The function I am using is 

```
vector<string> tokenise(string s){

}

```

Firstly, I intend to split the string into substrings, in which case the string is always arithmetic expression (e.g. "100+5") and there could be some whitespaces.

"100+5" is needed to convert to "100", "+", "5" 

After conversion, the substrings will be stored in a vector and return it. I am struggling with the fist step and using the subscript to loop over a string. The type of the value returned is char, so there is no way to put it in the vector.