Why don't the two else if statements work? [closed]

I'm trying to make a program that asks the user to input characters untill '@' is inserted and at the end it should show biggest small letter, capital letter and digit as well as how many of each were inserted. Here's what I tried:

#include <iostream>

int main()
{
    char a; //character that user has to insert
    std::cin>>a;
    char small=a;
    char big=a;
    char digit=a;
    int i=0,j=0,k=0; //counters for small letters, capital letters and digits
    while (a!='@')
    {

        if (a>='a' || a<='z')
        {
            if (a>small)
            {
            small=a; //grabs the biggest small letter
            }
            i++;
        }
        else if (a>='A' || a<='Z')
        {
            if (a>big)
            {
                big=a; //grabs the biggest capital letter
            }
            j++;
        }
        else if (a>='0' || a<='9')
        {
            if (a>digit)
            {
                digit=a; //grabs the biggest digit
            }
            k++;
        }
        std::cin>>a;
    }
    std::cout<<small<<'\t'<<i<<std::endl;
    std::cout<<big<<'\t'<<j<<std::endl;
    std::cout<<digit<<'\t'<<k<<std::endl;
    return 0;
}

When I run this the first if gives me biggest small letter but the counter doesn't only count characters whitin the (a>='a' || a<='z') range, instead it counts everything I insert and the 2 else if statements give me the first character inserted and the counters don't work at all. What am I doing wrong and why doesn't this work?