I have just started to code in Prolog, and I cannot do what I would like to.
Basically I have 2 input: a start date "2018/02/14" and a duration "99 days/months". I would like to know if there would be a way to predict the end date from this, in PROLOG:
date(X,Y,Z):-is_a_date(X),is_a_duration(Y),...
The main problem is about date format...
Any tips ?
The main problem is about date format...
Prolog uses terms to model data. Terms can be atoms or numbers, or any compound terms obtained by applying functors or operators to other terms. / is an operator. So 2018/02/14 is a perfectly fine Prolog term:
?- Date = 2018/02/14.
Date = 2018/2/14.
This is different from most other programming languages, in which something like the above would be an expression that would be evaluated to a value. For example, entering 2018/2/14 into a Python prompt gives back 72.0714 because it thinks we want it to compute a number. But in Prolog the example above is not asking for a number; it simply says that 2018/02/14 is some data made up of three numbers separated by / signs.
To operate on Prolog data structures, we use unification:
?- Date = 2018/02/14, Date = Year/Month/Day.
Date = 2018/2/14,
Year = 2018,
Month = 2,
Day = 14.
We unified the date with another, similar term Year/Month/Day. Here, the leaves of the term are variables. Unification will bind these variables to the numbers in the date terms at the corresponding positions. That is all!
So (using meaningful variable and predicate names, which are very important in Prolog) you can write your predicate as:
startdate_duration_enddate(Year/Month/Day, Duration, EndDate) :-
... .
Related
I've looked for help on the internet for the following, but I could not find a satisfying answer: for an assignment, I need to plot the time series of a certain variable (the term spread in percentages), with years on the x-axis.
However, we use daily data. Does anybody know a convenient way in which this can be done? The 'date' variable that I've got is formulated in the following way: 20111017 represents the 17th of October 2011.
I tried to extract the first 4 numbers of the variable 'date', by using the substr(date, 1, 4) command, but the message 'type mismatch' popped up. Also, I'm not quite sure if it gives the right information if I only use the years to plot daily data (over the years). It now gives the following graph, which doesn't look that nice.
Answering the question in your title.
The date() function expects a string. If your variable with value 20111017 is in a numeric format you can convert it like this: tostring datenum , gen(datestr).
Then when using the date() function you must provide a mask that tells Stata what format the date string is in. Below is a reproducible example you can run to see how this works.
* Example generated by -dataex-. For more info, type help dataex
clear
input float datenum
20111016
end
* Convert numberic varaible to string
tostring datenum , gen(datestr)
* Convert string to date
gen date = date(datestr, "YMD")
* Display date as date
format date %td
If this does not help you, try to provide a reproducible example.
This adds some details to the helpful answer by #TheIceBear.
As he indicates, one way to get a Stata daily date from your run-together date variable is convert it to a string first. But tostring is just one way to do that and not essential. (I have nothing against tostring, as its original author, but it is better suited to other tasks.)
Here I use daily() not date(): the results are identical, but it's a good idea to use daily(): date() is all too often misunderstood as a generic date function, whereas all it does is produce daily dates (or missings).
To get a numeric year variable, just divide by 10000 and round down. You could convert to a string, extract the first 4 characters, and then convert to numeric, but that's more operations.
clear
set obs 1
gen long date = 20111017
format date %8.0f
gen ddate = daily(strofreal(date, "%8.0f"), "YMD")
format %td ddate
gen year = floor(date/10000)
list
+-----------------------------+
| date ddate year |
|-----------------------------|
1. | 20111017 17oct2011 2011 |
+-----------------------------+
* date is in %td format
gen date1 = real(string(mofd(daily(date, "DMY")), "%tmCYN"))
* type mismatch error
tostring date, gen(dt)
gen date1 = real(string(mofd(daily(dt, "DMY")), "%tmCYN"))
* the code runs but generates no results
tostring date, gen(dt)
gen date2=date(dt, "YMD")
* the code runs but generates no results
If a date variable has a display format %td it must be numeric and stored as some kind of integer. The display format is, and is only, an instruction to Stata on how to display such integers. Confusions about conversion often seem to hinge on a misunderstanding about what format means, as format is an overloaded word in computing, referring variously to file format (as in graphics file format, .png or jpg or whatever); data layout (as in wide or long layout, structure or format); variable or storage type; and (here) display format. There could well be yet other meanings.
