# Determining the dates of days

I'm trying to use WeatherData[] to give me (not sure how) the temperature, humidity, wind-speed, cloud cover... at a certain location.

I only want the information for weekdays (e.g. Monday-Friday) for the last 83 years.

How do I make a list of all these dates in a format (DateList or DateString) that WeatherData[] can use?

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Search for DayOfWeek in the documentation. –  b.gatessucks Jun 12 '12 at 13:28

After testing the performance of my first answer, along with J.M.'s suggestion, I don't think it will be fast enough. Here is another approach: filtering after acquisition.

This gives all "Temperature" data for Chicago in 2011, filtering out all Saturdays and Sundays.

Select[
WeatherData["Chicago", "Temperature", {{2011, 1, 1}, {2011, 12, 31}}],
! MatchQ[DateString[#[[1]], "DayName"], "Saturday" | "Sunday"] &
]


If you only want one point for each day, use: {{2011, 1, 1}, {2011, 12, 31}, "Day"}

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No need for the Calendar package. The built-in DateString[{2011, 1, 1}, {"DayName"}] will do (and is slightly faster too). Note that it returns day names as strings instead of symbols. –  Sjoerd C. de Vries Jun 12 '12 at 21:03
@Sjoerd thanks! Updated. –  Mr.Wizard Jun 12 '12 at 21:27

For variation, here is method that doesn't require the Calendar package and uses a function which returns days of the week ranging from 0 for Sunday to 6 for Saturday:

GaussDay[y_Integer, m_Integer, d_Integer] :=
With[{yDigits = IntegerDigits[y - Boole[m < 3]},
With[{ y1 = FromDigits[yDigits[[1 ;; 2]]],
y2 = FromDigits[yDigits[[3 ;; 4]]]},
Mod[(d + Floor[2.6 ( Mod[m + 9, 12] + 1) - 0.2] + y2 +
Quotient[y2, 4] + Quotient[y1, 4] - 2 y1), 7]]]


And in play:

Select[WeatherData["Chicago",
"Temperature", {{2011, 1, 1}, {2011, 12, 11}}],
MemberQ[Range@5, GaussDay @@ #[[1, 1 ;; 3]]] &]


For comparison, this took about 8.5 seconds to select approx 130,000 measurements when asked for 83 years of data.

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Floor[y2/4] is better expressed as Quotient[y2, 4]. –  Guess who it is. Jun 12 '12 at 15:53
This is a good solution. As a rule of thumb, never use Mathematica's built-in date functions unless you absolutely have to. By my tests, they are two orders of magnitude slower than Visual Basic's comparable functions, and even farther behind compiled languages. –  Michael Stern Jun 12 '12 at 15:53
Also, I prefer Larsen's method myself for day-of-week computations: larsen[{yr_Integer, mo_Integer, da_Integer}] := Module[{y = yr, m = mo, d = da, f, q}, If[m < 3, y--; m += 12]; f = If[y >= 1752 && m >= 9 && d >= 14, Quotient[y, 400] - Quotient[y, 100], 5]; q = d + 2 m + 1 + Quotient[3 (m + 1), 5] + y + Quotient[y, 4] + f; Mod[q, 7] + 1]. No digit twiddling necessary; just integer arithmetic. –  Guess who it is. Jun 12 '12 at 15:59
@J.M. Thank you. –  image_doctor Jun 12 '12 at 16:00
If[m < 3, IntegerDigits[y - 1], IntegerDigits[y]] might be better written as IntegerDigits[y - Boole[m < 3]]. –  Guess who it is. Jun 12 '12 at 16:01

Needs["Calendar"]


Find a Sunday:

DayOfWeek[{1950, 1, 1}] (* Sunday *)


Create a list of offsets from that day for weekdays:

weeks = 10;
offsets = Join @@ Array[Range@5 + 7 # &, weeks, 0];


Generate resolved days from offsets:

weekdays = DaysPlus[{1950, 1, 1}, #]& /@ offsets;


Confirm:

Tally[DayOfWeek /@ weekdays]

{{Monday, 10}, {Tuesday, 10}, {Wednesday, 10}, {Thursday, 10}, {Friday, 10}}

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After which, OP can do something like Outer[WeatherData["Chicago", #1, #2] &, {"Temperature", "Humidity", "WindSpeed", "CloudCoverFraction"}, weekdays, 1]. –  Guess who it is. Jun 12 '12 at 13:43
@J.M. I'll try this, thanks! –  wizlog Jun 12 '12 at 13:44
@J.M. okay; I was leaving something for the OP to figure out. ;-) –  Mr.Wizard Jun 12 '12 at 13:44
@Mr.Wizard I'm sure in the long run I'd appreciate it, but right now... No. (I'm just in a hurry.) –  wizlog Jun 12 '12 at 13:46
Wizard: I figured that use of Outer[]` is not too well-known among beginners, so I figured it should be pointed out... :) –  Guess who it is. Jun 12 '12 at 13:46