jumping inside loop

C l开发者_开发技巧anguage allows jumping inside loop. What would be the use of doing so?

  if(n > 3) {

    i = 2;
    goto inner;
  }

   /* a lot of code */

   for(i = 0; i < limit ;i ++) {

    inner:
    /* ... */
   }

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If you've ever coded in Assembler (ASM), then you'll know that GOTOs are pretty standard, and required, actually. The C Language was designed to be very close to ASM without actually being ASM. As such, I imagine that "GOTO" was kept for this reason.

Though, I'll admit that GOTOs are generally a "bad idea, mmmkay?" in terms of program flow control in C and any other higher level language.

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It's certainly a questionable construct. A design that depends on this behavior is probably a poor design.

You've tagged this as C++, but C++ (intelligently, IMO) doesn't allow you to jump inside a loop where a variable was declared in the first part of the for statement:

int main()
{
    int q = 5;

    goto inner;

    for (int i = 0; i < 4; i++)
    {
        q *= 2;
inner:
        q++;
        std::cout << q << std::endl;
    }    
}

g++ output:

l.cpp: In function ‘int main()’:
l.cpp:12: error: jump to label ‘inner’
l.cpp:7: error:   from here
l.cpp:9: error:   crosses initialization of ‘int i’

Initializing i before the loop allows the program to compile fine (as would be expected).

Oddly, compiling this with gcc -std=c99 (and using printf instead) doesn't give an error, and on my computer, the output is:

6
13
27
55

as would be expected if i were initialized outside the loop. This might lead one to believe that int i = 0 might be simply "pulled out" of the loop initializer during compilation, but i is still out of scope if tried to use outside of the loop.

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From http://en.wikipedia.org/wiki/Duff%27s_device

In computer science, Duff's device is an optimized implementation of a serial copy that uses a technique widely applied in assembly language for loop unwinding.

...

Reason it works
The ability to legally jump into the middle of a loop in C.