C++ String轉int用法及代碼示例

// C++ program to demonstrate 
// working of stoi()
// Work only if compiler supports
// C++11 or above. Because STOI()
// was added in C++ after 2011
#include <iostream>
#include <string>
using namespace std;
// Driver code
int main()
{
  string str1 = "45";
  string str2 = "3.14159";
  char str3[] = "31337 geek";
  // type of explicit type casting
  int myint1 = stoi(str1);
   
  // type of explicit type casting
  int myint2 = stoi(str2);
     
  // type of explicit type casting
  int myint3 = stoi(str3);
  cout << "stoi(\"" << str1 << 
          "\") is " << myint1 << '\n';
    cout << "stoi(\"" << str2 << 
            "\") is " << myint2 << '\n';
    cout << "stoi(\"" << str3 << 
            "\") is " << myint3 << '\n';
    return 0;
}

// C++ program to demonstrate the
// functioning of the atoi() function
#include <iostream>
#include <stdlib.h>
using namespace std;
int main()
{
    char* str1 = "141";
    char* str2 = "3.14";
    int res1 = atoi(str1);
    int res2 = atoi(str2);
    cout << "atoi(" << str1 << ") is " << res1 << "\n";
    cout << "atoi(" << str2 << ") is " << res2 << "\n";
    return 0;
}

// C++ program to demonstrate the
// use of a stringstream to
// convert string to int
#include <iostream>
#include <sstream>
using namespace std;
// Driver code
int main()
{
    string s = "12345";
    // object from the class stringstream
    stringstream geek;
    // inserting string s in geek stream
    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 + 1 : " << x + 1;
    return 0;
}

// C++ program to demonstrate
// the working of sscanf() to
// convert a string into a number
#include <iostream>
using namespace std;
int main()
{
    const char* str = "12345.0000046";
    float x;
    sscanf(str, "%f", &x);
    cout << "The value of x : " << x << endl;
    return 0;
}

// C++ Program to convert String
// into number using for loop
#include <bits/stdc++.h>
using namespace std;
int main()
{
    string number = "13";
    int i = 0;
    // Traversing string
    for (char c : number) {
        // Checking if the element is number
        if (c >= '0' && c <= '9') {
            i = i * 10 + (c - '0');
        }
        // Otherwise print bad output
        else {
            cout << "Bad Input";
            return 1;
        }
    }
    cout << i;
}

#include <stdio.h>
#include <stdlib.h>
int main() {
  char str[] = "12345";
  char* endptr;
  long int value = strtol(str, &endptr, 10);
  if (endptr == str) {
    printf("No digits were found\n");
  } else if (*endptr != '\0') {
    printf("Invalid input: %s\n", str);
  } else {
    printf("The value is %ld\n", value);
  }
  return 0;
}