在C / C++中将字符串转换为数字

有两种将字符串转换为数字的常用方法：

使用stringstream类或sscanf()
stringstream()：这是将数字字符串转换为int，float或double的简单方法。以下是使用stringstream将字符串转换为int的示例程序。

CPP

// A program to demonstrate the use of stringstream
#include 
#include 
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;
}

输出：

Value of x : 12345

// A stringstream is similar to input/output
// file stream. We need to declare a stringstream
// just like an fstream, for example: 
stringstream ss;

// and, like an fstream or cout, 
// we can write to it:
ss << myString; or 
ss << myCstring; or
ss << myInt;, or float, or double, etc.

// and we can read from it:
ss >> myChar; or
ss >> myCstring; or
ss >> myInt;  

总而言之，stringstream是一种方便的操作字符串。
sscanf()是类似于scanf()的C样式函数。它从字符串而不是标准输入中读取输入。

CPP

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

输出：

Value of x : 12345

同样，我们可以分别使用％f和％lf读取float和double。

使用stoi()或atoi()进行字符串转换
stoi()： stoi()函数将字符串作为参数并返回其值。以下是一个简单的实现：

CPP

// C++ program to demonstrate working of stoi()
// Work only if compiler supports C++11 or above.
#include 
#include 
using namespace std;
 
int main()
{
    string str1 = "45";
    string str2 = "3.14159";
    string str3 = "31337 geek";
 
    int myint1 = stoi(str1);
    int myint2 = stoi(str2);
    int myint3 = stoi(str3);
 
    cout << "stoi(\"" << str1 << "\") is "
         << myint1 << '\n';
    cout << "stoi(\"" << str2 << "\") is "
         << myint2 << '\n';
    cout << "stoi(\"" << str3 << "\") is "
         << myint3 << '\n';
 
    return 0;
}

输出：

stoi("45") is 45
stoi("3.14159") is 3
stoi("31337 geek") is 31337 

atoi()： atoi()函数将字符数组或字符串字面量作为参数并返回其值。以下是一个简单的实现：

CPP

// For C++11 above
#include 
#include 
using namespace std;
 
int main()
{
    const char* str1 = "42";
    const char* str2 = "3.14159";
    const char* str3 = "31337 geek";
 
    int num1 = atoi(str1);
    int num2 = atoi(str2);
    int num3 = atoi(str3);
 
    cout << "atoi(\"" << str1 << "\") is " << num1 << '\n';
    cout << "atoi(\"" << str2 << "\") is " << num2 << '\n';
    cout << "atoi(\"" << str3 << "\") is " << num3 << '\n';
 
    return 0;
     
}

C

#include 
#include 
int main()
{
 
    char* str1 = "42";
    char* str2 = "3.14159";
    char* str3 = "31337 geek";
 
    int myint1 = atoi(str1);
    int myint2 = atoi(str2);
    int myint3 = atoi(str3);
 
    printf("stoi(%s) is %d \n", str1, myint1);
    printf("stoi(%s) is %d \n", str2, myint2);
    printf("stoi(%s) is %d \n", str3, myint3);
 
    // This Code is Contributed by Harshit Srivastava
    return 0;
}

输出：

atoi("42") is 42
atoi("3.14159") is 3
atoi("31337 geek") is 31337 

stoi()vs atoi()

• atoi()是旧的C样式函数。在C++ 11中添加了stoi()。
• atoi()仅适用于C样式的字符串(字符数组和字符串字面量)，stoi()适用于C++的字符串和C样式的字符串
• atoi()仅接受一个参数，并返回整数值。

int atoi (const char * str); 

• stoi()最多可以包含三个参数，第二个参数用于起始索引，第三个参数用于输入数字的基数。

int stoi (const string&  str, size_t* index = 0, int base = 10); 

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