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我有一个list.txt文件。

它包含大约 100 条记录，但如果用户cin是一个字母，例如 A，我只想在循环中显示所有包含 A 的记录。

记录是以换行格式记录的，在shell命令中我们使用A*，但是在C++中，我们怎么做呢？

例子：

Alfred
Alpha
Augustine
Bravo
Charlie
Delta
4

3 回答 3

3

Here's a bunch of ways to do it, chose the one you like more ;)

Crappy solution with strings and streams:

std::vector< std::string > vec;//this will hod the file data
std::ifstream ifs("test.txt");//the input file stream
std::string tmp;//a temporary string

while( ifs >> tmp )//reading the whole data from the file
    vec.push_back(tmp);

for( int i = 0; i < vec.size(); i++ )
    if(vec[i][0] == 'a')//vec[i][0] stands for "the first symbol of element number i in vector"
        std::cout << vec[i] << std::endl;//outputting the string if it starts with 'a'

If you have c++11, you can replace the for with this range-based for:

for( std::string & s : vec )
    if(s.at(0) == 'a')
        std::cout << s << std::endl;

Or, you can complicate things further and replace the for with std::copy_if and lambdas from c++11 (IMO it's much too complicated and hard to read for such a simple occasion, but still I'll include it):

//this will copy all strings starting with 'a' into res vector.
std::vector< std::string > res;
std::copy_if(vec.begin(), vec.end(), back_inserter(res), [](const std::string & s){ return s[0]=='a'; } );

If you don't need to store the strings anywhere, it's easier:

std::vector< std::string > vec;
std::ifstream ifs("test.txt");
std::string tmp;

while( ifs >> tmp )
    if( tmp.at(0) == 'a' )
        std::cout << tmp;

A more old-school solution without using streams of strings:

FILE * f = fopen("test.txt", "r");//opening the file
if( !f )//checking in case it didn't open
    return -1;

char buffer[255];//buffer for the strings being read from file
while( !feof(f) )
{
    fgets(buffer, 255, f);//getting a string
    if(buffer[0] == 'a')//printing if it starts with 'a'
        printf("%s", buffer);
}
fclose(f);//don't forget to close the file
于 2012-07-26T06:27:37.840 回答
3

这是一个相当优雅，可能更惯用的解决方案：

#include <algorithm> //for copy_if
#include <cctype> //for tolower
#include <fstream> //for ifstream
#include <iostream> //for cout, cin
#include <iterator> //for istream_iterator, ostream_iterator
#include <string> //for string

int main() {
    char letter;
    std::cout << "Enter the letter to look for: ";
    std::cin >> letter; //I didn't validate it

    std::ifstream fin ("names.txt");

    std::istream_iterator<std::string> ifbegin (fin); //begin file iter
    std::istream_iterator<std::string> ifend; //end file iter
    std::ostream_iterator<std::string> obegin (std::cout, " "); //begin out iter

    std::copy_if (ifbegin, ifend, obegin, //copy from file to output if
        [letter] (const std::string &str) { //capture letter
            return std::tolower (str [0]) == std::tolower (letter);
        } //copy if starts with upper/lower case of entered letter
    );
}

请注意，它确实需要 C++11copy_if和 lambda。这将输出文件中以输入字母的大写/小写开头的每个名称，用空格分隔。它在数据排序时的执行与数据未排序时的执行相同。

但是，正如 Luc 在下面指出的那样，这将读取带有空格的行的单独名称。如果你想解决这个问题，你需要一个operator>>读取一行的自定义替换。

第 1 步：创建替换：

struct Line {
    std::string text; //note I made this public to save time
    operator std::string() const {return text;} //less work later
};

第 2 步：修改operator>>以读取结构的一行：

std::istream &operator>> (std::istream &in, Line &line) {
    std::getline (in, line.text); //get whole line
    return in;
}

第 3 步：更改迭代器以使用我们的自定义结构。请注意，最后一个保留一个字符串，因为它可以隐式转换为一个。让我们也用换行符分隔打印，这样我们就可以知道它是一行，而不是一个单词：

std::istream_iterator<Line> ifbegin (fin); //begin file iter
std::istream_iterator<Line> ifend; //end file iter
std::ostream_iterator<std::string> obegin (std::cout, "\n"); //begin out iter

第 4 步：更改 lambda 以满足我们的需要：

[letter] (const std::string &line) {
        return !line.empty() //we introduced the possibility of "" 
               && (std::tolower (line [0]) == std::tolower (letter));
}

4个简单的步骤后，我们就完成了！

于 2012-07-26T06:39:16.887 回答
0

如果您确实需要新列表，我建议您使用 remove_copy_if 或 for_each 使用检查谓词的 lambda 表达式（如果您不需要）。你能详细说明一下你所说的“表演”是什么意思吗？

于 2012-07-26T06:24:04.313 回答