如何从字符串中提取整数?C++

因为这行不是以数字开头的。在提取数字之前，您需要跳过#operation=部分。

您应该检查提取和getline的结果，以帮助确定当这些失败时出了什么问题。

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个行数。要么从零开始循环，要么在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。

如果您尝试读取的文件中的实际文本以 "#operation=1" 开头，并且您希望从中1数字，则不能使用简单输入运算符。它将首先读取字符'#'，该字符不是数字，因此解析将失败，并且不会初始化k。如果k没有初始化，它将具有不确定的值，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';

但这并不能解决您的问题，就像我上面说的，您不能在这里使用简单的输入运算符。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字符'='并获取一个子字符串以尝试提取数字。

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;
}

编辑：一种方法与您尝试使用 atoi（） 而不是流的方式来做到这一点并不接近。

#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，但我喜欢从字符串中过滤掉数字;

#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;
    }
}

// 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;
}