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73 Generating a cross-reference table
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Once we know how to count how often words occur in the input, a logical next step is to write a program to generate a cross-reference table that indicates where each word occurs in the input This extension requires several changes to our basic program First, instead of reading a word at a time, we'll need to read a line at a time, so that we can associate line numbers with words Once we're reading lines instead of words, we'll need a way to break each line into its constituent words Fortunately, we already wrote such a function, named split, in 611/103 We can use this function to turn each input line into a vector<string>, from which we can extract each word Rather than using split directly, we're going to make it a parameter to the cross-reference function That way, we leave open the possibility of changing the way we find the words on a line For example, we could pass the find_urls function from 613/105, and use the cross-reference function to see where URLs appear in the input As before, we will use a map with keys that are the distinct words from the input This time, however, we will have to associate a more complicated value with each key Instead of keeping track of how often the word occurs, we want to know all the line numbers on which the word occurred Because any given word may occur on many lines, we will need to store the line numbers in a container When we get a new line number, all we will need to do is append that number to those that we already have for that word Sequential access to the container elements will suffice, so we can use a vector to keep track of line numbers Therefore, we will need a map from string to vector<int> With these preliminaries out of the way, let's look at the code:
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// find all the lines that refer to each word in the input map<string, vector<int> > xref(istream& in, vector<string> find_words(const string&) = split) { string line; int line_number = 0; map<string, vector<int> > ret; // read the next line while (getline(in, line)) { ++line_number; // break the input line into words vector<string> words = find_words(line); // remember that each word occurs on the current line for (vector<string>::const_iterator it = wordsbegin(); it != wordsend(); ++it) ret[*it]push_back(line_number); } return ret; }
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Both the return type and the argument list of this function deserve attention If you look at the declaration of our return type and the local variable ret, you will see that we carefully wrote >> instead of >> The compiler needs that space, because if it sees >> without intervening spaces, it will assume that it is looking at an >> operator, rather than at two separate > symbols
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In the argument list, notice that find_words defines a function parameter, which captures our intent to pass to xref the function to use to split the input into words The other interesting thing is that we say = split after the definition of find_words, which indicates that this parameter has a default argument When we give a parameter a default argument, we're saying that callers can omit that argument if they wish If they supply an argument, the function will use it If they omit the argument, the compiler will substitute the default Thus, users can call this function in either of two ways:
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xref(cin); xref(cin, find_urls); // uses split to find words in the input stream // uses the function named find_urls to find words
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The function body starts by defining a string variable, named line, which will hold each line of input as we read it, and an int variable, named line_number, to hold the line number of the line that we are currently processing The input loop calls getline ( 57/91) to read a line at a time into line As long as there is input, we increment the line counter and then process each word in the line We begin that processing by declaring a local variable named words, which will hold all the words from line, and initialize it by calling find_words That function will be either our split function ( 611/103), which splits line into its component words, or another function that takes a string argument and returns a vector<string> result We continue with a for statement that visits each element in words, updating the map each time through words The for header should be familiar: It defines an iterator, and marches that iterator sequentially through words The statement that forms the body of the for may be hard to understand on first reading,
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ret[*it]push_back(line_number);
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so we'll pick it apart a bit at a time The iterator it denotes an element of words, and so *it is one of the words in the input line We use that word to index our map The expression ret[*it] returns the value stored in the map at the position indexed by *it That value is a vector<int>, which holds the line numbers on which this word has appeared so far We call that vector's push_back member to append the current line number to the vector As we saw in 72/125, if this is the first time we've seen this word, then the associated vector<int> will be value-initialized Value-initialization of class types is a bit complicated, as we'll see in 95/164; what we need to know is that vectors are value-initialized the same way that variables of type vector are created when we don't give them an initial value explicitly In both cases, the vector is created without any elements Thus, when we insert a new string key into the map, it will be associated with an empty vector<int> The call to push_back will append the current line number to this initially empty vector Having written the xref function, we can use it to generate a cross-reference table:
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int main() { // call xref using split by default map<string, vector<int> > ret = xref(cin); // write the results for (map<string, vector<int> >::const_iterator it = retbegin(); it != retend(); ++it) { // write the word cout first secondbegin(); cout write the first line number ++line_it; // write the rest of the line numbers, if any while (line_it != it->secondend()) { cout write a new line to separate each word from the next cout
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