main()
{

int choice;
printf("enter 1 for addition,2 for subtraction............")
if(choice==1)
{
int sum = add(x,y);
..
.
}
else
if(choice ==2)
{
int diff = sub(x,y);
.
.
}
else
if
{
.
}
-----------------------------------------------------------------
and so on.You can also google "syntax of if-else" for more help.Hope that was helpful