Analyzing strings in Software

Print QR Code ISO/IEC18004 in Software Analyzing strings
61 Analyzing strings
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In 582/94, we used a loop to concatenate two character pictures:
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for (vector<string>::const_iterator it = bottombegin(); it != bottomend(); ++it) retpush_back(*it);
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We noted that this loop was equivalent to inserting a copy of the elements of bottom at the end of ret, an operation that vectors provided directly:
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retinsert(retend(), bottombegin(), bottomend());
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This problem has an even more general solution: We can separate the notion of copying elements from that of inserting elements at the end of a container, as follows:
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copy(bottombegin(), bottomend(), back_inserter(ret));
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Here, copy is an example of a generic algorithm, and back_inserter is an example of an iterator adaptor A generic algorithm is an algorithm that is not part of any particular kind of container, but instead takes a cue from its arguments' types about how to access the data it uses The standard library's generic algorithms usually take
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iterators among their arguments, which they use to manipulate the elements of the underlying containers So, for example, the copy algorithm takes three iterators, which we'll call begin, end, and out, and copies all the elements in the range [begin, end) to a sequence of elements starting at out and extending as far as necessary In other words,
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copy(begin, end, out);
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has the same effect as
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while (begin != end) *out++ = *begin++;
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except that the while body changes the values of the iterators, and copy doesn't Before we describe iterator adaptors, we should note that this loop depends on the use of the postfix version of the increment operators These operators differ from the prefix versions, which we have used up to now, in that begin++ returns a copy of the original value of begin, incrementing the stored value of begin as a side effect In other words,
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it = begin++;
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is equivalent to
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it = begin; ++begin;
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The increment operators have the same precedence as *, and they are both right-associative, which means that *out++ has the same meaning as *(out++) Thus,
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*out++ = *begin++;
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is equivalent to the more verbose
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{ *out = *begin; ++out; ++begin; }
Let's return to iterator adaptors, which are functions that yield iterators with properties that are related to their arguments in useful ways The iterator adaptors are defined in <iterator> The most common iterator adaptor is back_inserter, which takes a container as its argument and yields an iterator that, when used as a destination, appends values to the container For example, back_inserter(ret) is an iterator that, when used as a destination, appends elements to ret Therefore,
copy(bottombegin(), bottomend(), back_inserter(ret));
copies all of the elements of bottom and appends them to the end of ret After this function completes, the size of ret will have increased by bottomsize() Notice that we could not call
// error ret is not an iterator copy(bottombegin(), bottomend(), ret);
because copy's third argument is an iterator, not a container Nor could we call
// error - no element at retend() copy(bottombegin(), bottomend(), retend());
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This latter mistake is particularly insidious, because the program will compile What it does when you try to run it is another story entirely The first thing copy will try to do is assign a value to the element at retend() There's no element there, so what the implementation will do is anybody's guess Why is copy designed this way Because separating the notions of copying elements and expanding a container allows programmers to choose which operations to use For example, we might want to copy elements on top of elements that already exist in a container, without changing the container's size As another example, which we shall see in 622/112, we might want to use back_inserter to append elements to a container that are not merely copies of another container's elements
611 Another way to split
Another function that we can write more directly using the standard algorithms is split, which we saw in 56/88 The hard part of writing that function was dealing with the indices that delimited each word in the input line We can replace the indices by iterators, and use standard-library algorithms to do much of the work for us:
// true if the argument is whitespace, false otherwise bool space(char c) { return isspace(c); } // false if the argument is whitespace, true otherwise bool not_space(char c) { return !isspace(c); } vector<string> split(const string { typedef string::const_iterator iter; vector<string> ret; iter i = strbegin(); while (i != strend()) { // ignore leading blanks i = find_if(i, strend(), not_space); // find end of next word iter j = find_if(i, strend(), space); // copy the characters in [i, j) if (i != strend()) retpush_back(string(i, j)); i = j; } return ret; }
This code uses a lot of new functions, so it will take a bit of explanation The key idea to keep in mind is that it implements the same algorithm as the original, using i and j to delimit each word in str Once we've found a word, we copy it from str, and push the copy onto the back of ret This time, i and j are iterators, not indices We use typedef to abbreviate the iterator type, so that we can use iter instead of the longer string::const_iterator Although the string type does not support all of the container operations, it does support iterators Therefore, we can use the standard-library algorithms on the characters of a string, just as we can use them on the elements of a vector