Given a date like 20/2/2002, we like to convert it to: Wed. Feb 20, 2002. The process involves validating the input to ensure it has the proper format and then extracting from it the day, month, and year as strings. Assuming the extracted fields are indeed numeric, we convert them to integers and then verify that the date exists; e.g. it is not the 31st of September or the 29th of February in a non-leap year. Once properly validated, we employ the Zeller algorithm to find the day of the week (dow) and then output the result looking up the month name and the day name.
Here is how the sought system should behave:
Enter the date ...
test
Invalid format!
Enter the date ...
test/
Invalid format!
Enter the date ...
t/mm/yyyy
Non-numeric values!
Enter the date ...
12/45/98
None existent date!
Enter the date ...
10/2/1983
Thu. Feb 10, 1983
It would be a mistake to attempt this using the un-structured approach. This would yield messy code that is hard to debug and understand. Instead, we break the problem into pieces as shown in the following structure chart:
Here is the full main program:
program LongDate
implicit none
character*10 date, day, month, year, readDate
integer*2 d, m, y
logical*1 ok, existDMY
do while (.true.)
ok = .true.
date = readDate()
call checkFormat(date, day, month, year, ok)
if (.not. ok) then
print*, "Invalid format!"
else
call checkValue(day, month, year, d, m, y, ok)
if (.not. ok) then
print*, "Non-numeric values!"
else
if (existDMY(d, m, y)) then
call printDate(d, m, y)
else
print*, "None existent date!"
ok = .false.
end if
end if
end if
if (ok) then
exit
end if
end do
end
The job description of each subprogram is as follows:
readDate()
checkFormat
true
. checkValue
false
. existDMY
false
. isLeap
false
. isDigit
false
else return true
.
s2n
printDate
dow
·
· mm = m
· yy = y
· if (m .le. 2) then
· mm = m + 12
· yy = y - 1
· end if
· tmp1 = 26*(mm+1) / 10
· tmp2 = 125*yy / 100
· tmp3 = d + tmp1 + tmp2 - yy / 100 + yy / 400 - 1
dow = mod(tmp3, 7)
It would also be a mistake if these subprograms are written
in full, put together, and then tested. Instead, write only one of them, any
one, and then write a small main program that uses it. Such a main program is
known as a driver or a test harness and it enables you to test each
piece before building the whole system. Only when all pieces are tested
individually, and only then, would you put them together and test. You can put
everything in one giant source file but it is recommended that those
subprograms that can be useful for other applications (such as s2n
or isLeap
) be
placed in separate files and compiled using f2lib
.
Fall2002/HR