292: goto

2024-03-26T17:06:21Z

TRSNS: /* Explanation */ Fixed a borked link. Linking to a scan of the original.

{{comic
| number = 292
| date = July 20, 2007
| title = goto
| image = goto.png
| titletext = Neal Stephenson thinks it's cute to name his labels 'dengo'
}}

==Explanation==
''{{w|Goto}}'' is a construct found in many computer languages that causes control flow to go from one place in a program to another, without returning. Once common in computer programming, its popularity diminished in the 1960s and 1970s as focus on {{w|structured programming}} became the norm. {{w|Edsger W. Dijkstra}}'s article [https://dl.acm.org/doi/10.1145/362929.362947 "Go To Statement Considered Harmful"] in particular contributed to the decline of ''goto.''

Often, people learning programming are told that ''goto'' is bad and should be avoided, but frequently, they are not given a good reason. [[Cueball]], as one of these people, sees no harm in using ''goto'' to avoid rewriting much of his program, most likely written in the C programming language. As a result, he is attacked by a [[velociraptor]]. Velociraptor attacks are a running joke (and fear) often expressed in [[xkcd]].

The name ''main_sub3'' of the ''goto'' destination also shows bad programming style: it is an unmeaningful name suggesting that at least two similarly unmeaningful names are used for other ''goto'' marks and it is contradictory in that a chunk of code can either belong to the main program or to a subroutine, but not both.

The title text refers to {{w|Neal Stephenson}}, an author of cyberpunk novels. A label is used in many programming languages to refer to a point in a program that a goto instruction can jump to. The joke is that one of Stephenson's characters in ''{{w|Cryptonomicon}}'' is named Goto Dengo. When said out loud, "Dengo" sounds like "Then go."

==Transcript==
:[Sideways view of Cueball sitting at computer, thinking.]
:Cueball: I could restructure the program's flow
:Cueball: or use one little 'GOTO' instead.

:[Cueball starts typing.]
:Cueball: Eh, screw good practice. How bad can it be?
:Text on computer: <tt>goto main_sub3;</tt>
:<nowiki>*Compile*</nowiki>

:[We now have a view from behind Cueball. Cueball looks at the computer.]

:[A raptor jumps into the panel, pushing Cueball off his chair.]

==Trivia==
* Like all velociraptors in xkcd, the velociraptor in this comic is specifically the man-sized movie monster from ''{{w|Jurassic Park (film)|Jurassic Park}}.'' Real velociraptors were more like [http://commons.wikimedia.org/wiki/File:Vraptor-scale.png carnivorous turkeys.] The beast devouring Cueball might actually be ''[http://commons.wikimedia.org/wiki/File:Dromie_scale.png Deinonychus antirrhopus.]'' The beast may also be ''[https://en.wikipedia.org/wiki/Utahraptor Utahraptor.]''
* This comic heads the official [http://php.net/manual/en/control-structures.goto.php PHP manual page on goto].

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

2687: Division Notation

2022-10-20T19:32:42Z

TRSNS: /* Explanation */

{{comic
| number = 2687
| date = October 19, 2022
| title = Division Notation
| image = division_notation_new_2x.png
| imagesize = 235x310px
| noexpand = true
| titletext = Science tip: Scientists hardly ever use the two-dot division sign, and when they do it often doesn't even mean division, but they still get REALLY mad when you repurpose it to write stuff like SALE! ALL SHOES 30÷ OFF!
}}

==Explanation==
{{incomplete|Created by a GROUP OF SCHOOLCHILDREN DIVIDED AMONGST THEMSELVES. Do NOT delete this tag too soon.}}

This comic pokes fun at some of the ways to write the {{w|Division (mathematics)|division}} operation in math. In this comic, [[Randall]] has used A as the dividend (the number being divided) and B as the divisor (the number that A is divided by). Division is the fourth simplest arithmetic operation in mathematics, after addition, subtraction, and multiplication.[https://plato.stanford.edu/entries/principia-mathematica/#PartIVRelaArit]

The first two of the seven notations shown are the {{w|division sign}} (÷) and the {{w|long division}} notation used for {{w|short division}} and {{w|long division}} in beginning arithmetic. (Note: division typography is only used in some countries, and there are [https://en.wikipedia.org/wiki/Long_division#Notation_in_non-English-speaking_countries different notations in the non-English speaking world]). These methods of division are often used by school children because the ÷ sign is what most people use when first learning division, and the short division format is usually the first algorithm learned for dividing arbitrary dividends, typically starting with the easier abbreviated short division form.

