Mon., 21 Sept. 2015 (delayed slides) Conditional and unconditional - - PowerPoint PPT Presentation

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Mon., 21 Sept. 2015 (delayed slides) Conditional and unconditional - - PowerPoint PPT Presentation

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Mon., 21 Sept. 2015 (delayed slides) Conditional and unconditional branches The go to statement and Dijkstras letter In-class exercise: the C goto statement Conditional Branches Familiar to most novice programmers: if and

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SLIDE 1

Mon., 21 Sept. 2015 (delayed slides)

Conditional and unconditional branches The “go to” statement and Dijkstra’s letter In-class exercise: the C “goto” statement

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SLIDE 2

Conditional Branches

Familiar to most novice programmers: “if” and “if-else” statements “switch” statements (More about these on Wednesday) Basic idea: if (condition) then … else … It wasn’t always quite this easy, though ...

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SLIDE 3

The Good Old Days of FORTRAN

Here is part of an old FORTRAN program:

if (i+j-k)10,20,30

10 print *,"i+j-k is negative" go to 40 20 print *,"i+j-k is zero" go to 40 30 print *,"i+j-k is positive" 40 stop end

Evaluate i+j-k and take

  • ne of three branches:

statement 10 if i+j-k < 0, statement 20 if i+j-k = 0, statement 30 if i+j-k > 0 (You can run this in the lab

  • - look for file “arith-if.for” in

the repository and follow instructions in comments.)

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SLIDE 4

The Good Old Days of FORTRAN

Things were slightly better with relational operators: if (i+j .lt. k)go to 10 if (i+j .eq. k)go to 20 print*,"i+j > k" go to 30 10 print*,"i+j < k" go to 30 20 print*,"i+j = k" 30 stop

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SLIDE 5

The “go to” Statement

Notice how often “go to” appears in these two

  • programs. This is an UNCONDITIONAL
  • branch. Most early programming languages

had “go to” statements. Later languages like C also adopted them. They were easy to misuse, however:

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SLIDE 6

The “go to” Statement

(Contrived) Example (in C):

for (i = 0; i < 5; i++) { if (i==3) goto OUTSIDE; INSIDE: printf(“inside\n”); } goto FINISH; OUTSIDE: printf(“outside\n”); goto INSIDE; FINISH: ... OUTPUT: inside inside inside

  • utside

inside inside

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SLIDE 7

The “go to” Statement

Edsger W. Dijkstra (world famous computer scientist -- “Dijkstra’s Algorithm”, etc.) wrote a letter to the Communications of the ACM in 1968:

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The “go to” Statement

HUGE response. Letter is now famous; many imitations. “Considered harmful” essays appear about almost every topic in computer science:

  • XMLHttpRequest Considered Harmful
  • Csh Programming Considered Harmful
  • Turing Test Considered Harmful
  • Considered Harmful Essays Considered Harmful
  • … etc. ...
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SLIDE 9

The “go to” Statement

But why? C example shows that we can “break

  • ut of scope” with a goto (the for-loop block

might have its own local variables); we can write incomprehensible code (“spaghetti code”); and many other problems arise. IN-CLASS EXERCISE: write some spaghetti code–get it out of your system! (See website.)