Go to statement considered harmful. Author: Edsger W. Dijkstra, Technological University, Eindhoven, The Netherlands View colleagues of Edsger W. Dijkstra . “GOTO considered harmful” needs to be read in its historical context, where structured programming (for Thanks for posting Dijkstra answer. One of the comments did mention Dijkstra’s article Go to Statements Considered Harmful, but aside from that mention, there was much.

Author: Akinonos Zulkikinos
Country: Honduras
Language: English (Spanish)
Genre: Medical
Published (Last): 15 November 2007
Pages: 149
PDF File Size: 14.5 Mb
ePub File Size: 2.39 Mb
ISBN: 892-8-48083-366-6
Downloads: 43193
Price: Free* [*Free Regsitration Required]
Uploader: Mataxe

The set of flow control statements and clauses provided should be powerful and flexible enough so that a programmer can express his ideas clearly and succintly without having to resort to the use of extraneous control variables or to rearrange his code unnaturally just to get around the syntactical restrictions of the language. Note that this example is also in the title of a slightly cranky letter to the editor – it’s probably not an accident that the first example that came to hand of “considered harmful” in a pre-Dijkstra title was of this type.

He was probably right in this choice however, as the paper got enough attention to have a significant and lasting influence though we are not doing analytical verification to anywhere near the extent he demanded. This movement, which began in the late s, was similar in spirit to the earlier movement in mathematics known as Hilbert’s programmeexpressed by David Hilbert, which was intended to codify all of mathematics in a complete and all-inclusive set of laws derived from nothing more than the building blocks of the natural numbers and the rules of simple logic and arithmetic.

A loop with a continue statement must take the conditions at the continue statement into account at the beginning of the loop. Dobb’s encourages readers to engage in spirited, healthy debate, including taking us to task.

My point was that in many domains goto’s are useful, both for solving problems, getting performance and for code clarity. I’m sure it’s not the earliest example of this phrase used in a headline or title, either — I chose it only as a convenient illustration of susage a couple of decades before the date of Dijkstra’s paper.

Dijkstra mentions the design of the case or select control flow structure as proposed by Hoare and Wirth. Purists say this sullies an otherwise logically clean control construct, while pragmatists say that it allows for more efficient code in many difficult programming situations. However, he states, once the programmer has written a working program, the actual execution of the program is entirely under the control of the machine itself.

Read a header block from the opened file. Some languages provide special syntax specifically for terminating a loop in the middle of its body:. One can regard and appreciate the clauses considered as bridling its use.

What Dijkstra said was harmful about goto statements | Dr Dobb’s

Other languages such as Modula-2 and Oberon also do not provide goto, but appear to lack enough flow control constructs to make it convenient to write early loop exits and exception handling code; it would seem that such languages were linguistic experiments that took Dijkstra’s maxim too far and failed.


Here it is, used in OpenBSD’s httpd: A major goal of all of this effort was to create a nomeclature that could be used not just for actual programming languages, but which also could be used directly for mathematical formulations of programming algorithms. I entered the undergraduate C. Usually, people take into account as well the values of some well chosen variables, but this is out of the question because it is relative to the progress that the meaning of these values is to be understood!

Optimizers have also gotten better over the years–it would be fairly easy to transform the idiomatic find-loop pattern based on flags to the one that uses the goto. A large part of the debate that followed the publication of “GOTO Considered Harmful”, and which smolders to this day, concerned whether there are any corner cases where gotos are the better choice. Again, Dijkstra states the obvious, that once a program is written and running, hramful programmer no dijktsra has any control over the actual execution.

Several structured programming languages do not provide goto statements at all, including Modula-2, Modula-3, Oberon, Eiffel, and Java, on the assumption that the other flow control mechanisms they do provide are sufficient for all programming tasks and thus goto statements should never be needed. There’s also been a shift to using basic block-based optimizers, which means that the optimizer sees a loop statement and something built entirely of gotos as completely identical anyways.

My second remark is that our intellectual powers are rather geared to master static relations and that our powers to visualize processes evolving in time are relatively poorly developed.

It is more difficult in some languages especially some older languages to write multiple if-then sequences, considerwd the construct ends up looking like the following code, which is functionally equivalent but harder to read:. In my current dijkstrra I use both. Dijkstea primitive languages such as FORTRAN, COBOL, C, Pascal, and the like may provide some kinds of primitive exception handling mechanisms, but their use does not guarantee that the execution state is cleanly preserved or that allocated resources will be properly released.

Allowing unstructured gotos to change the course of execution could cause that invariant to become invalid i. However, they do not corrupt or obfuscate the program execution state. This type of use of goto statements is generally accepted as a “correct” use of goto.

More efficient implementations are possible, such as using an index into a jump table, or rearranging the comparisons to emulate an dikjstra binary search, etc. A common programming problem is handling loop-and-a-half constructs, i.

There was never any real evidence that Goto was harmful, as the article states people accepted it was harmful without any proof.

Some structured programming languages do not provide goto statements at all. But without rules that enforce these limitations, the goto statement provided by a language cannot be said to be truly well-structured.


Some examples of inadequate flow control constructs are discussed in more detail below. Note that, as before, the clean-up operations are performed in the reverse order that their corresponding allocation operations are performed. Another common use for goto statements is in the handling of exceptionsor what Dijkstra called abortion clauses.

Save a copy of a specified file name.

I’d Consider That Harmful, Too

Essentially, Dijkstra argues that the “unbridled use” of goto statements in a program obscures the execution state and history of the program, so that at any given moment the values of the call stack and loop iteration stack are no longer sufficient to determine the value of the program variables. So now, instead of merely having to inspect the loop to understand what it does, we must inspect the entire program.

For reasons of realism I don’t wish to exclude them: Today, of course, the two terms are still both used in the context of programming, but whereas error means as it always has a mistake produced by either a person or a machine, the term bug has come to mean something more specific, applied only to the realm of man-made systems, particularly programmable computers, to denote a specific failure in design or unexpected execution result.

A sufficiently dogmatic “considered harmful” essay can end a debate in favor of the viewpoint the essay considers harmful. AdmiralAsshat on Feb 8, Good programming language design dictates that a language should provide a sufficiently complete and powerful set of flow control constructs in order to make it relatively easy to write efficient code for any programming task.

We introduce you to Apple’s new Swift programming language, discuss the perils of being the third-most-popular mobile platform, revisit SQLite on Androidand much more! Some languages allow for more complicated flow control by providing clauses for terminating a loop iteration early a break statement or skipping the rest of the loop body and forcing the next loop iteration a continue statement. Dijkstra, like most of his colleages in the early formative days of serious computer dijkstda, was an academic with heavy mathematical training.

Most programming languages of the time did not support the basic control flow statements that we take for granted today, or only provided harmcul limited forms of them. In order to exit a loop early without using such mechanisms, a trade-off must be made in which an additional flag boolean variable to signal completion of the loop is used.

This is the Harmfjl of goto: It is hard to end this with a fair acknowledgment. Most structured programming languages provide some kind of multiple selection control construct variously known as harrmfulselectswitchexamineinspectchoosewhenetc.