Usage of string.find()

See it on my own blog https://dyingdown.github.io/2019/09/02/string-find-usage/
Every time I use find function in C++ , I always search its usage after using it for so many times. So I decide to make a summary of some commonly used usages.

C++ string.find()

Before seeing the usage directly, we need to know a data type size_type.

Size_type is a type defined by the string class type and the vector class type to hold the length of any string object or vector object. The standard library type defines size_type as unsigned. The abstract meaning of string is a string, the abstract meaning of size() is the size of a string, and the abstract meaning of string::size_type is a unit of size unit.

The followings are the usage.

#include <string>
#include <iostream>

using namespace std;

int main(){
    string s;
    string flag;
    char a;
    string::size_type position;
    ...
    return 0;
}

the subscript position of flag in s

position = s.find(flag);
if(position != s.npos){
    // the position exists
} else {
    // the position does not exist
}

Subscript position of flag in s start from 3 subscript

position = s.find(flag, 3);

First occurrence of the subscript position of character a in s

position = s.find_first_of(a);

All locations where a appears in s

position = 0;
int i = 1;
while(position = s.find_fisrt_of(a, position)) != string::npos){
    ...
    position ++;
    i ++;
}

Find the position in the flag that does not match the first one of s

position = flag.find_first_not_of(s);

Last occurrence of flag in s

position = s.rfind(flag)

The reason why I use this function is that I came cross a problem on LuoGu P1533 number reversion and I have another article that solve this problem.