undefined behaviour in std::string.substr

I want to get a substring from a string . I do this in new for_each loop with lambda and string.substr() .for the first time ,it runs correctly but after that it don't work correctly

here is my code:-

suppose eqn = "1231-78*78-433-67+89/32" and

std::for_each(eqn.begin(),eqn.end(),[&](char ch)
{
    if(isdigit(ch) || ch=='.')
        i++;
    else if(isOperator(ch))
    {
        std::string temp = eqn.substr(li,i);
        double tempN = std::stod(temp);
        NumOp.insert(std::pair <double ,char> (tempN,ch));
        li=i+1;i++;
    }
});

now here is what I see in my QtCreator's debug:-

enter image description here

it should give me "78*" substring from string "1231-78*78-433-67+89/32" but i am getting "78*78-4" which is causing so much destruction and I can't figure out.please help me cause I was like waking since all night