GATE CSE 1989 | Question: 3-i

Question

An unrestricted use of the "go to" statement is harmful because of which of the following reason (s):

• A. It makes it more difficult to verify programs.
• B. It makes programs more inefficient.
• C. It makes it more difficult to modify existing programs.
• D. It results in the compiler generating longer machine code.

Comments

Why option c must not be one of the correct options, actually too many goto’s for the same label makes it difficult to know from where the program is redirected to this line, and hence this validates option A to be true. But then it also makes it evident that modifying such a code will become difficult too.

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Same question : 

https://gateoverflow.in/2442/gate-cse-1994-question-1-5

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@amansrivastava905 Option C is directly related to option A but option A is more correct.

Until you don't know that whether the program is rightly coded what's the use of modifying it.

Answer 1

Solution : (A) It makes it more difficult to verify programs.

Proof of correctness : https://en.wikipedia.org/wiki/Goto#Criticism
(1st Paragraph, last 4 lines.)

Option B: goto has no role in making a program inefficient. Adding a "goto" introduces a branch instruction but assuming same program logic this can never be avoided and even when replaced by a proper structural construct like for/while/if -- will still have the branch instruction.

Option C: Actually using "goto" kind of makes it easy to modify an existing program as to make the control flow from say line number 'x' to line number 'y', we can simply do a "goto". This is much harder to do in a proper structural way.

Option D:  This is also false as goto introduces only a single "JUMP" instruction (machine code will be its equivalent; OPCODE for JMP followed by the destination address) and equivalent structural constructs will have longer machine codes. 

Comments

In the given link by you it is said that - unrestricted GOTO is harmful

Is unrestricted goto is like a goto without any condition i.e

some code

goto 7

some code

line 7

or unrestricted goto means using goto too much?

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Probably the most famous criticism of GOTO is a 1968 letter by Edsger Dijkstra called "Go To Statement Considered Harmful".[3][6] In that letter Dijkstra argued that unrestricted GOTO statements should be abolished from higher-level languages because they complicated the task of analyzing and verifying the correctness of programs (particularly those involving loops).

So, unrestricted goto means goto without conditions.

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Thanks for the Wikipedia reference.

Answer 2

Correct Answer everywhere: OPTION A

My Answer: OPTION C

While certainly, Option A is correct one and the sources like https://en.wikipedia.org/wiki/Goto#Criticism are evidence of its correctness. And had Option C be something else, then Option A would have been the default choice.

But, Option C, modifying programs means: ANALYZING + ALTERING the code.

Changing any line of code in a program with UNRESTRICTED NUMBER OF gotos  necessitates fully analyzing whole program again for verification. As might somewhere at the end of the function, there would be a goto targeting the line we have modified.

So, it is just a contest between Option A and Option C. And for me C is more suitable.

Talking about others:

Option B: It makes programs more inefficient.

1. Some programs like DESIGNING STATE MACHINES in c would be much efficient with goto.

2. Multi-level loops could be much efficient with goto then with break,continue and if-elses.

3. Can be optimal too:

In Donald Knuth's Structured Programming with go to Statements, which analyzes many common programming tasks and finds that in some of them GOTO is the optimal language construct to use.(same wiki link)

Option C: It results in the compiler generating longer machine code.

When a goto is converted to ASSEMBLY CODE, just a JMP #address, would be sufficient. Also concrete examples would be:

1. end-of-function garbage collection, where a function has more than one exit points and at each exit point we have to perform garbage collection could be replaced by a goto statement at all these exit points targeting to a place at the end-of-the-function, thus using only one place for garbage collection.

2. Low-level performance improvements

This is only valid in perf-critical code, but goto statements execute very quickly and can give you a boost when moving through a function. This is a double-edged sword, however, because a compiler typically cannot optimize code that contains gotos.

Note that in all these examples, gotos are restricted to the scope of a single function.

(StackOverflow)

3. Reduces Code Size too:

Many Programmers, including Linux Kernel designer and coder Linus Torvalds or software engineer and book author Steve McConnell, also object to Dijkstra's point of view, stating that GOTOs can be a useful language feature, improving program speed, size and code clarity, but only when used in a sensible way by a comparably sensible programmer. (same wiki link)

Answer 3

Option A is correct 

Source :- Go To Statement Considered Harmful.pdf - Google Drive

Answer 4

C should be correct

In black-box usage of GOTO wouldn't matter as in option A.

Answer 5

Option A is correct 

Source :- Go To Statement Considered Harmful.pdf - Google Drive