Programming Languages - PowerPoint Presentation

Slide1

Programming Languages

Meeting 12

November 18/19, 2014Slide2

Short Exam Results

Computer programming requires precise notation:

Missing ;

Wrong case

Mismatched parentheses

There were many sloppy answers to questions. The reprise on the next slides gives you the opportunity to be precise.Slide3

Short Exam Reprise

Submit by Friday, November 20, 2014, no later than 5:00 p.m. a text file attached to an email message containing the implementation in Lisp using our development rules of the functions:

reverse

concat

IGSlide4

Short Exam AnswersSlide5

GetFloat Project

Code checked

Comments

getc

implemented

getfloat

implemented

input file specification

Output checked

File of correct floats

File of floats with line numbers

File of floats mingled with character strings

File with no floats, but some look temptingly goodSlide6

Matrix Project

Helpfulness of

LispWorks

Bug when functions are redefined

Solution: Define

myfirst

,

mylast

Primitives

and

functionals

submitted?

Questions?Slide7

Preparations for Tonight

Unix cluster access?Slide8

Scripting Languages

Originally, tools for

Quick, dirty programs

Rapid prototyping for text-based computation

Glue between other programs

File format conversion

Evolved to mainstream programming tools

Check current estimates on the number of lines of

Javascript

code doing productive workSlide9

Characteristics

Strings as the basic, maybe only, data type

Associative arrays (hashes?) as the basic structured data type

Regular expressions as a principle programming structure

Minimal use of types and declarations

Usually interpreted rather than compiledSlide10

Examples

Unix shell

AWK

Perl

Python

Tcl

Javascript

VBScript, Jscript

PHPSlide11

Our Approach

Look at AWK and Perl first

Determine the syntactical structure of each language, starting with AWK

Use experimentation to discover the semantics

Work several exercises on each language during class timeSlide12

AWK

Age: 38 years

Developed at Bell Labs by

Al

Aho

, Brian Kernighan, Peter Weinberger

Aside: Where are each of these computing pioneers now?

Intended for simple manipulation of, and data extraction, from text files

38 years ago data existed in text files almost exclusivelySlide13

AWK Examples

Print all lines longer than 80 characters.

length > 80

Replace the second field in each line by its logarithm.

{ $2 = log($2); print }

Add up the numbers in the first field and report the sum and average.

{ sum += $1 }

END { print sum, sum/NR }Slide14

AWK Syntax

<program> ::= [<

begstate

>] <

stateseq

> [<

endstate

>]

<

stateseq

> ::= <statement> {<statement>}

<statement> ::= <pattern> |

{

<action>

}

|

<pattern>

{

<action>

}

<

begstate

> ::=

BEGIN {

<action>

}

<

endstate

> ::=

END {

<action>

}Slide15

Heart of AWK

<pattern>

A regular expression

A numeric expression

A combination of the previous two

<action>

Executable code, similar in structure to CSlide16

Inferred Control Structure

Previous languages: sequence

Execute S1, then S2, then S3, …

AWK: a triply nested for-loop surrounding an if-then. Specifically

for each file

for each input line

for each pattern

if pattern matches input line then

actionSlide17

Program Development

Use your favorite text editor to create an AWK program file.

Run the program

awk

–f

myprog

[file1 file2 … ]

OR

for really short programs (one line)

awk

‘program’ [file1 file2 …]Slide18

Some Interesting Data

From the course website, under Resources, download the four data files

Flight Aware data

Moby Dick extract

Web form data

Classic AWK data on countries (these data come from the 1985 AWK manual)Slide19

Some AWK Programs

Try each of these programs using

flightaware.txt

as the input file

{ print NR, $0 }

{ $1 = NR; print }

Note that <action> is surrounded by { } and may consist of several statements

separated

by ;Slide20

More Programs

{print $NF}

/Cancelled/

END {print NR}Slide21

AWK Built-Ins

NF – number of fields on a line

NR – number of lines (records) in a file, actually the number of the current line read in the file

$k – name of the

k

th

field, count starts at

1

$0 – name of the current line

FILENAME – name of current input fileSlide22

More Built-Ins

FS – field separator, typically reset in the BEGIN action if something other than space is needed

OFS – output field separator

RS – record separator

ORS – output record separator

OFMT – output format Slide23

Your Turn

Write AWK programs to:

Determine how many flights were cancelled

Find the latest arriving flight from those in the list

List the different aircraft used for this flight number.