help !!!

Hello! im a newbie in C programing as well as Postgresql. The above example
is workin but im having difficulty regarding assigning the result of Query
Select. here's the program

int
main()
{
char UserName[50]; /* holds user
state code */
char query_string[256]; /* holds
constructed SQL query */
PGconn *conn; /* holds
database connection */
PGresult *res; /* holds
query result */

conn = PQconnectdb("dbname=grade"); /* connect
to the database */

if (PQstatus(conn) == CONNECTION_BAD) /* did the
connection fail? */
{
fprintf(stderr, "Connection to database failed.\n");
fprintf(stderr, "%s", PQerrorMessage(conn));
exit(1);
}

printf("Enter Username: "); /* prompt user
for a state code */
scanf("%s", UserName);

sprintf(query_string, /* create an
SQL query string */
"SELECT username, value
FROM table1
where test = 'grade' and username = '%s'", UserName);

res = PQexec(conn, query_string); /* send the
query */

if (PQresultStatus(res) != PGRES_TUPLES_OK) /* did the
query fail? */
{
fprintf(stderr, "SELECT query failed.\n");
PQclear(res);
PQfinish(conn);
exit(1);
}

printf("\n\n\n");
printf("Username : %s\n\n\n", PQgetvalue(res, 0, 0));
printf("Grade : %s \n\n\n", PQgetvalue(res, 0, 1));

PQclear(res); /* free
result */

PQfinish(conn); /*
disconnect from the database */

return 0;
}

############################################################################
########

hello how can i assign to certain variable the output of PQgetvalue(res, 0,
1)? example i want to do like this ...

a = PQgetvalue(res, 0, 1)
printf("Grade : %s \n\n\n", a );

and then after assigning the value of PQgetvalue(res, 0, 1) to a then ...

b = a \ 2;

but it is possible to divide the output of "a" where it is a var and "b" is
integer?