How do I sort a chart by last names alphabetically from a inputted text file?

I have to create a program that: - define class cust(I already did this part) - create an array of cust (size 10)to hold the data of the class. (done) - read the date from a file(everything from the class - already done) - call a function print cust(this function will print all the data in table format)....first, last, state, sales history(0,1,2), units - Read the data from a file (you may break up the strings and numeric values any way you choose (separate lines may be easier) - Make a function call to a function called printcust. This function will print all the data for all of the customers (in tabular format). - Make a function call to a function called sortname which will sort the cust array alphabetically by last name. - Make a function call to the function printcust. - Make a function call to a function called sortsales. This function will sort the above array in descending order of the total sales. - Call the function printcust

I have attached what I have so far. All I need to do is get help with the next two tables which will sort by last name, alphabetically and then sort by total sales in descending order.

    #include <iostream>
    #include <string>
    #include <fstream>
    #include <sstream>
    #include <algorithm> // for std::sort

    using namespace std;

    class customer{
  public:
  string first;
  string last;
  string state;
  double sHistory[3]; // Sales history for three years.
  double totalSales; // Total sales (adding all three years together)
  int purchaseUnits;
  };

    void printcust(customer[], int);
    void sortname(customer[], int);
    void sortsales(customer[], int);


    int main () 
    {

fstream infile;
customer cust; 
customer custarray[10];

infile.open("data.txt");
int i = 0;

while(infile)
{
    infile >> custarray[i].first;
    infile >> custarray[i].last;
    infile >> custarray[i].state;
    infile >> custarray[i].sHistory[0];
    infile >> custarray[i].sHistory[1];
    infile >> custarray[i].sHistory[2];
    infile >> custarray[i].purchaseUnits;

custarray[i].totalSales = custarray[i].sHistory[0] + custarray[i].sHistory[1] + custarray[i].sHistory[2];
i++;
}


i = i - 1;

for(int a = 0; a < i; a++)
{
    cout << custarray[a].first << '\t' << custarray[a].last << '\t' << custarray[a].state << '\t';
    cout << custarray[a].sHistory[0] << " " << custarray[a].sHistory[1] << " " << custarray[a].sHistory[2];
    cout << '\t' << custarray[a].purchaseUnits << '\t' << custarray[a].totalSales;
    cout << endl << endl; 
}
    }