COMP26120: Algorithms and Imperative
Programming
Lecture 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, in
Eight 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 tide
By 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.”
COMP26120 Lecture C5
3/34
C.A.R.Hoare – 1980 ACM Turing Award Lecture.
”I was eventually persuaded of the need to design
programming notations so as to maximise the number of errors
which cannot be made, or if made, can be reliably detected at
compile time.
Perhaps this would make the text of programs longer. Never
mind!
Wouldn’t you be delighted if your Fairy Godmother offered to
wave her wand over your program to remove all its errors and
only made the condition that you should
write out and key in your whole program three times!”
COMP26120 Lecture C5
5/34
Review
?
COMP26120 Lecture C5
6/34
Lecture Outline
Language Design Philosophy
YAFIYGI
COMP26120 Lecture C5
7/34
Lecture C5: You are here
Language Design Philosophy
YAFIYGI
COMP26120 Lecture C5
Language Design Philosophy
8/34
Design Goals of Java
The Java Language Environment – May 1996
Simple, Object Oriented, and Familiar
Robust and Secure
Architecture Neutral and Portable
High Performance
Interpreted, Threaded, and Dynamic
Extensive compile-time + run-time checking
Strong typing – mainly Static
Eliminate dangerous language features:
automatic coercions, pointers, gotos, . . .
and redundant language features:
typedef, struct, union, . . .
“Sacrifice code speed to get coding speed”
COMP26120 Lecture C5
Language Design Philosophy
10/34
Design Goals of Java
The Java Language Environment – May 1996
Simple, Object Oriented, and Familiar
Robust and Secure
Architecture Neutral and Portable
High Performance
Interpreted, Threaded, and Dynamic
Extensive compile-time + run-time checking
Strong typing – mainly Static
Eliminate dangerous language features:
automatic coercions, pointers, gotos, . . .
and redundant language features:
typedef, struct, union, . . .
“Sacrifice code speed to get coding speed”
COMP26120 Lecture C5
Language Design Philosophy
10/34
Design Goals of Java
The Java Language Environment – May 1996
Simple, Object Oriented, and Familiar
Robust and Secure
Architecture Neutral and Portable
High Performance
Interpreted, Threaded, and Dynamic
Extensive compile-time + run-time checking
Strong typing – mainly Static
Eliminate dangerous language features:
automatic coercions, pointers, gotos, . . .
and redundant language features:
typedef, struct, union, . . .
“Sacrifice code speed to get coding speed”
COMP26120 Lecture C5
Language Design Philosophy
10/34
C
A low-level language for small computers
e.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
COMP26120 Lecture C5
Language Design Philosophy
12/34
C
A low-level language for small computers
e.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
COMP26120 Lecture C5
Language Design Philosophy
12/34
C
A low-level language for small computers
e.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
COMP26120 Lecture C5
Language Design Philosophy
12/34
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.
COMP26120 Lecture C5
Language Design Philosophy
14/34
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.
COMP26120 Lecture C5
Language Design Philosophy
14/34
Lecture C5: You are here
Language Design Philosophy
YAFIYGI
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
COMP26120 Lecture C5
YAFIYGI
17/34
Weak Typing
union oops {int a; float b; void * c;};
(struct person*) malloc (...)
Implicit Coercion; Explicit Cast
Change bits; Change type but not bits
COMP26120 Lecture C5
YAFIYGI
17/34
Weak Typing
union oops {int a; float b; void * c;};
(struct person*) malloc (...)
Implicit Coercion; Explicit Cast
Change bits; Change type but not bits
COMP26120 Lecture C5
YAFIYGI
17/34
to break or not to break
switch(x) {
default:
if (prime(x))
case 2: case 3: case 5: case 7:
process prime(x);
else
case 4: case 6: case 8: case 9: case 10:
process non prime(x);
}
COMP26120 Lecture C5
YAFIYGI
19/34
to break or not to break
switch(x) {
default:
if (prime(x))
case 2: case 3: case 5: case 7:
process prime(x);
else
case 4: case 6: case 8: case 9: case 10:
process non prime(x);
}
COMP26120 Lecture C5
YAFIYGI
19/34
to break or not to break
switch(x) {
default:
if (prime(x))
case 2: case 3: case 5: case 7:
process prime(x);
else
case 4: case 6: case 8: case 9: case 10:
process non prime(x);
}
COMP26120 Lecture C5
YAFIYGI
19/34
to break or not to break
switch(x) {
default:
if (prime(x))
case 2: case 3: case 5: case 7:
process prime(x);
else
case 4: case 6: case 8: case 9: case 10:
process non prime(x);
}
COMP26120 Lecture C5
YAFIYGI
19/34
, is an operator
for (i=0, j=2; i<j; i+=2, j++)
...
