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Parsing Input

In the introduction I defined "parsing" as the process of analyzing the structure of a sentence in a natural language or a statement in a formal language. For example, the compiler has to parse your program before it can translate it into machine language.

In addition, when you read input from a file or from the keyboard you often have to parse it in order to extract the information you want and detect errors.

For example, I have a file called distances that contains information about the distances between major cities in the United States. I got this information from a randomly-chosen web page

http://www.jaring.my/usiskl/usa/distance.html

so it may be wildly inaccurate, but that doesn't matter. The format of the file looks like this:

"Atlanta"       "Chicago"       700
"Atlanta"       "Boston"        1,100
"Atlanta"       "Chicago"       700
"Atlanta"       "Dallas"        800
"Atlanta"       "Denver"        1,450
"Atlanta"       "Detroit"       750
"Atlanta"       "Orlando"       400

Each line of the file contains the names of two cities in quotation marks and the distance between them in miles. The quotation marks are useful because they make it easy to deal with names that have more than one word, like "San Francisco."

By searching for the quotation marks in a line of input, we can find the beginning and end of each city name. Searching for special characters like quotation marks can be a little awkward, though, because the quotation mark is a special character in C++, used to identify string values.

If we want to find the first ppearance of a quotation mark, we have to write something like:

  int index = line.find ('\"');

The argument here looks like a mess, but it represents a single character, a double quotation mark. The outermost single-quotes indicate that this is a character value, as usual. The backslash (\verb+\+) indicates that we want to treat the next character literally. The sequence \verb+\"+ represents a quotation mark; the sequence \verb+\'+ represents a single-quote. Interestingly, the sequence \verb+
+ represents a single backslash. The first backslash indicates that we should take the second backslash seriously.

Parsing input lines consists of finding the beginning and end of each city name and using the substr function to extract the cities and distance. substr is an pstring member function; it takes two arguments, the starting index of the substring and the length.

void processLine (const pstring& line)
{
  // the character we are looking for is a quotation mark
  char quote = '\"';

  // store the indices of the quotation marks in a vector
  pvector<int> quoteIndex (4);

  // find the first quotation mark using the built-in find
  quoteIndex[0] = line.find (quote);

  // find the other quotation marks using the find from Chapter 7
  for (int i=1; i<4; i++) {
    quoteIndex[i] = find (line, quote, quoteIndex[i-1]+1);
  }

  // break the line up into substrings
  int len1 = quoteIndex[1] - quoteIndex[0] - 1;
  pstring city1 = line.substr (quoteIndex[0]+1, len1);
  int len2 = quoteIndex[3] - quoteIndex[2] - 1;
  pstring city2 = line.substr (quoteIndex[2]+1, len2);
  int len3 = line.length() - quoteIndex[2] - 1;
  pstring distString = line.substr (quoteIndex[3]+1, len3);

  // output the extracted information
  cout << city1 << "\t" << city2 << "\t" << distString << endl;
}

Of course, just displaying the extracted information is not exactly what we want, but it is a good starting place.
Home File Input/Output and pmatrices Parsing Input

Last Update: 2005-12-05