Day 12: Recipes I Dates, Times, Writing Files Suggested Reading: - - PowerPoint PPT Presentation

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Day 12: Recipes I Dates, Times, Writing Files Suggested Reading: - - PowerPoint PPT Presentation

Jan 29, 2024 333 likes •562 views

Computer Sciences 368 Introduction to Perl Day 12: Recipes I Dates, Times, Writing Files Suggested Reading: Perl Cookbook (2nd Ed.) Chapter 3: Dates and Times Chapter 7: File Access (esp. 7.11, 7.19) 2012 Summer Cartwright 1 Computer

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Day 12: Recipes I

Dates, Times, Writing Files Suggested Reading: Perl Cookbook (2nd Ed.) Chapter 3: Dates and Times Chapter 7: File Access (esp. 7.11, 7.19)

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Homework Review

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Homework Preview

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Forecast Sample

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··· <H1>Madison Forecast</H1>\n Local Madison Forecast\n 635 AM CDT THU JUL 26 2012\n <br><br><font size=+1><B>TODAY...</B></font>PARTLY

  • SUNNY. A 20 PERCENT CHANCE OF THUNDERSTORMS IN\n

THE AFTERNOON. HIGHS IN THE UPPER 80S. NORTHWEST WINDS 5 TO\n 10 MPH.\n <br><br><font size=+1><B>TONIGHT...</B> </font>PARTLY CLOUDY. CHANCE OF THUNDERSTORMS THROUGH AROUND\n MIDNIGHT...THEN SLIGHT CHANCE OF THUNDERSTORMS AFTER MIDNIGHT.\n LOWS IN THE MID 60S. NORTHWEST...\n

2012-07-26 06:35 UPPER 80S LOWER 60S

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Dates and Times

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

What Is So Hard About This?

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Dates

  • Different calendars
  • Historical calendar changes
  • Y2K, 2038 January 19

Times

  • Coordinated Universal Time (UTC) vs. time zones
  • Daylight saving time
  • Leap years
  • Leap seconds
  • Indiana (http://en.wikipedia.org/wiki/Time_in_Indiana)
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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Unix/POSIX/Epoch Time Seconds since 1970 January 01 @ 0:00 (UTC)

(more or less) Remaining challenge Unix time <–> Other time formats

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Standard Date/Time Functions

Caution: Read perldoc pages!!!

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localtime

get local YMDHMS from Unix time

gmtime

get UTC YMDHMS from Unix time

Time::Local::timelocal create Unix time from local YMDHMS Time::Local::timegm

create Unix time from UTC YMDHMS

POSIX::mktime

create Unix time from local YMDHMS

POSIX::strftime

format a Unix time

my ($sec, $min, $hour, $mday, $mon, $year, $wday, $yday, $isdst) = localtime(time); my $real_year = 1900 + $year;

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Parsing Dates and Times

  • Use regular expressions

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  • Use CPAN’s Date::Manip or Date::Manip::Date

if (m,(\d{1,2})/(\d{1,2})/(\d{4}),) { my $year = $3 - 1900; my $month = $1 - 1; my $mday = $2; } my $unixtime = timelocal(0, 0, 0, $mday, $month, $year);

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Time Interval Calculations

  • Use Unix time

– Convert to Unix time – Do math in seconds – Convert to meaningful output manually

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  • Use CPAN’s Date::Calc

use POSIX qw/floor/; use Time::Local qw/timelocal/; my $start = timelocal(0, 0, 11, 18, 6, 112); # Calculate interval since course started my $interval = time() - $start; my $minutes = floor($interval / 60); my $seconds = $interval - ($minutes * 60);

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Timing Events

  • Unix times: second-level resolution

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my $start = time(); sleep(rand(10)); my $end = time(); printf "Slept %f seconds\n", $end - $start;

  • Time::HiRes: much higher resolution

use Time::HiRes qw/gettimeofday/; my $start = gettimeofday(); sleep(rand(10)); my $end = gettimeofday(); printf "Slept %f seconds\n", $end - $start;

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Writing Files

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Seriously?

What is so hard about writing a file?

>_<

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Writing Files Robustly

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aka What if the power goes off in the middle of a file write?

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

  • pen(OUT, '>', $filename) or die("…");

print OUT "Header\n"; # Do some long calculation => @things [1000ms] foreach my $thing (@things) { # Format $thing => $display_thing [5ms each] print OUT $display_thing; } close(OUT);

$filename (OLD) $filename (NEW)

  • pen(OUT, '>', $filename) or die("…");

print OUT "Header\n"; # Do some long calculation => @things [1000ms] foreach my $thing (@things) { # Format $thing => $display_thing [5ms each] print OUT $display_thing; } close(OUT);

  • pen(OUT, '>', $filename) or die("…");

print OUT "Header\n"; # Do some long calculation => @things [1000ms] foreach my $thing (@things) { # Format $thing => $display_thing [5ms each] print OUT $display_thing; } close(OUT);

Writing Files: The Problem

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Perl Output Buffers

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  • pen(OUT, '>', $filename) ...

print OUT "first line\n"; sleep(30); foreach my $thing (@things) { sleep(10); print OUT calculate($thing); } close(OUT);

FILE Perl Output Buffer

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Flushing Your … Buffers

  • Set $| (dollar-pipe) to true
  • Affects all output buffers
  • Can significantly affect performance
  • Not a general solution, but sometimes useful

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#!/usr/bin/perl use strict; use warnings; $| = $ARGV[0]; # try 0, then try 1 print "Start of output... "; sleep(2); print "and now we are done!\n";

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Computer Sciences 368 Introduction to Perl

Atomic File Writes

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Key idea: Write to separate file, move into place

sub write_file { my ($filename, $contents) = @_;

  • pen(NEW, '>', "$filename.NEW") or return;

print NEW $contents or return; close(NEW) or return; rename($filename, "$filename.BAK") or return; rename("$filename.NEW", $filename) or return; return 1; }

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Better Temporary Files

Use File::Temp to open file and give name

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use File::Temp qw/tempfile/; my $temp_fh = tempfile(); my ($fh, $temp_filename) = tempfile(); print $fh $contents; close($fh); rename($filename, "$filename.BAK"); rename($temp_filename, $filename);

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Homework

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Forecast Sample

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··· <H1>Madison Forecast</H1>\n Local Madison Forecast\n 635 AM CDT THU JUL 26 2012\n <br><br><font size=+1><B>TODAY...</B></font>PARTLY

  • SUNNY. A 20 PERCENT CHANCE OF THUNDERSTORMS IN\n

THE AFTERNOON. HIGHS IN THE UPPER 80S. NORTHWEST WINDS 5 TO\n 10 MPH.\n <br><br><font size=+1><B>TONIGHT...</B> </font>PARTLY CLOUDY. CHANCE OF THUNDERSTORMS THROUGH AROUND\n MIDNIGHT...THEN SLIGHT CHANCE OF THUNDERSTORMS AFTER MIDNIGHT.\n LOWS IN THE MID 60S. NORTHWEST...\n

2012-07-26 06:35 UPPER 80S LOWER 60S

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Cartwright 2012 Summer

Computer Sciences 368 Introduction to Perl

Weather Analysis, Part I

  • Download current forecast
  • Parse forecast timestamp and convert to Unix time
  • Parse temperature forecasts
  • Save the data

– Use safe file-write pattern with tempfile() – Filename contains date of forecast timestamp – Record forecast timestamp and high/low predictions – If script is run twice in one day, overwrite

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