comp26120: algorithms and imperative...

COMP26120: Algorithms and ImperativeProgrammingLecture C5: C - You Asked For It, You Got It

Pete Jinks

School of Computer Science, University of Manchester

Autumn 2010

COMP26120 Lecture C5 1/34

Lewis Carroll – The Hunting of the Snark (an Agony, inEight Fits)

Fit the First – The Landing.

”Just the place for a Snark!” the Bellman cried,As he landed his crew with care;Supporting each man on the top of the tideBy a finger entwined in his hair.

”Just the place for a Snark! I have said it twice:That alone should encourage the crew.Just the place for a Snark! I have said it thrice:What I tell you three times is true.”

C.A.R.Hoare – 1980 ACM Turing Award Lecture.

”I was eventually persuaded of the need to designprogramming notations so as to maximise the number of errorswhich cannot be made, or if made, can be reliably detected atcompile time.

Perhaps this would make the text of programs longer. Nevermind!

Wouldn’t you be delighted if your Fairy Godmother offered towave her wand over your program to remove all its errors andonly made the condition that you shouldwrite out and key in your whole program three times!”

Review

Lecture Outline

Language Design Philosophy

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Lecture C5: You are here

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COMP26120 Lecture C5 Language Design Philosophy 8/34

Design Goals of Java

The Java Language Environment – May 1996

Simple, Object Oriented, and FamiliarRobust and SecureArchitecture Neutral and PortableHigh PerformanceInterpreted, Threaded, and Dynamic

Extensive compile-time + run-time checkingStrong typing – mainly StaticEliminate dangerous language features:

automatic coercions, pointers, gotos, . . .and redundant language features:

typedef, struct, union, . . .

“Sacrifice code speed to get coding speed”

A low-level language for small computerse.g. operating systems, embedded systems

“Don’t hide the hardware”

Speed, Efficiency >> Safety

A small language

The programmer is knowledgeable; trust the programmer.

Strengths: Efficiency, Power, Flexibility, Standard Library, Unix

Weaknesses: Error-prone, Difficult to understand

A small language

C Portability

Unportable:– undefined (incorrect and no default behaviour)

e.g. int overflow– implementation-defined (correct but compiler-dependant)

e.g. int >>– unspecified (correct but no default behaviour)

e.g. argument evaluation order

Portable:– conforming (always get sensible answers)

e.g. limits.h defines INT MAX etc.– strictly-conforming (always get same answers)

e.g. stdint.h defines int32 t INT32 MAX etc.

C Portability

Unportable:– undefined (incorrect and no default behaviour)

e.g. int overflow– implementation-defined (correct but compiler-dependant)

e.g. int >>– unspecified (correct but no default behaviour)

e.g. argument evaluation order

Portable:– conforming (always get sensible answers)

e.g. limits.h defines INT MAX etc.– strictly-conforming (always get same answers)

e.g. stdint.h defines int32 t INT32 MAX etc.

Lecture C5: You are here

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COMP26120 Lecture C5 YAFIYGI 15/34

Weak Typing

union oops {int a; float b; void * c;};

(struct person*) malloc (...)

Implicit Coercion; Explicit Cast

Change bits; Change type but not bits

Weak Typing

to break or not to break

switch(x) {default:if (prime(x))

case 2: case 3: case 5: case 7:process prime(x);

elsecase 4: case 6: case 8: case 9: case 10:

process non prime(x);}

, is an operator

for (i=0, j=2; i<j; i+=2, j++)...

a, b= b, a; // swap a and b

, is an operator

for (i=0, j=2; i<j; i+=2, j++)...

a, b= b, a; // swap a and b

= is an operator

e.g. char *s, *t;

while (*s++);

while (*s++ == *t++);

while (*s++ = *t++);

= is an operator

e.g. char *s, *t;

while (*s++);

while (*s++ == *t++);

while (*s++ = *t++);

= is an operator

e.g. char *s, *t;

while (*s++);

while (*s++ == *t++);

while (*s++ = *t++);

boolean?

int min=0, max=100, x;. . .min <= x <= max

’A’

getchar()

Operator overloading

(b) - (c);

(b) + (c);

(b) * (c);

(b) & (c);

(b) (c);

(b) - (c);

(b) + (c);

(b) * (c);

(b) & (c);

(b) (c);

(b) - (c);

(b) + (c);

(b) * (c);

(b) & (c);

(b) (c);

(b) - (c);

(b) + (c);

(b) * (c);

(b) & (c);

(b) (c);

White space

a+++++b;

char *s[]= {"A", "B", "C" "D", "E",};

White space

a+++++b;

char *s[]= {"A", "B", "C" "D", "E",};

Synonyms – “Syntactic Sugar”

a=a+1; a+=1; a++; ++a;

(*a).b; a->b;

a[3]; *(a+3); *(3+a); 3[a];

for (A; B; C) {D;

A;while(B) {D; C;

a=a+1; a+=1; a++; ++a;

(*a).b; a->b;

a[3]; *(a+3); *(3+a); 3[a];

for (A; B; C) {D;

A;while(B) {D; C;

a=a+1; a+=1; a++; ++a;

(*a).b; a->b;

a[3]; *(a+3); *(3+a); 3[a];

for (A; B; C) {D;

A;while(B) {D; C;

a=a+1; a+=1; a++; ++a;

(*a).b; a->b;

a[3]; *(a+3); *(3+a); 3[a];

for (A; B; C) {D;

A;while(B) {D; C;

Lecture Review

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