A date displayed as 30jan2015 is stored as an integer, namely
. display mdy(1, 30, 2015)
20118
and a glance at help data types shows that your variable date could be stored as an int, float, long or double. All would work, although int is least demanding of memory. You would need (e.g.) to run describe date to find out which type is being used in your case, but nothing to come in this answer depends on knowing that type. Note that finding out what Stata is doing and thinking can be illuminated by running display with simple, single examples.
Your question is ambiguous.
Want to change display format? If you wish merely to see your dates in a display format exemplified by 20150130 then consulting help datetime display formats shows that the display format is as tested here with display, which can be abbreviated all the way down to di
. di %tdCCYYNNDD 20118
20150130
so
format date %tdCCYYNNDD
is what you need. That instructs Stata to change the display format, but the numbers stored remain precisely as they were.
Want such dates as variables held as integers? If you want the dates to be held as integers like 20150130 then you could convert it to string using the display format above, and then to a real value. A minimal sandbox dataset shows this:
. clear
. set obs 1
Number of observations (_N) was 0, now 1.
. gen date = 20118
. gen wanted = real(strofreal(date, "%tdCCYYNNDD"))
. format wanted %8.0f
. l
+------------------+
| date wanted |
|------------------|
1. | 20118 20150130 |
+------------------+
A display format such as %8.0f is needed to see such values directly.
Another method is to generate a large integer directly. You need to be explicit about a suitable storage type and (as just mentioned) need to set an appropriate format, but it can be got to work:
. gen long also = 10000 * year(date) + 100 * month(date) + day(date)
. format also %8.0f
Want such dates as variables held as strings? This is the previous solution, but leave off the real(). The default display format will work fine.
. gen WANTED = strofreal(date, "%tdCCYYNNDD")
. l
+-----------------------------+
| date wanted WANTED |
|-----------------------------|
1. | 20118 20150130 20150130 |
+-----------------------------+
I have not used tostring here but as its original author I have no bias against it. The principles needed here are better illustrated using the underlying function strofreal(). The older name string() will still work.
Turning to your code,
tostring date, gen(dt)
will just put integers like 20118 in string form, so "20118", but there is no way that Stata can understand that alone to be a daily date. You could have run tostring with a format argument, which would have been equivalent to the code above. The advantage of tostring would only be if you had several such variables you wished to convert at once, as tostring would loop over such variables for you.
I can't follow why you thought that conversion to a monthly date or use of a monthly date display format was needed or helpful, as at best you'd lose the information on day of the month. Thus at best Stata can only map a monthly date back to the first day of that month, and at worst a monthly date (here 660) could not be understood as anything you want.
. di mofd(20118)
660
. di %td mofd(20118)
22oct1961
. di %td dofm(mofd(20118))
01jan2015
There is no shortcut to understanding how Stata thinks about dates that doesn't involve reading the needed parts of help datetime and help datetime display formats.
Yet more explanation and examples can be found at https://www.stata-journal.com/article.html?article=dm0067
I want to assign the current year in a YY format to either a macro or data set variable.
I am able to use the automatic macro variables &sysdate or &sysdate9 to get the current date. However, extracting the year in a YY format is proving to be a nightmare. Below are some examples of what I've been trying.
There exists the YEARw. format. But when I try to use it I get errors or weird results. For instance, running
data _null_;
yy = year(input("&sysdate9.", year2.));
put yy=;
run;
produces the error
ERROR 48-59: The informat YEAR was not found or could not be loaded.
If I try to format the variable in the output, I get 1965 instead of the current year. The following
data _null_;
yy = year(input("&sysdate9.", date9.));
put yy= yy year2.;
run;
outputs
yy=2016 65
Please help.
This works to get you the 2-digit year number of the current year:
DATA _NULL_;
YEAR = PUT(TODAY(),YEAR2.);
PUT YEAR;
RUN;
/* Returns: 16 */
To breakdown what I am doing here:
I use TODAY() to get the current date as a DATE type. &SASDATE needs to be converted to a DATE, but also it is the date that the SAS session started. TODAY() is the current date.
PUT allows us to pass in a non-character (numeric/date) value, which is why it is used with TODAY() as opposed to INPUT.
I think it is worth exploring the issues here in more detail.
First, Formats are patterns for converting numeric values to a human readable format. That's what you want to do here: convert a date value to a human readable format, in this case to a year.
