Question

我从txt文件（文件中的第一行）获取此行：

#operation=1(create circle and add to picture) name X Y radius.

为什么这段代码不取整数1并将其放入k？

Circle Circle::CreateCirc(Circle c){
    int k;
    ifstream myfile("cmd.txt");
    if (!myfile.is_open())
        cout<<"Unable to open the requested file "<<endl;
    string line,line2="create circle";
    for (int i=1;i<countrows();i++)
    {
        getline(myfile,line);
        if (line.find(line2)!=string::npos)
        {
             istringstream ss(line);
             ss>>k;
             cout<<k<<endl;

        }

    }
    return c;

}

而是我获得地址记忆...帮助PLZ

Answer 1

因为该行不以数字开头。在提取数字之前，您需要跳过#operation=部分。

您应该检查提取结果和getline，以帮助确定这些结果失败时出现的问题。

此外，如果countrows()返回文件中预期的行数，那么您的循环将错过最后一行。从零开始循环，或者在i <= countrows()时循环;或者，如果要处理文件中的每一行，只需循环while (getline(myfile,line))即可。

Answer 2

如果您尝试阅读的文件中的实际文本以"#operation=1"开头，并且您希望编号为1，则无法使用简单输入运算符。它将首先读取字符'#'，这不是数字，因此解析将失败并且k将不会被初始化。如果k未初始化，则它将具有不确定的值，并且读取该值将导致未定义的行为和看似随机的输出。

您需要检查提取是否有效：

if (ss >> k)
    std::cout << k << '\n';

这不会解决你的问题，就像我上面说过的那样，你不能在这里使用简单的输入算子。您需要使用其他方法解析字符串。一种方法可能是找到相等的字符'='并在此之后获取子字符串以尝试提取数字。

Answer 3

编辑：使用atoi()而不是流来尝试使用它的方式更接近一点。

#include <iostream>
#include <cstdlib> // for atoi()
int main(){

    std::string str = "#operation=1(create circle and add to picture) name X Y radius.";
    int k;

    std::string line=str, line2="(create circle";

    std::size_t fnd = line.find(line2);
    if (fnd!=std::string::npos)
    {
         k = atoi(&str[fnd-1]); // int atoi(const char *str) == argument to integer
         std::cout<< k << " " << str[fnd-1] << str[fnd] << " ";
    }
}

有几种方法可以从字符串中提取整数，但我想过滤掉字符串中的数字;

#include <iostream>

int main(){

    std::string str = "#operation=1(create circle and add to picture) name X Y radius.";
    int k = 0;

    // an array of our base10 digits to filter through and compare
    const char digit[] = {'0','1','2','3','4','5','6','7','8','9'};

    for(int s_filter = 0; s_filter<str.size(); ++s_filter){
        for(int d_filter = 0; d_filter<10; ++d_filter){
        // filter through each char in string and
        // also filter through each digit before the next char

            if(digit[d_filter] == str[s_filter]) {
            // if so the char is equal to one of our digits

                k = d_filter;// and d_filter is equal to our digit
                break;

            } else continue;
        }
    }
    switch(k) {
        case 1:
            std::cout<< "k == 1";
            // do stuff for operation 1..
            return 0;
        case 2:
            std::cout<< "k != 1";
            // do more stuff
            break;
        //case 3: ..etc.. etc..
        default:
            std::cout<< "not a digit";
            return 1;
    }
}

Answer 4

试试这个：

Circle Circle::CreateCirc(Circle c){
 const std::streamsize ALL = std::numeric_limits< std::streamsize >::max(); // #include <limits> needed
 int k;
 ifstream myfile("cmd.txt");
 if (!myfile.is_open())
    cout<<"Unable to open the requested file "<<endl;
 for (int i=1;i<countrows(); ++i, myfile.ignore(ALL,'\n') ) // skip rest of the line
 {
    if( myfile.ignore(ALL,'=') >> k  )
    {
         cout<<k<<endl;
    }
    else
        break; // read error
 }
 return c;
}

Answer 5

// find_num.cpp (cX) 2015 adolfo.dimare@gmail.com
// http://stackoverflow.com/questions/21115457/

#include <string> // std::string
#include <cctype> // isnum

/// Find the number in 'str' starting at position 'pos'.
/// Returns the position of the first digit of the number.
/// Returns std::string::npos when no further numbers appear within 'str'.
/// Returns std::string::npos when 'pos >= str.length()'.
size_t find_num( const std::string str, size_t pos ) {
    size_t len = str.length();
    bool   isNegative = false;
    while ( pos < len ) {
        if ( isdigit(str[pos]) ) {
            return ( isNegative ? pos-1 : pos );
        }
        else if ( str[pos]=='-' ) {
            isNegative = true;
        }
        else {
            isNegative = false;
        }
        ++pos;
    }
    return std::string::npos;
}

#include <cassert>     // assert()
#include <cstring>     // strlen();

int main() {
    std::string str;

    str = "";
    assert( std::string::npos == find_num( str, 0 ) );
    assert( std::string::npos == find_num( str, 9 ) );

    str = "#operation=1(create circle and add to picture) name X Y radius.";
    assert( strlen("#operation=") == find_num( str, 0 ) );

    str = "abcd 111 xyx 12.33 alpha 345.12e-23";
    ///    0123456789.123456789.123456789.123456789.
    assert(     5 == find_num( str, 0 ) );
    assert(            13 == find_num( str, 5+3 ) );
    assert(                        25 == find_num( str, 20 ) );

    str = "abcd-111 xyx-12.33 alpha-345.12e-23";
    ///    0123456789.123456789.123456789.123456789.
    assert(    4 == find_num( str, 0 ) );
    assert(           12 == find_num( str, 5+3 ) );
    assert(                       24 == find_num( str, 20 ) );

    str = "-1";
    assert( 0 == find_num( str, 0 ) );
    assert( 1 == find_num( str, 1 ) );
    assert( std::string::npos == find_num( str, 2 ) );
    assert( std::string::npos == find_num( str, strlen("-1") ) );

    return 0;
}

如何从字符串中提取整数？ C ++

5 个答案: