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Welcome to PLAB

Course Staff

Teacher: Nir Friedman

Teaching Assistants: Yoseph Barash Liad Blumrosen Michael Okun

Communications

WWW:http://www.cs.huji.ac.il/~plab

Email: Personal questions should be sent only to plab@cs

Newsgroups: local.course.plab.stud local.course.plab.ta (moderated)

Course Objectives

Procedural programming language (C). Pointers (C/C++) Generic programming (C++ templates) Design patterns (STL, streams, and more) Practice of programming:

Style Testing & Debugging Efficiency & Portability Modularity

Books “The C Programming Language” , 2nd Edition,

Brian W. Kernighan & Dennis M.Ritchie “C++ Programming Language”, 3rd Edition,

Bjarne Strousrtup “The C++ Primer”, Stanley Lippman “C++ for Java Programmers”, Timothy Budd “C++ How to Program”, Harvey Deitel & Paul Deitel “The Practice of Programming”, Brian W. Kernighan

& Rob Pike “Programming Pearls” 2nd Edition, Jon Bentley

Course Grading

6-7 programming exercises Mid-term exam Final exam Final grade 60% exercises and 40% exam.

Shortage in computers - don’t wait for the last minute.

History

C C++ JAVA

70’s – Development of UNIX. (Richie+Kernigham – Bell labs)

80’s – Large efficient code. (Stroustrup – Bell labs)

90’s – Language for the web. (Sun Microsystems)

Simple to convert to machine code.

Advanced Programming

Fast and Efficient Secure and Safe

Welcome to Plab Easy to avoid bugsEasy to Debug

C – Design Decisions

“Bare bones” – the language leaves maximal flexibility with the programmer Efficient code (operating systems) Full control on memory & CPU usage

High-level Type checking High-level constructs

Portable Standard language definition Standard library

C – Warning Signs

No run-time checks Array boundary overruns Illegal pointers

No memory management Programmer has to manage memory

C++ - OO extension of C

Classes & methods OO design of classes

Generic programming Template allow for code reuse

Stricter type system Some run-time checks & memory control

First Program in C

// This is a comment

// This line defines standard I/O library

#include <stdio.h>

// main – name of the main part of the program

int

main()

{ // {…} define a block

printf("Hello class!\n");

return 0;

}

Compiling & Running…

> g++ -o hello hello.c> hello

Hello class!

>

Second Program#include <stdio.h>

int

main()

{

int i; // declares i as an integer

int j = 0; // declares j as an integer, and initializes it to 0

// for( initial ; test condition ; update step )

for( i = 0; i < 10; i++ )

{

j += i; // shorthand for j = j + i

printf("%d %d %d\n", i, j, (i*(i+1))/2); }

return 0;

}

Running…

> g++ -o loop loop.c

> loop

0 0 0

1 1 1

2 3 3

3 6 6

4 10 10

5 15 15

6 21 21

7 28 28

8 36 36

9 45 45

Character Input/Output

#include <stdio.h>

int

main()

{

char c;

while( (c = getchar()) != EOF )

putchar(c);

return 0

}

Print header#include <stdio.h>

#define HEADER 10

int main()

{

int n = 0;

char c;

while( ((c = getchar()) != EOF) && (n < HEADER) )

{

putchar(c);

if( c == '\n' )

n++;

}

return 0;

}

Functions

C allows to define functions

Syntax:int

power( int a, int b )

{

…

return 7;

}

Return type Parameter declaration

Return statement

Procedures

Functions that return void

void

proc( int a, int b )

{

…

return;

}

Return w/o value(optional)

Example – printing powers#include <stdio.h>

int

power( int base, int n )

{

int i, p;

p = 1;

for( i = 0; i < n; i++ )

p = p * base;

return p;

}

int

main()

{

int i;

for( i = 0; i < 10; i++ )

printf("%d %d %d\n",

i,

power(2,i),

power(-3,i) );

return 0;

}

Functions Declaration“Rule 1”: A function “knows” only functions which were defined above it.

void funcA(){ ...}

void funcB(){ funcA();}

void funcC(){ funcB(); funcA(); funcB();}

void funcA(){ ...}

void funcB(){ funcC();}

void funcC(){ funcA();}

Error: funcC is not known yet.