Informats, on the other hand, convert human readable information to numeric values. That's not what you're doing here; you have a value already.
Second, put matches with Formats, and input matches with informats, exclusively.
Third, you get close in your last try: but you misuse the year format. Formats are basically value mappings, so they map every possible numeric value in their range (sometimes "all values" is the range, sometimes not) to a display value (string). You need to know what kind of value is expected on the input. YEARw. expects a date value as input, not a year value: meaning input is "number of days from 1/1/1960", mapped to "year". So you cannot take a value you've already mapped to a year value and map it again with that method; it will not make any sense.
Let's look at it:
data _null_;
yy = year(input("&sysdate9.", date9.));
put yy= yy year2.;
run;
yy contains the result of the year function - 2016. Good so far. Now, you need the 2 digit year (16); you can get that through mod function, if you like, or put/substr/input:
data _null_;
yy = input(substr(put(year(input("&sysdate9.", date9.)),4.),3,2),2.);
put yy=;
run;
mod is probably easier though since it's a number. But of course you could've used year:
data _null_;
yy = put(input("&sysdate9.", date9.),year2.);
put yy=;
run;
Now, yy is character, so you could wrap that with input(...,2.) or leave it character depending on your purposes.
Finally - a use note on &sysdate9.. You can easily make this a date without input:
"&sysdate9."d
So:
yy = put("&sysdate9."d,year2.);
That's called a date literal (and "..."dt and "..."t also work for datetime,time). They require things in the standard SAS formats to work properly.
And as pointed out in Nicarus' answer, today() is a bit better than &sysdate9 since it is guaranteed to be today. If you're running this in batch or restart your session daily, this won't matter, but it will if you have a long-running session.
Apply the year function to the date variable
Convert to string
Take last 2 digits
EDIT: change input to PUT
Year = substr(put(year(today()), 4.), 3);
In Pig Latin, is there a built-in function to find the Month End date for a given date ? For example, if the given date is '2015-03-15', the month end date returned should be '2015-03-31' and if given date is '2015-04-15', the month end date should be '2015-04-30'.
This is how you do it:
REGISTER /usr/lib/pig/piggybank.jar;
DEFINE ISOToMonth org.apache.pig.piggybank.evaluation.datetime.truncate.ISOToMonth();
%declare END_OF_MONTH SubtractDuration(AddDuration(ToDate(ISOToMonth('2015-03-15')),'P1M'),'P1D')
A = LOAD 'DummyFileWithOneRow.txt' USING PigStorage(',') AS (f1:chararray, f2:chararray);
result = FOREACH A GENERATE
f1 AS f1,
$END_OF_MONTH AS end_of_month;
DUMP result
The result of this run is:
(1,2015-03-31T00:00:00.000Z).
You can now convert this result to your desired format.
You can do this calculation as part of the foreach on the loaded values.
The ordinary way to do such things, if you do not find that the language in question already has a built-in set of functions to "do such things," is to ... in this case:
Determine the first day of the current month. ("Month/01/Year" This is the only step that you "do by hand.")
Add "one month" to that. (There should be some kind of "DateAdd()" function in your language...)
Finally, using the same function, "subtract one day."
December 15th => December 1st => January 1st (of next year) => December 31st (of this year).
But first, look carefully. "Accountants want to do this sort of thing all the time." There is usually a pretty-good, sometimes very-good, set of functions to do date-manipulation. (And if they're not built-in to the language, there's often a contributed library of "goodies" that someone else already wrote and perfected.)
I am trying to compare two dates.
But the temp always returns true.
Can you explain where i am going wrong
temp = (Format(CDate("27-Aug-09"), "dd-mmm-yy") > Format(CDate("07-Jul-12"), "dd-mmm-yy"))
You're formatting the values according to dd-mmm-yy - which is actually the format they're in to start with. So you're just comparing the strings "27-Aug-09" and "07-Jul-12"... at which point "2" is later than "0", so the comparison finishes really quickly.
I suspect you can just get rid of the Format calls, to compare the dates:
temp = (CDate("27-Aug-09") > CDate("07-Jul-12"))
That's assuming that CDate can handle the input, of course. (I expect that part is fine.)
If you really want to compare strings, you need to convert the dates into a naturally sortable format, e.g. yyyy-mm-dd.