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Breaking Out of Nested Loops

From inside a loop, nested within another loop [etc], returning to the base code block.

Python Qt/Cpp 
    

  1. r = 10
    2. 

  2. n = 240
    3. 

  3. for x in range(r):
    4. 

  4.         for y in range(r):
    5. 

  5.                 for z in range(r):
    6. 

  6.                         # Start reading here
    7. 

  7.                         if x * y * z == n:
    8. 

  8.                                 print(x,y,z)
    9. 

  9.                                 # 5 6 8
    10. 

  10.                         break
    11. 

  11.                 else:
    12. 

  12.                         continue
    13. 

  13.                 break
    14. 

  14.         else:
    15. 

  15.                 continue
    16. 

  16.         break
    17. 

    

  1. const int r(10 );
    2. 

  2. const int n(240);
    3. 

  3. for ( int x = 0; x < r; x++ ) {
    4. 

  4. for ( int y = 0; y < r; y++ ) {
    5. 

  5. for ( int z = 0; z < r; z++ ) {
    6. 

  6.         // Start reading here
    7. 

  7.         if ( x * y * z == n ) {
    8. 

  8.                 qDebug() << x << y << z;
    9. 

  9.                 goto finished;
    10. 

  10.         }
    11. 

  11. }}} finished:
    12.
Read as:
  • if [ expression is true ]
  • print x y z
  • 'break'out of loop z [ ignore the else ]
  • 'break'out of loop y [ ignore the else ]
  • 'break'out of loop x.
  • if [ expression is true ]
  • debug out x y z
  • and goto finished [ label ]
Benefits:
  • In python, you can place an "else" statement after a loop ( "for" or "while" ) which gets called if the loop exits without a 'break' statement. This is a novell construct absent in c++."
  • This code, is clean, and in this particular case, no extra indentation was needed.
  • the "goto" label is particularly descriptive: "[ I am finished ] goto finish line."
Drawbacks:
  • This "else" feature is esoteric and unknown towards most Python developers.
  • The "else" mistakenly seems like the continuation to the "if" statement above.
  • This feature does not exist in any other language, making this a non-transferable idiom.
  • Indentation structure is bumpy and disorienting.
  • None.

Final Thoughts