15.9. Text Queries Revisited

As a final example of inheritance, we’ll extend our text-query application from §12.3 (p. 484). The classes we wrote in that section let us look for occurrences of a given word in a file. We’d like to extend the system to support more complicated queries. In our examples, we’ll run queries against the following simple story:

Alice Emma has long flowing red hair.
Her Daddy says when the wind blows
through her hair, it looks almost alive,
like a fiery bird in flight.
A beautiful fiery bird, he tells her,
magical but untamed.
"Daddy, shush, there is no such thing,"
she tells him, at the same time wanting
him to tell her more.
Shyly, she asks, "I mean, Daddy, is there?"

Our system should support the following queries:

• Word queries find all the lines that match a given string:

Executing Query for:
Daddy Daddy occurs 3 times
(line 2) Her Daddy says when the wind blows
(line 7) "Daddy, shush, there is no such thing,"
(line 10) Shyly, she asks, "I mean, Daddy, is there?"

• Not queries, using the ~ operator, yield lines that don’t match the query:

Executing Query for: ~(Alice)
~(Alice) occurs 9 times
(line 2) Her Daddy says when the wind blows
(line 3) through her hair, it looks almost alive,
(line 4) like a fiery bird in flight.
...

• Or queries, using the | operator, return lines matching either of two queries:

Executing Query for: (hair | Alice)
(hair | Alice) occurs 2 times
(line 1) Alice Emma has long flowing red hair.
(line 3) through her hair, it looks almost alive,

• And queries, using the & operator, return lines matching both queries:

Executing query for: (hair & Alice)
(hair & Alice) occurs 1 time
(line 1) Alice Emma has long flowing red hair.

Moreover, we want to be able to combine these operations, as in

fiery & bird | wind

We’ll use normal C++ precedence rules (§4.1.2, p. 136) to evaluate compound expressions such as this example. Thus, this query will match a line in which both fiery and bird appear or one in which wind appears:

Executing Query for: ((fiery & bird) | wind)
((fiery & bird) | wind) occurs 3 times
(line 2) Her Daddy says when the wind blows
(line 4) like a fiery bird in flight.
(line 5) A beautiful fiery bird, he tells her,

Our output will print the query, using parentheses to indicate the way in which the query was interpreted. As with our original implementation, our system will display lines in ascending order and will not display the same line more than once.