Ludzie pragną czasami się rozstawać, żeby móc tęsknić, czekać i cieszyć się z powrotem.
NewFind.cpp isn’t the best solution to the case sensitivity problem, so we’ll revisit it when we examine string comparisons.
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Finding in reverse
Sometimes it’s necessary to search through a string from end to beginning, if you need to find the data in “last in / first out “ order. The string member function rfind( ) handles this job.
//: C17:Rparse.cpp
// Reverse the order of words in a string
#include <string>
#include <iostream>
#include <vector>
using namespace std;
int main() {
// The ';' characters will be delimiters
string s("now.;sense;make;to;going;is;This");
cout << s << endl;
// To store the words:
vector<string> strings;
// The last element of the string:
int last = s.size();
// The beginning of the current word:
int current = s.rfind(';');
// Walk backward through the string:
while(current != string::npos){
// Push each word into the vector.
// Current is incremented before copying to
// avoid copying the delimiter.
strings.push_back(
s.substr(++current,last - current));
// Back over the delimiter we just found,
// and set last to the end of the next word
current -= 2;
last = current;
// Find the next delimiter
current = s.rfind(';', current);
}
// Pick up the first word - it's not
// preceded by a delimiter
strings.push_back(s.substr(0, last - current));
// Print them in the new order:
for(int j = 0; j < strings.size(); j++)
cout << strings[j] << " ";
} ///:~
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Here’s how the output from Rparse.cpp looks:
now.;sense;make;to;going;is;This
This is going to make sense now.
rfind( ) backs through the string looking for tokens, reporting the array index of matching characters or string::npos if it is unsuccessful.
Finding first/last of a set
The find_first_of( ) and find_last_of( ) member functions can be conveniently put to work to create a little utility that will strip whitespace characters off of both ends of a string. Notice it doesn’t touch the original string, but instead returns a new string:
//: C17:trim.h
#ifndef TRIM_H
#define TRIM_H
#include <string>
// General tool to strip spaces from both ends:
inline std::string trim(const std::string& s) {
if(s.length() == 0)
return s;
int b = s.find_first_not_of(" \t");
int e = s.find_last_not_of(" \t");
if(b == -1) // No non-spaces
return "";
return std::string(s, b, e - b + 1);
}
#endif // TRIM_H ///:~
The first test checks for an empty string; in that case no tests are made and a copy is returned.
Notice that once the end points are found, the string constructor is used to build a new string from the old one, giving the starting count and the length. This form also utilizes the “return value optimization” (see the index for more details).
Testing such a general-purpose tool needs to be thorough:
//: C17:TrimTest.cpp
#include "trim.h"
#include <iostream>
using namespace std;
string s[] = {
" \t abcdefghijklmnop \t ",
"abcdefghijklmnop \t ",
" \t abcdefghijklmnop",
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"a", "ab", "abc", "a b c",
" \t a b c \t ", " \t a \t b \t c \t ",
"", // Must also test the empty string
};
void test(string s) {
cout << "[" << trim(s) << "]" << endl;
}
int main() {
for(int i = 0; i < sizeof s / sizeof *s; i++)
test(s[i]);
} ///:~
In the array of string s, you can see that the character arrays are automatically converted to string objects. This array provides cases to check the removal of spaces and tabs from both ends, as well as ensuring that spaces and tabs do not get removed from the middle of a string.
Removing characters from strings
My word processor/page layout program (Microsoft Word) will save a document in HTML,
but it doesn’t recognize that the code listings in this book should be tagged with the HTML