a, b= b, a; // swap a and b
COMP26120 Lecture C5
YAFIYGI
21/34
, is an operator
for (i=0, j=2; i<j; i+=2, j++)
...
a, b= b, a; // swap a and b
COMP26120 Lecture C5
YAFIYGI
21/34
= is an operator
e.g. char *s, *t;
while (*s++);
while (*s++ == *t++);
while (*s++ = *t++);
COMP26120 Lecture C5
YAFIYGI
23/34
= is an operator
e.g. char *s, *t;
while (*s++);
while (*s++ == *t++);
while (*s++ = *t++);
COMP26120 Lecture C5
YAFIYGI
23/34
= is an operator
e.g. char *s, *t;
while (*s++);
while (*s++ == *t++);
while (*s++ = *t++);
COMP26120 Lecture C5
YAFIYGI
23/34
boolean?
int min=0, max=100, x;
...
min <= x <= max
COMP26120 Lecture C5
YAFIYGI
25/34
char?
’A’
getchar()
COMP26120 Lecture C5
YAFIYGI
27/34
Operator overloading
(b) - (c);
(b) + (c);
(b) * (c);
(b) & (c);
(b)
COMP26120 Lecture C5
(c);
YAFIYGI
29/34
Operator overloading
(b) - (c);
(b) + (c);
(b) * (c);
(b) & (c);
(b)
COMP26120 Lecture C5
(c);
YAFIYGI
29/34
Operator overloading
(b) - (c);
(b) + (c);
(b) * (c);
(b) & (c);
(b)
COMP26120 Lecture C5
(c);
YAFIYGI
29/34
Operator overloading
(b) - (c);
(b) + (c);
(b) * (c);
(b) & (c);
(b)
COMP26120 Lecture C5
(c);
YAFIYGI
29/34
White space
a+++++b;
char *s[]= {"A", "B", "C" "D", "E",};
COMP26120 Lecture C5
YAFIYGI
31/34
White space
a+++++b;
char *s[]= {"A", "B", "C" "D", "E",};
COMP26120 Lecture C5
YAFIYGI
31/34
Synonyms – “Syntactic Sugar”
a=a+1;
a+=1;
(*a).b;
a[3];
a++;
++a;
a->b;
*(a+3);
*(3+a);
3[a];
for (A; B; C) {
D;
}
A;
while(B) {
D; C;
}
COMP26120 Lecture C5
YAFIYGI
33/34
Synonyms – “Syntactic Sugar”
a=a+1;
a+=1;
(*a).b;
a[3];
a++;
++a;
a->b;
*(a+3);
*(3+a);
3[a];
for (A; B; C) {
D;
}
A;
while(B) {
D; C;
}
COMP26120 Lecture C5
YAFIYGI
33/34
Synonyms – “Syntactic Sugar”
a=a+1;
a+=1;
(*a).b;
a[3];
a++;
++a;
a->b;
*(a+3);
*(3+a);
3[a];
for (A; B; C) {
D;
}
A;
while(B) {
D; C;
}
COMP26120 Lecture C5
YAFIYGI
33/34
Synonyms – “Syntactic Sugar”
a=a+1;
a+=1;
(*a).b;
a[3];
a++;
++a;
a->b;
*(a+3);
*(3+a);
3[a];
for (A; B; C) {
D;
}
A;
while(B) {
D; C;
}
COMP26120 Lecture C5
YAFIYGI
33/34
Lecture Review
Language Design Philosophy
YAFIYGI
COMP26120 Lecture C5
YAFIYGI
34/34