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Introduction (Sections 1.1-1.3)

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Title: Introduction (Sections 1.1-1.3)

1
Introduction(Sections 1.1-1.3)

CSCI 431 Programming Languages
Fall 2003

A modification of slides developed by Felix
Hernandez-Campos at UNC Chapel Hill
2
Programming Languages

What is a programming language?

3
Programming Languages

A language used to express instructions to a
computer.
Definition of legal Programs (Syntax)
Meaning of a Program (Semantics)
Style of Programming (Pragmatics)
Example, Consider the following statement
set xi to xi 1
This is clearly intended to denote the increment
of an array element. How would we translate this
statement to a variety of different languages,
and what would it mean?

4
In C (circa 1970), we would write this as

xi xi 1
This performs a hardware lookup for the address
of x and adds i to it. The addition is a hardware
operation, so it is dependent upon the hardware
in question. This resulting address is then
referenced (if it's legal - which it might not
be), 1 is added to the bit-string stored there
(again, as a hardware addition, which can
overflow), and the result is stored back to that
location. However, no attempt has been made to
determine that x is even a vector and that xi
is a number.

5
In Scheme (1975), this would be transcribed as

(vector-set! x i ( (vector-ref x i) 1))
This does all the things the corresponding C
operation does, but in addition it also (a)
checks that the object named x is indeed an
array, (b) makes sure it is within the bounds of
the array, (c) ensures the dereferenced location
contains a number, and (d) performs abstract
arithmetic (so there will be no overflow'').

6
Finally, in Java (circa 1991), one might write

xi xi 1
which looks identical to the C code. However,
the actions performed are those performed by the
Scheme code, with one major difference the
arithmetic is not as abstract. It is defined to
be done as if the machine were a 32-bit machine,
which means we can always determine the result of
an operation, no matter which machine we execute
the program on, but we cannot have our numbers
grow arbitrarily large.

7
Programming Languages

What is a programming language?
Abstraction of virtual machine

8
A simple algorithm for testing primality in Java

public static boolean isprime (int n)
int d
for (d 2 d lt n d)
if (n d 0)
return false
return true

9
In Intel X86 Assembler

.globl isprime
isprime
pushl ebp set up procedure entry
movl esp,ebp
pushl esi pushl ebx
movl 8(ebp),ebx fetch arg n from stack
movl 2,esi set divisor d 2
cmpl ebx,esi compare n,d
jge true jump if d gt n
loop
movl ebx,eax set n into ....
cltd ... dividend register
idivl esi divide by d
testl edx,edx remainder 0?
done
leal -8(ebp),esp clean up and exit
popl ebx
popl esi

10
Machine Code Characteristics

Explicit registers for values and intermediate
results.
Low-level machine instructions to implement
operations.
Control flow based on labels and conditional
branches.
Explicit memory management (e.g., stack
management for procedures).

11
General Characteristics of HLLs

Complex Expressions (Arithmetic, Logical, ...)
Structured Control Operators (Loops,
Conditionals, Cases)
Composite Types (Arrays, Records, etc.)
Type Declarations and Type Checking
Multiple storage classes (global/local/heap)
Procedures/Functions, with private scope, maybe
first-class
Maybe abstract data types, modules, objects, etc.
Maybe high-level control mechanisms (Exceptions,
Back-tracking, etc.)

12
Programming Languages

What is a programming language?
Donald Knuth
Programming is the art of telling another human
being what one wants the computer to do

int sum(int x) int sum 0 n 0
while (n lt x.length) sum xn
return sum
00101010101010 10101011111010 11101010101110 00101
010101010 ...
13
The Number of Programming Languages