Direct files - Example

------------------------------------------------------------
-- Program to update a file of bank account balances
-- when a withdrawal is made.
--
-- Uses a direct file to contain the balance records.
--
-- Adapted from Skansholm pp 527-8.
--
------------------------------------------------------------

with TEXT_IO, DIRECT_IO; use TEXT_IO, DIRECT_IO;
procedure make_transaction is

    type account_record is record
        number  : POSITIVE;
        balance : FLOAT;
    end record;

    package account_io is new DIRECT_IO (account_record);
    use account_io;

    package int_io is new INTEGER_IO(INTEGER); use int_io;

    accfile : account_io.FILE_TYPE;  -- account records
    acc     : account_record;        -- working storage
    target  : POSITIVE;              -- account no sought
    amount  : FLOAT;                 -- withdrawal amount
    found   : BOOLEAN;               -- right record found?
    first   : account_io.count;      -- for binary chop
    last    : account_io.count;      -- for binary chop
    mid     : account_io.count;      -- for binary chop


begin
    OPEN (accfile, INOUT_FILE, "accounts.dat");

    PUT ("Enter account number : ");
    GET (target); SKIP_LINE;

    -- use binary chop to find the record in the file
    found := false;
    first := 1;
    last  := SIZE (accfile);
    while (not found) and (first <= last) loop
        mid := (first + last) / 2;
        READ (accfile, acc, mid);
        if target = acc.number then
            found := true;
        elsif target < acc.number then
            last := mid - 1;
        else
            first := mid + 1;
        end if;
    end loop;

    if found then
        PUT ("Balance is ");
        PUT (acc.balance, EXP=>0,FORE=>7,AFT=>2); NEW_LINE;
        PUT ("Enter amount to withdraw: ");
        GET (amount); SKIP_LINE;
        if amount < balance then
            acc.balance := acc.balance - amount;
            WRITE (accfile, acc, mid);
            PUT ("Resulting balance = ");
            PUT (acc.balance, EXP=>0,FORE=>7,AFT=>2);
            NEW_LINE;
        else
            PUT_LINE ("Not enough funds available");
        end if;
    else
        PUT_LINE ("No such account found");
    end if;

    CLOSE (accfile);
end make_transaction;