Errata list

Errors in the third edition, second printing

• The publishers preparation of the PDF unfortunately led to a loss of certain characters. Tilde and hat characters are missing. The correct download URL for example files etc. reads (note the tilde)

  http://www.simula.no/~hpl/scripting
  

  In chapter 8.2.2 on regular expressions, the hat character is missing and replaced by a space. This is obviously wrong. The original PDF file prepared by the author is correct and can be used for reference.

Errors in the first edition

• Gnuplot is now in version 4.0 with a lot of syntax changes compared with version 3.7. The examples in the book work smoothly if you install Gnuplot 3.7.3. A few changes are necessary for Gnuplot 4.0. For example, if the PNG terminal makes use of the new GD support, the color option in set term png small color in simviz1.py-like scripts is no longer valid. The scripts in scripting-src.tgz and scripting-src.zip have already been updated, but the particular set term png small color commands in the book are only valid with Gnuplot 3.7 or the old PNG support. At present, I don't know how well the Python Gnuplot module works with Gnuplot 4.0.
• Page 56, bottom: ymin = None; ymin = None should be ymin = None; ymax = None
• Page 76: A correct root function, also valid for complex coefficients, could be like this:

  def roots(a, b, c):
      """
      Return two roots of the quadratic algebraic equation
      ax^2 + bx + c = 0, where a, b, and c may be complex.
      """
      import cmath      # complex functions
      q = b*b - 4*a*c
      r1 = -(b - cmath.sqrt(q))/(2*a)
      r2 = -(b + cmath.sqrt(q))/(2*a)
      # r1 and r2 are complex because cmath.sqrt returns complex,
      # convert to real if possible:
      if r1.imag == 0.0:  r1 = r1.real
      if r2.imag == 0.0:  r2 = r2.real
      if r1 == r2:  r2 = None  # use r2=None to indicate double root
      return r1, r2 
  

• Page 80: In the first for loop, xlist[1] should be xlist[i].
• Page 87: The try-except statement gets a printout from the try block even if except is invoked. A better solution is

  try:  # program was found if program_path is defined
      print '%s found in %s' % (program, program_path)
  except:      
      print '%s not found' % program
  

• Page 90: In the first for loop, filestr = file.read().replace should be filestr = f.read().replace.
• Page 96, next last paragraph: "... but the list is then [4,3,1] ... and index 1 is then the elmeent 3" should read "... but the list is then [4,2,1] ... and index 1 is then the elmeent 2".
• Page 101: elif should be else.
• Page 103: With the sum function in Python 2.3, a more compact version of statistics can be written

  def statistics(*args):
      return sum(args)/float(len(args)), min(args), max(args)
  

• Page 104: somefunc(a, 'copy', 'True') should be somefunc(a, 'copy', True), and in the printout 'install': 'yes' should be 'install': 'no'.
• Page 106: [0.0]*n should be [0.0]*10.
• Page 123: The definition of sequence should have inc=1.0 (not inc=0.1).
• Page 131: The True and False values were exchanged. Here is the correct interactive code segment:

  >>> x = array([0.1,-0.4,2,5,6,-3])
  >>> u = x < 0
  >>> u                               # u[i] is 1 if x[i]<0
  array([0, 1, 0, 0, 0, 1])
  >>> bool(u)                         # result of if u:
  True
  >>> bool(zeros(6,Float))            # is [0,0,0,...] true?
  False                               # no
  >>> bool(array([-1,0]))
  True    # at least one element != 0 gives a true array
  

  The boolean evaluation of an array only works with Numeric, not with numarray.
• Page 145: g.vectorized_eval(f) should be g.vectorized_eval(myfunc).
• Page 152: yc = sin(x) should be yc = sin(xc). The x variable is 2 (from the previous page) and hence outside the xc grid unless we set (e.g.) xc = seq(0, 4, 0.05) .
• Page 192: In Exercise 5.3, the MyFunc1 class displayed in detail does not contain a t attribute, which makes the example in the main function somewhat confusing. The remedy is to ignore the assignment f.t = M_PI.
• Page 184: Newer SWIG versions give a TypeError when we pass s to a pointer argument in the hw3 function.
• Page 190: The %apply statement must have s_ not just s.
• Page 231: The option -screenplot should be removed from the cmd variable.
• Page 237: InputFields(root, status) should be InputFields(root, status_line).
• Page 368: Method coor_mapping in class TransFunc must take self as first argument. Moreover, the a parameter has fallen out of formulas and the implementation (the original code is for a=0). The return sum in integrate also needs the "Jacobian factor" 0.5*h. The corrected code becomes

  def integrate(integrator, a, b, f, n):
      # integrator is an instance of a subclass of Integrator
      sum = 0.0
      h = (b-a)/float(n)
      g = TransFunc(f, h, a)
      for j in range(1, n+1):
          g.j = j
          sum += integrator.eval(g)
      return 0.5*h*sum
  
  class TransFunc:
      def __init__(self, f, h, a):
          self.f = f;  self.h = h;  self.a = a
  
      def coor_mapping(self, xi):
          """Map local xi in (-1,1) in interval j to global x."""
          return self.a + (self.j-0.5)*self.h + 0.5*self.h*xi
  
      def __call__(self, xi):
          x = self.coor_mapping(xi)
          return self.f(x)
  

All the errors above are corrected in the second edition of the book.