stringstream及其应用

1.介绍

stringstream 将字符串对象与流相关联，允许从字符串中读取，就好像它是一个流（如 cin）

clear() — 清除流
str() — 获取和设置其内容存在于流中的字符串对象。
运算符 << — 将字符串添加到 stringstream 对象。
运算符 >> — 从 stringstream 对象中读取内容。

2.应用

2.1 计算字符串中的单词数

// CPP program to count words in a string using stringstream.
#include <bits/stdc++.h>
using namespace std;
int countWords(string str)
{
    // breaking input into word using string stream
    stringstream s(str); // Used for breaking words
    string word; // to store individual words
    int count = 0;
    while (s >> word)
        count++;
    return count;
}
// Driver code
int main()
{
    string s = "geeks for geeks geeks "
            "contribution placements";
    cout << " Number of words are: " << countWords(s);
    return 0;
}

2.2 打印字符串中单个单词的频率

// CPP program to demonstrate use of stringstream
// to count frequencies of words.
#include <bits/stdc++.h>
using namespace std;
void printFrequency(string st)
{
    // each word it mapped to it's frequency
    map<string, int> FW;
    stringstream ss(st); // Used for breaking words
    string Word; // To store individual words
    while (ss >> Word)
        FW[Word]++;
    map<string, int>::iterator m;
    for (m = FW.begin(); m != FW.end(); m++)
        cout << m->first << " -> "
            << m->second << "\n";
}
// Driver code
int main()
{
    string s = "Geeks For Geeks Ide";
    printFrequency(s);
    return 0;
}

2.3 使用 Stringstream 从字符串中删除空格

代码一（更优）

// C++ program to remove spaces using stringstream
#include <bits/stdc++.h>
using namespace std;
// Function to remove spaces
string removeSpaces(string str)
{
    stringstream ss;
    string temp;
    // Storing the whole string
    // into string stream
    ss << str;
    // Making the string empty
    str = "";
    // Running loop till end of stream
    while (!ss.eof()) {
        // Extracting word by word from stream
        ss >> temp;
        // Concatenating in the string to be
        // returned
        str = str + temp;
    }
    return str;
}
// Driver function
int main()
{
    // Sample Inputs
    string s = "This is a test";
    cout << removeSpaces(s) << endl;
    s = "geeks for geeks";
    cout << removeSpaces(s) << endl;
    s = "geeks quiz is awesome!";
    cout << removeSpaces(s) << endl;
    s = "I love     to     code";
    cout << removeSpaces(s) << endl;
    return 0;
}

代码二

// C++ program to remove spaces using stringstream
// and getline()
#include <bits/stdc++.h>
using namespace std;
// Function to remove spaces
string removeSpaces(string str)
{
    // Storing the whole string
    // into string stream
    stringstream ss(str);
    string temp;
    // Making the string empty
    str = "";
    // Running loop till end of stream
    // and getting every word
    while (getline(ss, temp, ' ')) {
        // Concatenating in the string
        // to be returned
        str = str + temp;
    }
    return str;
}
// Driver function
int main()
{
    // Sample Inputs
    string s = "This is a test";
    cout << removeSpaces(s) << endl;
    s = "geeks for geeks";
    cout << removeSpaces(s) << endl;
    s = "geeks quiz is awesome!";
    cout << removeSpaces(s) << endl;
    s = "I love     to     code";
    cout << removeSpaces(s) << endl;
    return 0;
}

2.4 C/C+ + 中字符串到数字的转换

将字符串转换为数字有两种常用的方法： 使用 stringstream 类或 sscanf()

stringstream() ：这是将数字字符串转换为整数、浮点数或双精度数的简单方法。
以下是使用 stringstream 将 string 转换为 int 的示例程序。

// A program to demonstrate the use of stringstream
#include <iostream>
#include <sstream>
using namespace std;
int main()
{
    string s = "12345";
    // object from the class stringstream
    stringstream geek(s);
    // The object has the value 12345 and stream
    // it to the integer x
    int x = 0;
    geek >> x;
    // Now the variable x holds the value 12345
    cout << "Value of x : " << x;
    return 0;
}

sscanf()是一个类似于 scanf() 的 C 风格函数。它从字符串而不是标准输入中读取输入。

#include<stdio.h>
int main()
{
    const char *str = "12345";
    int x;
    sscanf(str, "%d", &x);
    printf("\nThe value of x : %d", x);
    return 0;
}