under Donald Knuth in his mountain hideaway.

[Donald Knuth is standing with a pointing stick at a chalk board with graph traversal patterns on it.]

Man 1 (Narrating) : For four years she studied algorithms.

Knuth: Child—

[Knuth whips around slashing the stick like a sword. Elaine jumps and lands on the stick.]

Knuth: Why is A* search wrong in this situation?

<<swish>>

Elaine: Memory usage!

Knuth: What would you use?

Elaine: Dijkstra's algorithm!

[They are outside both working on a chalkboard with a separator down the middle so they cannot look at each other's work. Elaine is no longer wearing her hair in a ponytail.]

Man 1 (Narrating) : Until one day she bested her master

Knuth: So our lower bound here is 0(n log n)

Elaine: Nope. Got it in 0(n log (log n))

Man 1 (Narrating) : And left.

{{comic discussion}}
[[Category:Comics featuring Cueball]]

[[Category:Comics featuring Donald Knuth]]

File:delta p.png

2012-09-11T05:48:37Z

Readerboy7: Readerboy7 uploaded a new version of "File:delta p.png"

== Summary ==
XKCD Comic #969
== Licensing ==
{{XKCD file}}

969: Delta-P

2012-09-11T05:46:50Z

Readerboy7: Created page with "{{comic | number = 969 | date = | title = Delta-P | image = http://www.explainxkcd.com/wiki/index.php?title=File:delta_p.png | imagesize = 305px x 661px | tit..."

{{comic
| number = 969
| date =
| title = Delta-P
| image = http://www.explainxkcd.com/wiki/index.php?title=File:delta_p.png
| imagesize = 305px x 661px
| titletext = If you fire a Portal gun through the door of the wardrobe, space and time knot together, which leads to a frustrated Aslan trying to impart Christian morality to the Space sphere.
}}

==Explanation==
: the wardrobe that leads to Narnia is attached to an anchor. below there is a formula which shows the flow rate into the wardrobe. the idea is that the water will fly into narnis and hit the white witch, who will melt into sludge like the 'wicked witch of the west' from 'wizard of Oz'
: the title text seems to reference to the video game 'Portal'. I haven't played it so I can't tell, but it seems to say that firing one portal through another will twist space and time. please edit this further.
==Transcript==

Q = A(2gd)^(1
2)

:Q = flow rate
:A = area of opening
:d = ocean depth (2 km)
:g = Earth gravity

:Flow: ~400,000 liters

:Water jet velocity: ~200 m

:The White Witch didn't know what hit her.

{{comic discussion}}