The expression on the third line, A/B, is the way division is usually written in software code. The four simple arithmetic operations in programming usually are +, -, *, /. This line was not in the [[media:division_notation_2x.png|first version]] of the comic. This is most commonly seen in regular mathematics as it somewhat saves space, and is easy to type with the slash key. Additionally, it uses standard {{w|ASCII}} characters instead of sophisticated notation. A notable exception is {{w|APL_(programming_language)|APL}}, which uses an idiosyncratic character set modeled after traditional arithmetic.

The expression on the fourth line, a/b, is how division is usually written by hand. It is nearly identical to the fraction notation that follows, but the diagonal line allows each number to be bigger while still fitting into a single line of text. The third line's representation is the best approximation of this line's notation on a computer, without using more obscure Unicode characters. The Unicode character set can be used to accurately represent division as on the fourth line in plain text, using a small set of precomposed fractions (⅔, ⅕, etc.), regular numerals and [https://unicode-explorer.com/c/2044 U+2044 FRACTION SLASH] (e.g. 22⁄7, provided font support exists), or superscript and subscript numerals (e.g. ²²/₇, or ²²⁄₇ with the fraction slash). Using any of these requires a greater knowledge of Unicode, and the know-how (and, possibly, patience) to type them, so it is likely that only a Unicode enthusiast would type division like this.

The fifth notation is the way division is written in science: <table style="display: inline-table; line-height: 0.6em; vertical-align: middle; font-size:7pt; text-size-adjust: none;"><tr><td>A</td></tr><tr><td>B</td></tr></table>, i.e. the dividend on the top of the expression, over the divisor on the bottom with a horizontal line separating them. This is how a {{w|Fraction|fraction}} would be written. It has the advantage of clearly separating the numerator and denominator when they are longer expressions, such as polynomials, without needing to add parentheses. This format is mostly used in written and professionally typeset math, as it can't be typed without something like {{w|MathML}}, {{w|LaTeX}} or HTML tables.

The sixth, "fancy," notation uses a negative exponent. The exponent -1 is equivalent to {{w|Multiplicative inverse|reciprocation}}. It can be used to keep an entire division expression on one line. Note that AB-1 is equal to A/B only if A and B are in a commutative ring (and B has an inverse). If A and B are, for example, matrices, AB-1 is not the same in general as B-1A (and the notation A/B is never used in this case as it would be ambiguous).

The AB-1 format is also often used to express physical units.

The final form of notation declares a function. The writer defines a new function, F, that takes in the parameters A and B, before listing out the function's definition (trailing off in increasingly smaller text). Randall warns the reader they should escape while they still can, because both the function itself and the math environment as a whole are going to get relatively tedious. Integer division can be defined in terms of multiplicative inequalities and the remainder, or modulo ('%' in Python), operator. This situation is likely to occur in many sorts of algebra, where one might have to define what "division" means for two elements of a mathematical object such as a group, ring, or magma. One example would be an object G, such that, for two elements A and B of G, "A divided by B" is defined as an element C such that CB=A, or alternatively as an element C such that BC=A. These definitions will differ if multiplication in G is not commutative. Furthermore, if such a C is not unique, the function F(A,B) will need to include a method to select a unique value for "A divided by B" for each A and B. Thus, the F(A,B) in the comic might not even refer to a uniquely defined operation, but simply to the property of a function F(A,B) that is a valid division operation on G, given some definition of division. You were warned.

The title text discusses how the division sign (÷) has fallen out of favor in most professional contexts (the ISO-80000 guidelines even specify the symbol "should not be used") yet has resisted all efforts to repurpose it as a new function. Specifically, it pokes fun at how similar the division sign is to a percent sign. A scientist might be really mad at the use of the division sign as an alternative to a percent sign not just because it is the wrong symbol in that context, but also because of the tedium of modern consumerist practices.

==Transcript==
{{incomplete transcript| Really should have descriptions instead of 'span styles'... Consider giving it descriptive labels instead (or at least as well as)... Do NOT delete this tag too soon.}}
:Division notation
:A÷B
:B⟌A Schoolchild.
:A/B Software engineer.

A
B
 Scientist
:AB-1 Fancy scientist.
:F(A, B) such that F(G)= (text getting smaller) Oh no, run

{{comic discussion}}

[[Category:Math]]
[[Category:Science]]
[[Category:Programming]]
[[Category:Science tip]]

explain xkcd - User contributions [en]

292: goto

2687: Division Notation