Comparing dates in racket

Question

Are there any built in functions in racket to compare 2 dates?

If not can anyone tell me how to write a function to compare 2 dates in racket. I am very new to functional programming languages please help.

Spectrograph

Take a look at racket/date and srfi/19. If those are insufficient for your needs, take a look at the gregor package. – Surcingle

What code have you tried? What do are you using to represent dates? – Sterculiaceous

@benrudgers Hi actually I am unsure how to proceed so I have no code, I am using strings to represent dates – Spectrograph

Then it seems like you should have or soon have some code for parsing strings as dates. – Sterculiaceous

Answer 1

Built-in for Racket

Racket has a built-in date structure:

(struct date (  second
                minute
                hour
                day
                month
                year
                week-day
                year-day
                dst?
                time-zone-offset)

but not particularly good functions for dealing with dates programmaticly, that is if you want to know the date in five minutes, you have to do all the wrapping of minutes, hours, days, weeks, years, and daylight savings time yourself.

Course Grained Comparison

Comparison can be on dates can be done with eq? or equal? or eqv? just as with any other struct type.

#lang racket

require racket/date)

(define then (current-date))

(define now (current-date))

and used:

> (eq? then now)
#f
> (eq? then then)
#t

This is great if you care about nano-second granularity, not if you care about anything bigger like seeing if two dates are the same day.

Fine Grained Comparison

To compare dates at the level of days you have to write something like:

(define (same-day? date1 date2)
  (and (= (date-day date1)
          (date-day date2))
       (= (date-month date1)
          (date-month date2))
       (= (date-year date1)
          (date-year date2))))

That can be used in:

"scratch.rkt"> (same-day? then now)
#t

In All Seriousness

Working with dates is really hard if it you're doing work that really matters. Libraries like Joda Time exist in languages like Java when getting dates right matters. Don't launch missiles based on your home grown date library.

Sterculiaceous

Answer 2

To check if two objects are the same type and happen to look the same you use equal?. Scheme and Racket (the language) does time differently. Scheme has SRFI-19 while Racket has a date object

Scheme

#!r6rs
(import (rnrs base)
        (srfi :19))

(equal? (make-time time-utc 0 123)
        (make-time time-utc 0 123))
; ==> #t

// perhaps faster equality test (not guaranteed to be faster)
(time=? (make-time time-utc 0 123)
        (make-time time-utc 0 123))

; ==> #t

Racket

#!racket/base

(equal? (seconds->date 123) 
        (seconds->date 123)) 
; ==> #t

Forego

Answer 3

• If you already have absolute seconds, you can simply compare with the usual integer functions like =, <=, and so on.

• If you have date parts like month, day, year, then convert that to seconds so you can do the simple thing above. To do so:

  1. Create a Racket date struct.
  2. Convert it to seconds using date->seconds.

  Or more simply, use find-seconds, which is (roughly) the composition of these.

Untouchability