::  Posts  ::  RSS  ::  ◂◂RSS  ::  Contact

Significant Whitespace In Expressions

January 8th, 2011
ideas, math, programming  [html]
Order of operations can conflict with what an expression looks like it should mean. For example, the expression a * b+c looks like it means a*(b+c) but technically (and in most computer languages) means (a*b)+c. This seems analagous to the reasoning behind python using indentation instead of braces. The C code:
  if (x)
     if (y)
       z();
  else
     w();
looks like it should mean
  if (x) {
    if (y) {
      z();
    }
  }
  else {
    w();
  }
but actually means
  if (x) {
    if (y) {
       z();
    }
    else {
      w();
    }
  }
Python deals with this by treating the whitespace as significant, so:
  if x:
    if y:
      z()
    else:
      w()
and
  if x:
    if y:
       z()
  else:
    w()
both mean what they look like they should mean.

Some examples of how I would propose we do things instead:

  write                 to mean
  -----                 -------
  a+b                   a+b
  a + b*c               a+(b*c)
  a * b+c               a*(b+c)
  a+b * c+d             (a+b)*(c+d)
  a+b - c+d             (a+b)-(c+d)
  a+b  *  c+d - e+f     (a+b) * ((c+d) - (e+f))
  a  *  c + d + e       a*(c+d+e)
  a + b                 a+b, maybe yields warning
  a-b+c                 error
  a*b+c                 error
Algorithm:
for each number of spaces 0 on up:
  group any terms with that number of internal
  spaces by adding parens

Comment via: facebook

Recent posts on blogs I like:

Absolute scale corrupts absolutely

The Internet has gotten too big. Growing up, I, like many computery people of my generation, was an idealist. I believed that better, faster communication would be an unmitigated improvement to society. "World peace through better communication,"…

via apenwarr August 19, 2019

Traces

At naptime Anna listens to recordings of novels recorded by Jeff’s grandmother. It is the main way she will know Winnie, as it is the main way I have ever known Winnie. Some of the recordings are missing parts, and Suzie often fills in the first few sente…

via The whole sky August 18, 2019

One Night Ultimate Werewolf

I really like One Night Ultimate Werewolf (ONUW), and recommend trying it out. The Daybreak expansion is also good (but I’d skip the later games in the series). App For those who’ve played a lot and wish the night took 20 seconds rather than a few minutes…

via The sideways view August 18, 2019

more     (via openring)

More Posts:


  ::  Posts  ::  RSS  ::  ◂◂RSS  ::  Contact