7.STL中string的一些超常用函数 (附习题)

1.find

2.atoi

3.to_string

4.getline

【leetcode 习题】

387.字符串中的第一个唯一字符

125. 验证回文串

1.find

1.查找第一次出现的目标字符串：说明：如果查找成功则输出查找到的第一个位置，否则返回-1；

s1.find(s2)

#include <iostream>
#include <string>
using namespace std;
int main() {string s1 = "abcdef";string s2 = "de";int ans = s1.find(s2) ;   //在S1中查找子串S2cout << ans << endl;system ("pause");
}

2.查找从指定位置开始的第一次出现的目标字符串：

s1.find(s2, 2)

#include <iostream>
#include <string>
using namespace std;
int main() {string s1 = "abcdef";string s2 = "de";int ans = s1.find(s2, 2) ;   //从S1的第二个字符开始查找子串S2cout << ans << endl;system("pause");
}

2.atoi

atoi()函数的语法非常简单
int atoi (const char * str);
int integer type which is the type of returned value.

int整数类型，它是返回值的类型。

const char * is a constant char array that is equal to a string whose variable name is str.

const char *是一个常量char数组，它等于一个变量名称为str 。

作用：在C和C ++中将字符串/字符转换为整数 (Convert String/Char To Integer In C and C++)

可以参考如下实例：

#include <iostream>
#include <cstdlib>int main()
{const char *str1 = "57";const char *str2 = "314.159";const char *str3 = "52345 some text";const char *str4 = "some text 25";int mynum1 = std::atoi(str1);int mynum2 = std::atoi(str2);int mynum3 = std::atoi(str3);int mynum4 = std::atoi(str4);std::cout << "atoi(\"" << str1 << "\") is " << mynum1 << '\n';std::cout << "atoi(\"" << str2 << "\") is " << mynum2 << '\n';std::cout << "atoi(\"" << str3 << "\") is " << mynum3 << '\n';std::cout << "atoi(\"" << str4 << "\") is " << mynum4 << '\n';
}

If given string or char array is floating-point like “314.159” in only integer part will be converted where the result will be “314”
如果给定的字符串或char数组是浮点数，例如“ 314.159”，则仅整数部分将被转换，结果为“ 314”
If there are some non-numerical chars in the given char array or string they will not convert and there will be no error where only the integer part will be converted. As an example “52345 some text” will be converted 52345
如果给定的char数组或字符串中有一些非数字char，它们将不会转换，并且只有整数部分会被转换也不会出错。例如，“ 52345 some text”将被转换为52345
If the start of the char array or string and there are numerical characters after them this will be converted into 0 as an integer value. As an example “some text 25” will be converted into 0.
如果char数组或字符串的开头，并且后面有数字字符，则将其转换为0作为整数值。例如，“一些文本25”将转换为0。
If the numerical characters are between non-numeric characters the conversion result will be 0 too.
如果数字字符介于非数字字符之间，则转换结果也将为0。
If the char array or string starts with numeric character and after that, there is a non-numeric, and then numeric character only the starting numeric character will be converted. As an example “25 text 50” will be converted into 25.
如果char数组或字符串以数字字符开头，并且之后是非数字字符，则仅将起始数字字符转换为数字字符。例如，“ 25个文本50”将转换为25。

【总结来说】：字符转化为数字，是以起始为主要参考的

3.to_string

将数字常量转换为字符串，返回值为转换完毕的字符串

头文件：#include < string >（必须加）

string s = to_string(i); //将整数i转换为字符串表示形式

附：如果是string s = to_string(‘3’)，则结果是s = 51。
需要对s = s -48; 才能得到正确的结果。

#include <iostream>   
#include <string>     
using namespace std;int main()
{string pi = "pi is " + std::to_string(3.1415926);string perfect = to_string(1 + 2 + 4 + 7 + 14) + " is a perfect number";cout << pi << '\n';cout << perfect << '\n';system("pause");return 0;
}