Re: Algorithm for performing a rollup

From: (Stefan Ram)
18 Mar 2007 18:46:50 GMT
Chris <> writes:

My guess that there is a way to simplify the code a bit and get
a good general algorithm.

  I do not like the ?while( true ){ ... break; ... }?-style
  myself, but sometimes it is the last ressort, when the loop
  control logic is too complex to fit into the parentheses.

  The loop control with some embedded side-effects was put into
  the body of the loop:

/* begin of the control section of the outer loop */
while( true )
{ if( next != null ){ first = next; next = null; }
  else if( hasNext() )first = getNext(); else break;
  /* end of the control section of the outer loop */

  /* begin of the actual body of the outer loop */
  int count = 1;

        /* begin of the control section of the inner loop */
        while( true )if( !hasNext() ){ next = null; break; }
        else if( !first.equals( next = getNext() ))break; else
        /* end of the control section of the inner loop */

        /* begin of the actual body of the inner loop */
        /* end of the actual body of the inner loop */

  java.lang.System.out.println( "\"" + first + "\", " + count );
  /* end of the actual body of the outer loop */ }

  The problem reminds me of the question how to structure a read
  loop, where the user has to redo his input if it is not valid
  without repeating the input statement ?i = input()? within the
  source code, as in:

i = input(); while( invalid( i )){ i = input(); }

  This appears in languages without a ?do{ ... }while( ...)?-loop
  or when additional constraints are given.

  It can be solved in the same way, using a ?dirty? ?while( true
  ){ ... break; ... }? style.

while( true ){ i = input(); if( valid( i ))break; }

  In fact, there was a time when such questions were research
  topics, namely, if I recall correct, Donald E. Knuth wrote an
  article about this input-loop in the 1960ies. Or, possibly, it
  was Niklaus Wirth, who wrote this.

I had to reformat it to understand it

  Feel free not to read the following rationale for my
  formatting style. I append it just for people who enjoy
  reading about formatting styles.

  One Way to Format Parentheses

  There are several different ways to format texts with braces
  and parentheses. One of them is being described here.

  Indentation within Braces

  An indentation of just one space often is too small to be seen
  clearly, because the natural width and form of characters
  often varies by an amount that is not very much smaller than a
  space. Therefore, the indentation should amount to at least
  two positions. In order not to waste horizontal spaces, an
  indentation of exactly two positions is chosen. This means,
  that the left position of the next level is two larger than
  the position of the directly enclosing level.

  Indentation by two positions within a block

{ ++x;
  ++x; }
^ ^
0 2

  Bad A small indentation by one position is not always visible

 ++x; }

  Good The indentation by two positions is visible clearly

{ ++x;
  ++x; }

  Bad A large indentation by more than two positions wastes
  horizontal space with no additional benefit

{ ++x;
     ++x; }

Spaces within braces

  In mathematics, there are often no spaces at the inner side of
  parentheses or braces in expressions, but spaces are used
  indeed at the inner side of braces in set notation, when the
  braces contain a description (not when they contain a list).
  Spaces in set notation

{ x | x > 2 }

  This style is adopted here: One space is written at the inner
  side of braces.

  Spaces at the inner side of parentheses within a block

{ ++x; }

  This style is consistent with the indentation by two
  positions, because only using this style, corresponding parts
  of two lines have the same position.

  Bad No space after the first brace, the two statements are

  ++x; }

  Good One space after the first brace, the two statements are
  properly aligned

{ ++x;
  ++x; }

  Bad Two spaces after the first brace, the two statements are

{ ++x;
  ++x; }

  There are some exceptions to this rule: No spaces are used
  within empty braces "{}" and between two or more closing
  braces of the same direction "}}", except, when the first one
  of them is part of an empty pair "{} }" (an empty pair of
  braces if treated like a single non-braces character).
  Unified rules for all Brackets

  For simplicity and uniformity, the rules from above apply to
  all kinds of brackets, including parentheses, braces (curly
  brackets), square brackets, and angle brackets.

  Spaces within parentheses and square brackets

{ y = f( x )+ g() + a[ 2 ]; }

  Binary operators are sorrounded by a space, but the space is
  omitted, when there already is a space on the other side of a
  sequence of bracket directly beside the operator: By this rule
  " )+" is written instead of " ) +".

  Representation of the Syntactical Structure

  A method declaration in Java consists of a head and a body.
  The following representation shows this structure:

  Good formatting according to the structure

void alpha() // head
{ beta(); } // body

  The following formatting is misleading, because the line break
  does not match the structural break:

  Bad line break within the body

void alpha() { // head and the beginning of the body
  beta(); } // the rest of the body

  This formatting also would make no sense for blocks within
  blocks. So it is often not used for such blocks. Therefore
  even the adopters of this style can not use it uniformly.

  Left Braces Look Like "bullets"

  There is a well known style to publish lists in typography
  using bullets sticking out on the left, looking like this:
  Common list representation with bullets in typography

o This is the first point
  of this list, it is written
  here just as an example.

o Here is another entry

o This is another example given
  just as an example to show
  an example

  The braces of the beginnings of blocks stand out on the left
  just the same, when the formatting being described here is
  used, so they look quite naturally as beginning-of-a-block
  markers, when one is used to the typographical list notation:

Left braces look like bullets to mark blocks

{ printf(); printf();
  printf(); printf(); printf();
  printf(); printf(); }

{ printf(); printf(); }

{ printf(); printf(); printf();
  printf(); printf();
  printf(); }


  Someone wrote this C code:

  Code someone wrote

while( fgets( eingabe, sizeof eingabe, stdin ))
  if( sscanf( eingabe, "%d", &wert )!= 1 )
    fprintf( stderr, "Please enter a number!\n" );
    summe += wert;

  It amazes me that I can add braces by my style conventions
  without the need to change the position of any character of
  the given code:

  The code from above plus braces

while( fgets( eingabe, sizeof eingabe, stdin ))
{ if( sscanf( eingabe, "%d", &wert )!= 1 )
  { fprintf( stderr, "Please enter a number!\n" ); }
  { summe += wert; }}

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