Amendment to “Rule 1” : Use forward declarations.void funcC(int param); // or void funcC(int);

void funcA();{ ...}

void funcB(){ …. funcC(7);}

void funcC(int param){ ….}

Functions Declaration

Powers revisted#include <stdio.h>

// Forward decleration

int power( int m, int n);

int main()

{

int i;

for( i = 0; i < 10;i++ )

printf("%d %d %d\n",

i, power(2,i),

power(-3,i) );

return 0;

}

int

power( int base, int n )

{

int i, p;

p = 1;

while( n > 0 )

{

if( n % 2 )

p *= base;

base *= base;

n /= 2;

}

return p;

}

Program Style

Readability Common Sense Clarity Right focus

What’s in a name

Example#define ONE 1

#define TEN 10

#define TWENTY 20

More reasonable#define INPUT_MODE 1

#define INPUT_BUFSIZE 10

#define OUTPUT_BUFSIZE 20

What’s in a name

Use descriptive names for global variables

int npending = 0; // current length of input queue

Naming conventions vary numPending num_pending NumberOfPendingEvents …

What’s in a name

Comparefor( theElementIndex = 0;

theElementIndex < numberOfElements;

theElementIndex++ )

elementArray[theElementIndex] = theElementIndex;

andfor( i = 0; i < nelems; i++ )

elem[i] = i;

Use short names for locals

What’s in a name

Considerint noOfItemsInQ;

int frontOfTheQueue;

int queueCapacity;

…

The word “queue” appears in 3 different ways

Be Consistent Follow naming guidelines used by your

peers

What’s in a name

Use active name for functions

now = getDate()

Compare if( checkdigit(c) ) …

to if( isdigit(c) ) …

Accurate active names makes bugs apparent

Indentation

Use indentation to show structure

Compare for(n++; n <100; field[n++] = 0);

c = 0; return ‘\n’;

to for( n++; n <100; n++)

field[n] = 0;

c = 0;

return ‘\n’;

Expressions

Use parens to resolve ambiguity

Compareleap_year = y % 4 == 0 && y %100 != 0

|| y % 400 == 0;

toleap_year = ((y % 4 == 0) && (y %100 != 0))

|| (y % 400 == 0);

Statements

Use braces to resolve ambiguity

Compareif( i < 100 )

x = i;

i++;

toif( i < 100 )

{

x = i;

}

i++;

Idioms

Do not try to make code “interesting”

i = 0;

while( i <= n – 1 )

array[i++] = 1;

…

for( i = 0; i < n; )

array[i++] = 1;

…

for( i = n; --i >= 0; )

array[i] = 1;

…

for( i = 0; i < n; i++ )

array[i] = 1;

This is the common “idiom” that any programmer will recognize

Idioms

Use “else if” for multiway decisions

if ( cond1 )

statement1

else if ( cond2 )

statement2

…

else if ( condn )

statementn

else

default-statement

Idiomsif( x > 0 )

if( y > 0 )

if( x+y < 100 )

{

...

} else

printf(“Too large!\n" );

else

printf("y too small!\n");

else

printf("x too small!\n");

if( x <= 0 )

printf("x too small!\n");

else if( y <= 0 )

printf("y too small!\n");

else if( x+y >= 100 )

printf("Sum too large!\n" );

else

{

...

}

Comments

Don’t belabor the obvious// return SUCCESS

return SUCCESS;

// Initialize “total” to “number_received”

total = number_received;

Test: does comment add something that is not evident from the code

Comments

Introduce each function

// random: return a random integer in [0..r]

int random( int r )

{

return (int)floor(rand()*r);

}

CommentsA more elaborate function comment

//

// GammaGreaterThanOne( Alpha )

//

// Generate a gamma random variable when alpha > 1.

//

// Assumption: Alpha > 1

//

// Reference: Ripley, Stochastic Simulation, p.90

// Chang and Feast, Appl.Stat. (28) p.290

//

double GammaGreaterThanOne( double Alpha )

{

…

Comments

Don’t comment bad code – rewrite it!

…

// If result = 0 a match was found so return

// true; otherwise return false;

return !result;

Instead…

return matchfound;

Style recap

Descriptive names Clarity in expressions Straightforward flow Readability of code & comments Consistent conventions & idioms

Why Bother?

Good style: Easy to understand code Smaller & polished Makes errors apparent

Sloppy code bad code Hard to read Broken flow Harder to find errors & correct them