Question

程序中的循环似乎至少执行一次，即使子串没有出现也是如此。这是为什么？

#include <iostream>
#include <string>

using namespace std;

int countSubstrings(const string& original_string, const string& substr) {
    int number_of_ocurrences = 0;
    int i = 0;
    for (i = original_string.find(original_string, 0); i != string::npos;
     i = original_string.find(substr, i)) {
        number_of_ocurrences++;
        i++;
    }
    return number_of_ocurrences;
}

int main() {
    string input;
    while (1) {
        cout << "Enter a a line of text: ";
        getline(cin, input, '\n');
        cout << '\n';
        cout << "Number of ocurrences of the word needle: ";
        cout << countSubstrings(input, "needle") << '\n';
    }
}

Answer 1

最初，当你在for循环中设置i时，你有

original_string.find(original_string, 0)

因此，您正在搜索它将找到的字符串。我相信你的意思是

original_string.find(substr, 0)

Answer 2

下面的代码与您的问题没有直接关系，但是，我认为这可能是该概念（通过C ++中的字符串循环）和新手的一个很好的例子。

public static async Task<HttpResponseMessage> Run(HttpRequest req, ILogger log)
{
    //string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
    MemoryStream ms = new MemoryStream(); 
    await req.Body.CopyToAsync(ms);
    byte[] imageBytes = ms.ToArray();
    log.LogInformation(imageBytes.Length.ToString());


    // ignore below (not related)
    string finalString = "Upload succeeded";
    Returner returnerObj = new Returner();
    returnerObj.returnString = finalString;
    var jsonToReturn = JsonConvert.SerializeObject(returnerObj);

    return new HttpResponseMessage(HttpStatusCode.OK) {
        Content = new StringContent(jsonToReturn, Encoding.UTF8, "application/json")
    };
}

public class Returner
{
    public string returnString { get; set; }
}

此代码的输出是：

#include <iostream>
using namespace std;

int main (){
    string mainString     = "foo bar foo bar bar";
    string searchedString = "bar";

    int numberOfOccurences = 0;

    size_t position = mainString.find(searchedString);
    while( position != std::string::npos ){
        cout<< "There is '" << searchedString << "' at position '" << position <<"' in '"<<mainString <<"'"<<endl;
        position += searchedString.length();
        position = mainString.find(searchedString, position);

        numberOfOccurences++;
    }

    cout<<"In total, there are "<< numberOfOccurences <<" '"<< searchedString <<"'"<<endl;
    return 0;
}

有关更多信息，请访问以下链接。

string.find
  第一个匹配项的第一个字符的位置。   如果未找到匹配项，则该函数返回string :: npos。

std::string::npos
  此值用作字符串成员函数中len（或sublen）参数的值时，表示“直到字符串结尾”。

如何使用find（）C ++循环遍历字符串

2 个答案: