Lecture Notes for Advanced Programming II

28 March 2002 - A Thrice-Made Error

Three Stream I-O Problems

• The second assignment involved optimizing statement blocks.

  Input to port (between -- lines):
  --
  a = 1
  b = a + 1
  a = 2
  c = b + 1
  --
  Output from port (between -- lines):
  --
  a = 1
  b = 2
  a = 2
  c = 3
  --
  

• Some optimizing programs had output problems.

  • Either no output at all, or only one kind of output.

• What went wrong?

  • It was either a testing problem, or a coding problem.

The First Program

int main() {

  string line
  strvec bloc
  int block_changed = 0

  while true
    bloc.clear()
    while true
      getline(cin, line)
      if !cin.good()
        return 0
      if line.length() > 0
	bloc.push_back(line)
      else
       break

    if block.size() > 0
      do
	cons_fold(bloc)
	eval_bloc(bloc, block_changed)
      while block_changed == 1
      print(bloc)

What Went Wrong?

• This code produced no ouput.

  Input to port (between -- lines):
  --
  a = 1
  b = a + 1
  a = 2
  c = b + 1
  --
  Output from port (between -- lines):
  --
  --
  

• Exit at eof without processing the collected input.

• What's the assumption?

  • No input block is terminated by eof.

• The error behavior is wrong too.

What's the Fix?

• Separate out input and termination.

  • Separate ideas should be treated separately.

  • The program almost does this already.

• Careful programmers know and check their assumptions.

  • The assert macro is useful for this.

    while true
      getline(cin, line)
      if !cin.good()
        assert(bloc.empty());
        return 0
      if line.length() > 0
        bloc.push_back(line)
      else
       break
    

• Flags are usually bad news. Avoid them.

  • Flags quickly lead to unintelligable control structures.

The First Program Reconfigured

int main() {

  string line
  strvec bloc

  while true
    bloc.clear()
    while true
      getline(cin, line)
      if cin.good() and line.length() > 0
	bloc.push_back(line)
      else
       break

    if block.size() == 0
      break

    do cons_fold(bloc)
    while eval_bloc(bloc)
    print(bloc)

The Second Program

int main()

  vector<A> av
  string line

  while(true)
    getline(cin, line)
    if cin.eof() break

    if line != ""
      A ab
      ab.expression = line
      ab.get_elements()

      if !ab.isvalid()
	cerr << "Input errors\n"
	exit(1)

      av.push_back(ab)

    else
      E_Optimization(av)
      av.clear()

What Went Wrong?

• No output produced for any input.

  Input to port (between -- lines):
  --
  x = y + 1
  y = z + 1
  z = x + 1
  --
  Output from port (between -- lines):
  --
  --
  

• On eof, data already read is ignored.

  while(true)
    getline(cin, line)
    if cin.eof() break
  

• The program intertwines three separate issues: input, validation, and computation.

• Good programmers keep separate issues separate.

How to Fix

• Isolate each component in its own code block.

• Provide minimal, simple, and obvious connections between components.

The Second Program Reconfigured

int main()

  vector<A> av
  string line

  while true
    vector<string> input
    while getline(cin, line) and (line != "")
      input.push_back(line)
    if input.empty()
      break
    if check(input, av)
      E_Optimization(av)

bool check(input, av) 
  av.clear()
  for each l line in input
    A ab
    ab.expression = l
    ab.get_elements()
    if !ab.isvalid()
      return false
    av.push_back(ab)
  return true

The Third Program

int main()

  vector<string> outputVect
  vector <Code_line> cVect
  bool errorFlag = false

  while getline(cin, str)
    if s.empty()
      if !cVect.empty()
	if !errorFlag
	  process_block(cVect, outputVect)
      if errorFlag
	outputVect.push_back(error)
      errorFlag = false
    else if !errorFlag
      Code_line cln
      if process_line (s, cln)
	cVect.push_back(cln)
      else
	errorFlag = true

  if cin.eof()
    if !cVect.empty()
      if !errorFlag
	process_block(cVect, outputVect)
      else
	outputVect.push_back(emsg)
      
  for (i = 0; i < outputVect.size(); i++)
    cout << outputVect[i]

What Went Wrong?

• This code produced output, but only if the input was correct.

  
  Input to port (between -- lines):
  --
  x = y + 1
  y = z + 1
  z = x + 1
  --
  Output from port (between -- lines):
  --
  x = y + 1 
  y = z + 1 
  z = x + 1 
  --
  
  Input to port (between -- lines):
  --
  x = y @[ z
  --
  Output from port (between -- lines):
  --
  --
  
  

• This code is way complex.

  • Everything's intertwined with everything else.

  • Flags.

  • Duplicate code.

How to Fix?

• This code is strange. What's going on?

  • This code outputs everything at once, at the end.

  • No documentation about this.

  • Notice how hard it is to see this in the code.

• Move error processing into process_block().

  • Treating errors as just another input value.

The Third Program Reconfigured

int main()

  vector<string> outputVect

  do
    vector <Code_line> cVect
    while getline(cin, s) and !s.empty()
      Code_line cln
      if process_line (s, cln)
	cVect.push_back(cln)
      else
	cVect.push_back(error_message)

    if !cVect.empty()
      process_block(cVect, outputVect)
  while !cin.eof()

  for (i = 0; i < outputVect.size(); i++)
    cout << outputVect[i]

Points to Remember

• Do one thing at a time.

  • Input input, then check input, then process input, then output input.

  • Combining simple things makes them complicated.

• If you're using flags, you're probably doing something wrong.

  • Combining simple things, most likely.

• Get the I-O right.

  • This is not easy; don't make it harder.

• Know your assumptions; check your assumptions.

  • Use the assert macro.

• Duplicate code is bad code.

This page last modified on 25 April 2002.