Lexical Elements, Operators, and the C System

1
Lexical Elements, Operators, and the C System
2
Outline

• Characters and Lexical Elements
• Syntax Rules
• Comments
• Keywords
• Identifiers
• Constants
• String Constants

3
Outline (continued)

• Operators and Punctuations
• Precedence and Association of Operators
• Increment and Decrement Operators
• Assignment Operators
• Example
• The C System (preprocessor and standard library)

4
Characters and Lexical Elements

• Rule of program is called syntax
• The program that check the legality of C code is
  called the compiler
• Compiler collects the characters of the program
  into tokens
• Six tokens keywords, identifiers, constants,
  string constant, operators, and punctuations

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The Compilation Process
C Program
Group characters into token
Translate tokens into target code
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Characters and Lexical Elements

• Basic characters being used in a C program
• These characters are collected by the compiler
  into syntactic units called tokens.

Lowercase a b c z Uppercase A B
C Z Digits 0 1 2
9 Others - / ( )
lt gt ' " !
\ , . / ? Space character
blank space, new-line, tab, etc.
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/ Read in two integers and print their sum.
/ include ltstdio.hgt int main(void) int
a, b, sum printf("Input two integers ")
scanf("dd", a, b) sum a b
printf("d d d\n", a, b, sum) return
0
Comment line using / /
Preprocessor directive which causes stdio.h to be
included, in order to use printf() and scanf()
Result
Input two integers 2 4 2 4 6
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main is identifier, as a function name () is an
operator int is a keyword , , , and , are
punctuations a,b,and sum are identifiers. printf
and scanf are identifiers as function names " is
punctuation Sequence characters in the double
quotes is a string constant is an operator for
memory address and are operators, one for
assignment and another one for arithmetic adding
C compiler ignore white space
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Comments

• Comments are not token. Compiler ignores the
  comments statement
• You can put comment statement in either blank
  line or white space after the C statement, such
  as
• Different comments

area lengthheight /calculate area
of rectangle /
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• / a comment /
• / another comment /
• //
• /
• A comment can be written in this fashion
• to set it off from the surrounding code.
• /
• /
• If you wish, you can
  
• put comments in a box.
• /

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Keywords

• C has less reserved keywords

auto do goto signed unsigned break double if
sizeof void case else int static volatile c
har enum long struct while const extern regi
ster switch continue float return typedef def
ault for short union
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Identifier --- naming

• Identifier is a token
• It composed of a sequence of letters, digits, and
  underscore _
• Identifier is case-sensitive (i.e., age is
  different from Age)

notme 101_south -plus add another
k _id (not advised) kamanidentifiers so_am_i
legal
illegal
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Identifier --- naming

• Be unique identifier
• No keyword can be used as identifier
• Size of identifier depends on systems (at least
  31 in ANSI C).
• Always choose meaningful identifier for naming
  variables, such as tax_rate, price,
• Underscore is used to create a single name, but
  has the meaning of word, such as C_class_student.

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Constants

• Character constant
• Integer constant
• Floating number constant

'a', 'G', '\n', '\t',
43, -54, 3424
132.34, 13.23e2
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String Constants

• string constants are identified by double quote "

" a string of text" "" " " "a bc" "/ this
is not a comment /" "a string with double quotes
\" within" "a single backslash \\ is in this
string" "abd" "efg"
/ "this is not a string" / "and Neither is this
" 'dgashgahg'
illegal
legal
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Operators and Punctuators

• Arithmetic operators ( - / )
• and has four meanings one is as arithmetic
  operator for addition, the second is for changing
  the sign of value, such as

c -a b
The third one is used as increment or decrement
operator such as and The fourth one is used
as additional operator
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Precedence and Associativity of Operators

• Operators have rules of precedence and
  associativity that are used to determine how
  expressions are calculated and what is the order
  of operations
• Example

1 (23)
1 23
(12)3
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Precedence and Associativity of Operators

