Dijkstra Paper

(A Look Back at) Go To Statement Considered Harmful Edsger Dijkstra wrote a Letter to the Editor of Communications in 1968, criticizing the excessive use of the go to statement in course of studyming languages. Instead, he encouraged his fellow computer scientists to keep back out structured scheduleming. The letter, originally entitled A Case Against the Goto Statement, was published in the March 1968 issue under the headline Go To Statement Considered Harmful. It would become the close leg residuumary CACM Letter of all clipping Considered Harmful would develop into an iconic catch-all.Dijkstras comments sparked an editorial debate that spanned these pages for over 20 years. In honor of the occasion, we republish here the original letter that started it all. Editor For a number of years I open been familiar with the observation that the quality of programmers is a decreasing feed of the density of go to statements in the programs they produce. More recently I discovered wh y the use of the go to statement has such disastrous effects, and I became win over that the go to statement should be abolished from all higher level programming languages (i. e. verything except, perhaps, plain machine code). At that time I did not attach in addition much importance to this discovery I at once submit my considerations for publication because in very recent discussions in which the subject turned up, I have been urged to do so. My first remark is that, although the programmers activity ends when he has constructed a correct program, the military operation taking place under control of his program is the true subject matter of his activity, for it is this process that has to accomplish the desired effect it is this process that in its alive(p) behavior has to satisfy the desired specifications.Yet, once the program has been made, the making of the corresponding process is delegated to the machine. My second remark is that our intellectual powers are rather geare d to master static relations and that our powers to visualize processes evolving in time are relatively poorly developed. For that reason we should do (as wise programmers aware of our limitations) our utmost to shorten the conceptual gap between the static program and the dynamic process, to make the correspondence between the program (spread out in text space) and the process (spread out in time) as trivial as possible.Let us now consider how we hindquarters characterize the give of a process. (You may think about this question in a very concrete manner say that a process, considered as a time succession of actions, is stopped after an arbitrary action, what data do we have to fix in order that we can redo the process until the very same point? ) If the program text is a pure concatenation of, say, assignment statements (for the purpose of this discussion regarded as the descriptions of ace actions) it is sufficient to point in the program text to a point between two concomi tant action descriptions. In the absence of go to statements I can permit myself the syntactic ambiguity in the last three words of the previous sentence if we parse them as successive (action descriptions) we mean successive in text space if we parse as (successive action) descriptions we mean successive in time. ) Let us 7 PAUL WATSON communication theory OF THE ACM January 2008/Vol. 51, No. 1 Forum call such a pointer to a suitable place in the text a textual index. When we include conditional clauses (if B then A), alternative clauses (if B then A1 else A2), choice clauses as introduced by C.A. R. Hoare (casei of (A1, A2, , An)), or conditional expressions as introduced by J. McCarthy (B1__ E1, B2 __ E2, , Bn __ En), the fact re primary(prenominal)s that the take place of the process remains characterized by a private textual index. As soon as we include in our language procedures we must admit that a single textual index is no longer sufficient. In the case that a textu al index points to the interior of a procedure body the dynamic progress is only characterized when we also give to which call of the procedure we refer.With the inclusion of procedures we can characterize the progress of the process via a season of textual indices, the length of this sequence being equal to the dynamic depth of procedure calling. Let us now consider repeat clauses (like, while B repeat A or repeat A until B). Logically speaking, such clauses are now superfluous, because we can express repetition with the aid of recursive procedures. For reasons of realism I dont wish to exclude them on the adept hand, repetition clauses can be implemented quite comfortably with present day finite equipment on the early(a) hand, the reasoning pattern known as induction makes us well quipped to retain our intellectual grasp on the processes generated by repetition clauses. With the inclusion of the repetition clauses 8 textual indices are