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calls the list version of erase and assigns the list iterator returned from erase into iter The implementations of the increment and erase operations will surely differ from their vector counterparts
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551 Some important differences
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One important way in which the operations on lists differ from those on vectors is the effect of some of the operations on iterators For example, when we erase an element from a vector, all iterators that refer to the element erased, or subsequent elements, are invalidated Using push_back to append an element to a vector invalidates all iterators referring to that vector This behavior follows from the fact that erasing an element moves the following elements, and appending to a vector might reallocate the entire vector to make room for the new element Loops that use these operations must be especially careful to ensure that they have not saved copies of any iterators that may be invalidated Inappropriately saving the value of studentsend() is a particularly rich source of bugs For lists, on the other hand, the erase and push_back operations do not invalidate iterators to other elements Only iterators that refer to the element actually erased are invalidated, because that element no longer exists We have already mentioned that list class iterators do not support full random- access properties We'll have much more to say about iterator properties in 821/144 For now, what's important to know is that, because of this lack of support, we cannot use the standard-library sort function to sort values that are stored in a list Because of this restriction, the list class provides its own sort member function, which uses an algorithm that is optimized for sorting data stored in a list Thus, to sort elements in a list, we must call the sort member
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list<Student_info> students; studentssort(compare);
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rather than the global sort function
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vector<Student_info> students; sort(studentsbegin(),studentsend(), compare);
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as we do for vectors It is worth noting that because the compare function operates on Student_info objects, we can use the same compare function to sort a list of Student_info records that we used to sort a vector of them
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552 Why bother
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The code that extracts records for failing students is a good example of the effect of data structure choices on performance The code accesses elements sequentially, which generally implies that a vector is the best choice On the other hand, we also delete elements from the interior of the container, thus favoring lists As with any performance-related question, the data structure that is "best" depends on whether performance even matters Performance is a tricky subject that is generally outside the scope of this book, but it is worth noting that the choice of data structure can have a profound effect on a program's performance For small inputs, lists are slower than vectors For large inputs, a program that uses vectors in an inappropriate way can run much more slowly than it would if it were to use lists It can be surprising how quickly performance degrades as the input grows To test our programs' performance, we used three files of student records The first file had 735 records The second file was ten times as big, and the third, ten times bigger than that, or 73,500 records The following table records the time, in seconds, that it took to execute the programs on each file size:
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File size 735 7,350 73,500 list 01 08 88 vector 01 67 5971
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For the file with 73,500 records, the list version of the program took less than nine seconds to run, whereas the vector version took nearly ten minutes The discrepancy would have been even greater had there been more failing students
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56 Taking strings apart
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Now that we've seen some of what we can do with containers, we're going to turn our attention back to strings Until now, we've done only a few things with strings: We've created them, read them, concatenated them, written them, and looked at their size In each of these uses, we have dealt with the string as a single entity Often, this kind of abstract usage is what we want: We want to ignore the detailed contents of a string Sometimes, though, we need to look at the specific characters in a string As it turns out, we can think of a string as a special kind of container: It contains only characters, and it supports some, but not all, of the container operations The operations that it does support include indexing, and the string type provides an iterator that is similar to a vector iterator Thus, many of the techniques that we can apply to vectors apply also to strings For example, we might want to break a line of input into words, separated from each other by whitespace (space, tab, backspace, or the end of the line) If we can read the input directly, we can get the words from the input trivially After all, that's exactly how the string input operator executes: It reads characters up to the whitespace character However, there are times when we want to read an entire line of input and examine the words within that line We'll see examples in 73/126 and 742/131 Because such an operation might be generally useful, we'll write a function to do it The function will take a string and return a vector<string>, which will contain an entry for each whitespace-separated word in that string In order to understand this function, you need to know that strings support indexing much the same way as vectors do So, for example, if s is a string that contains at least one character, the first character of s is s[0], and the last character of s is s[ssize() - 1] Our function will define two indices, i and j, that will delimit each word in turn The idea is that we will locate a word by computing values for i and j such that the word will be the characters in the range [i, j) For example,
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Once we have these indices, we'll use the characters that they delimit to create a new string, which we will copy into our vector When we are done, we will return the vector to our caller:
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vector<string> split(const string& s) { vector<string> ret; typedef string::size_type string_size; string_size i = 0; // invariant: we have processed characters [original value of i, i) while (i != ssize()) { // ignore leading blanks // invariant: characters in range [original i, current i) are all spaces while (i != ssize() && isspace(s[i])) ++i; // find end of next word string_size j = i; // invariant: none of the characters in range [original j, current j)is a space while (j != ssize() && !isspace(s[j])) j++; // if we found some nonwhitespace characters
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This document is created with the unregistered version of CHM2PDF Pilot if (i != j) { // copy from s starting at i and taking j - i chars retpush_back(ssubstr(i, j - i)); i = j; } } return ret; }
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In addition to the system headers that we have already encountered, this code needs the <cctype> header, which defines isspace More generally, this header defines useful functions for processing individual characters The c at the beginning of cctype is a reminder that the ctype facility is part of C++'s inheritance from C The split function has a single parameter, which is a reference to a const string that we'll name s Because we will be copying words from s, split does not need to change the string As in 412/54, we can pass a const reference to avoid the cost of copying the string, while still ensuring that split will not change its argument We start off by defining ret, which will hold the words from the input string The next two statements define and initialize our first index, i As we saw in 24/22, string::size_type is the name for the appropriate type to index a string Because we need to use this type more than once, we start by defining a shorter synonym for this type, as we did in 322/43, to simplify the subsequent declarations We will use i as the index that finds the start of each word, advancing i through the input string one word at a time The test in the outermost while ensures that once we've processed the last word in the input, we'll stop Inside the while, we start by positioning our two indices First, we find the first non-space character in s that is at or after the position currently indicated by i Because there might be multiple whitespace characters in the input, we increment i until it denotes a character that is not whitespace There is a lot going on in this statement:
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while (i != ssize() && isspace(s[i])) ++i;
The isspace function is a predicate that takes a char and returns a value that indicates whether that char is whitespace The && operator tests whether both its operands are true, failing if either of them is false In this expression, the operation will succeed if i is not equal to the size of s (meaning that we have not reached the end of the string), and s[i] is a whitespace character In that case, we will increment i and check again As we described in 2422/26, the logical && operation uses a short-circuit strategy for evaluating its operands Unlike our earlier examples, this one relies on the short-circuit property of && The binary logical operations (operators && and ||) execute by testing their left-hand operands first If that test suffices to determine the overall result, then the right-hand operand is not evaluated In the case of the &&, the second condition is evaluated if and only if the first condition is true Thus, the condition in the while executes by first checking whether i != ssize() Only if this test succeeds does it use i to look at a character in s Of course, if i is equal to ssize(), then there are no more characters left to examine, and so we drop out of the loop Once we fall out of this while, we know either that i denotes a character that is not whitespace, or that we've run out of input without finding such a character Assuming that i is still a valid index, the next while will find the space that terminates the current word in s We start by creating our other index, j, and initializing it to the value of i The next while,
while (j != ssize() && !isspace(s[j])) ++j;