• Left-to-right rule as associativity rule

12-34-5
(((12)-3)4)-5
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Operator precedence and associativity
Operator associativity () (postfix)
--(postfix) left to right (unary)
(unary) (prefix) (prefix) right to
left / left to right
- left to right - /
etc. Right to left
-ab-c ((-a)b)-c
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Increment and Decrement Operators

• Increment and decrement by 1
• They are unary operators
• They both can be used as both postfix and prefix
• They have higher precedence

i cnt--
int_variable -- int_ variable int_variable i
nt_ variable--
777 (ab-1)
not
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Increment and Decrement Operators

• The difference between postfix and prefix

int a,b,c0 ac bc printf(d d d\n",
a,b,c)
i i ii1
ab-c-- ((a)b)-(c--) 7bd 7-((-b)(d
))
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Assignment Operator

• "" is assignment operator

variableright_side (expr) b2 c3 abc
abc0
a(b(c0))
a(b2) (c3)
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Assignment Operator

• Additional assignment operators

- / gtgt ltlt
variable opexpression
variable variable op expression
jk3 jj(k3)
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Assignment Operator

• More example
• Power of 2 example

ijk i(jk) ii(jk) jkm5 k(k(m5))
jj(k(m5))
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/ Some powers of 2 are printed. / include
ltstdio.hgt int main(void) int i 0, power
1 while (i lt 10) printf("-6d",
power 2) printf("\n") return 0
2 4 8 16 32 64 128 256
512 1024
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C System

• Preprocessor using preprocessor directive
• include define

include ltstdio.hgt incluseltctype.hgt include
"headerfile.h"
Search in other (System) places
Search in the same directory
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C System

• Usually, header file contains function
  definitions
• On UNIX, the header files are located in the
  directory /usr/include
• On Borland C, it may be found in c\bc\include
• Put function prototypes or function registration
  in the header file

int printf(const char format, ) int
scanf(const char format, )
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C System

• Standard library
• C has many useful built-in and useful functions
• The collection of functions is called "library"
• Example of using functions built in the existing
  library

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include ltstdio.hgt include ltstdlib.hgt int
main(void) int i, n printf("\ns\ns",
"Some randomly distributed integers
will be printed.", "How many do you want to
see? ") scanf("d", n) for (i 0 i lt
n i) if (i 10 0)
putchar('\n') printf("7d", rand())
printf("\n\n") return 0
Width of field for print is 7
Needs srand(time(Null))
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Result
16838 5758 10113 17515 31051 5627
23010 7419 16212 4086 2749 12767 9084
12060 32225 17543 25089 21183 25137 25566
26966 4978 20495
If you run the code again, you will get the same
result
16838 5758 10113 17515 31051 5627
23010 7419 16212 4086 2749 12767 9084
12060 32225 17543 25089 21183 25137 25566
26966 4978 20495
Why? We have not generate seed yet.
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C System

• Seed the random-number generator
• Include lttime.hgt
• Add the following line before using rand()
  function

srand(time(NULL)
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include ltstdio.hgt include ltstdlib.hgt include
lttime.hgt int main(void) int i, n,
seed / seedtime(NULL) srand(seed)
alternatively we can use the following line. /
srand(time(NULL))
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printf("\ns\ns", "Some randomly
distributed integers will be printed.",
"How many do you want to see? ") scanf("d",
n) for (i 0 i lt n i) if (i
10 0) putchar('\n')
printf("7d", rand()) printf("\n\n")
return 0
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Execution 1
3615 12261 17611 9250 27999 30399 22387
30095 19430 28749 18292 5300 27375 19186
31060 12893 17075 15879 8091 13940 21398
413 32213
Execution 2
5433 14959 18114 23611 7140 17969 17481
13574 3022 11455 13012 13697 8305 28395
10077 12335 20728 28449 15022 12791 14511
20328 21935