no longer sufficient to describe the dynam ic progress of the process. With each entry into a repetition clause, however, we can relate a socalled dynamic index, inexorably find out the ordinal number of the corresponding current repetition. As repetition clauses (just as procedure calls) may be utilise nestedly, we find that now the progress of the process can always be uniquely characterized by a (mixed) sequence of textual and/or dynamic indices.The main point is that the values of these indices are outside programmers control they are generated (either by the write-up of his program or by the dynamic evolution of the process) whether he wishes or not. They provide independent coordinates in which to describe the progress of the process. Why do we need such independent coordinates? The reason isand this seems to be inherent to sequential processesthat we can interpret the value of a variable only with respect to the progress of the process.If we wish to count the number, n say, of people in an initially quash room, we can achieve this by increasing n by one whenever we see person entering the room. In the inbetween moment that we have observed someone entering the room but have not yet performed the subsequent increase of n, its value equals the number of people in the room minus one The unbridled use of the go to statement has an immediate consequence that it becomes terribly hard to find a meaningful set of coordinates in which to describe he process progress. 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 unders to a faultd With the go to statement one can, of course, still describe the progress uniquely by a counter counting the number of actions performed since program start (viz. a kind of normalized clock). The difficulty is that such a coordinate, although unique, is utterly unhelpful.In such a coordinate system it becomes an extremely en tangled affair to define all those points of progress where, say, n equals the number of persons in the room minus one The go to statement as it stands is just too primitive it is too much an invitation to make a mess of ones program. One can regard and appreciate the clauses considered as bridling its use. I do not claim that the clauses mentioned are exhaustive in the sense that they will satisfy all needs, but whatever clauses are suggested (e. g. bortion clauses) they should satisfy the requirement that a programmer independent coordinate system can be maintained to describe the process in a helpful and manageable way. It is hard to end this with a fair acknowledgment. Am I to judge by whom my thinking has been influenced? It is fairly obvious that I am not uninfluenced by shit Landin and Christopher Strachey. Finally I should like to record (as I remember it quite distinctly) how Heinz Zemanek at the pre-ALGOL meeting in early 1959 in Copenhagen quite explic- January 2008/Vol. 1, No. 1 COMMUNICATIONS OF THE ACM itly expressed his doubts whether the go to statement should be treated on equal syntactic footing with the assignment statement. To a modest extent I blame myself for not having then drawn the consequences of his remark. The remark about the undesirability of the go to statement is far from new. I remember having read the explicit recommendation to hold back the use of the go to statement to alarm exits, but I have not been able to trace it presumably, it has been made by C. A. R. Hoare. In 1, Sec. 3. . 1. Wirth and Hoare together make a remark in the same direction in motivating the case construction Like the conditional, it mirrors the dynamic structure of a program more clearly than go to statements and switches, and it eliminates the need for introducing a large number of labels in the program. In 2 Guiseppe Jacopini seems to have proved the (logical) superfluousness of the go to statement. The exercise to translate an arbitrary escape d iagram more or less mechanically into a jumpless one, however, is not to be recommended.Then the resulting flow diagram cannot be expected to be more right-down than the original one. REFERENCES 1. Wirth, Niklaus, and Hoare, C. A. R. A contribution to the development of ALGOL. Comm. ACM 9 (June 1966), 413432. 2. Bohn, Corrado, and Jacopini, Guiseppe. Flow Diagrams, Turing machines and languages with only two formation rules. Comm. ACM 9 (May 1966) 366371. Coming Next Month in COMMUNICATIONS Alternate Reality Gaming IT Diffusion in Developing Countries Are People Biased in their Use of Search Engines?The Factors that Affect Knowledge-Sharing Behavior choice Scenarios to the Banner Years Municipal Broadband Wireless Networks The Myths and Truths about Wireless Security Managing Large Collections of Data Mining Models Women and Men in IT Alike or Different? EDSGER W. DIJKSTRA Technological University Eindhoven, The Netherlands Communications of the ACM March 1968, Vol. 11, No. 3, pg 1 47 COMMUNICATIONS OF THE ACM January 2008/Vol. 51, No. 1 9