CS 161
Introduction to Programming
and Problem Solving
Chapter
Hello World Program in C++
And Lexical Rules
Herbert G. Mayer, PSU
Status 10/18/2014
Syllabus
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Lexical Rules
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Identifiers
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Keywords
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Literals
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Scopes
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Numeric Limits, from <limits.h>
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Hello World in C and C++
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Some Data
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For Loops in C++
Lexical Rules
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A C program is an ASCII text files
ASCII: American Standard Code for Information
Interchange
This includes white space, such as blanks ‘ ‘, tabs,
carriage return, line-feed, etc.
Meaningful parts consist of letters, digits, and
special symbols, such as ‘+’ or ‘/’
This sequence of (usually) visible characters. Named
the source program, is logically partitioned into
tokens
A token is a complete lexeme in the C language, i.e.
one single part of a C program
Lexical Rules
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Sometimes the C rules for composing lexical
elements, are sufficient to separate one from another
token, e.g. min = 100; consists of 4 tokens:
identifier min, special symbol =, decimal integer
literal 100, and special symbol ;
It could also be written correctly as min=100;
Other times a deliberate separation of one token
from another is needed, done via white space, e.g. a
blank character
A C token must fit onto a single line, with the
exception of string literals, enclosed in a pair of “
Lexical Rules: Identifiers
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Identifiers, AKA user-defined identifiers in C are
composed of letters, digits, and underscore
characters ‘_’
Must begin with a letter or underscore
Are case sensitive; i.e. the 3 legal C identifiers
MaxValue, Maxvalue, maxvalue are all 3 distinct
names
Reserved keywords are symbols that adhere to the
lexical rules of identifiers, but are reserved, with
predefined meanings, and cannot be be used as
identifiers
Lexical Rules: Reserved Keywords
All keywords reserved in the C programming language are also carried over
into C++. There are 32 such inherited reserved words:
auto
break
case
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
return
short
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while
Lexical Rules: Reserved Keywords
There are 30 additional C++ reserved keywords that are not part of C, hence
new in C++. See below:
asm
bool
catch
class
const_cast
delete
dynamic_cast
explicit
false
friend
inline
mutable
namespace
new
operator
private
public
protected
reinterpret_cast
static_cast
template
this
throw
true
try
typeid
typename
using
virtual
wchar_t
Lexical Rules: Literals
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There are octal, decimal and hexadecimal numeric
literals, bool, char, and string literals in C++
Integer numeric literals of base 8, octal numbers, are
introduced via the leading ‘0’ digit, followed by any
number of octal digits ‘0’ .. ‘7’. Example: 0177, same
as 12710; note that overflow can occur, if the
resulting number is greater than the maximum
integral number the machine can handle
Integer numbers may have an optional negative sign,
e.g. -077
Integer decimal literals consist of any number of
decimal digits ‘0’ .. ‘0’, but may not have a leading ‘0’.
Examples: 128, or -255
Lexical Rules: Literals
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Integer numeric literals of base 16, hexadecimal
numbers, are introduced via the leading ‘0x’,
followed by any number of hex digits ‘0’ .. ‘9’, and
‘a’..’f’, or ‘A’..’F’. Here ‘a’ stands for the digit of value
1010 , ‘and ‘f’ stands for the digit of value 1510. Upperand lower-case are equivalent. Example: 0xff, same
0xFF, same as 25510
The boolean literals are true and false
Literals of type char are embedded in a pair of single
quotes, like ‘z’
Some char type literals are not printable, thus they
are encoded via an escape mechanism, introduced
by the \ followed by the desired symbol. E.g. ‘\n’
stands for carriage return, and ‘\t’ for horizontal tab
Lexical Rules: Floating-Point Literals
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Float (AKA floating-point) literals are numeric literals
with a possible fraction. They can have integer and
fractional parts, be signed, and be scaled; e.g.:
3.142592
+3.142592
-3.142592
0.3142592E1
31.42592E-1
//
//
//
//
//
for PI
for PI with redundant sign
negative PI
PI scaled by factor * 10
PI scaled by 10-1
Numeric Limits, from <limits.h>
Lexical Rules: Special Symbols as Operators
L
1
2
Description
Function call
( )
Array subscript
[ ]
Struct member
.
Struct dereference
expr++
Decrement (post)
expr--
Associativity
L
5
left-to-right
6
Description
Operator
Bitwise left shift
<<
Bitwise right shift
>>
Less than
<
Greater than
>
LT or equal to
<=
GT or equal to
>=
Equal to
==
Not equal to
!=
Associativity
left-to-right
left-to-right
Indirection
*
Reference (addr)
&
Unary plus
+
8
Bitwise AND
&
left-to-right
Unary minus
-
9
Bitwise XOR
^
left-to-right
Logical negation
!
10
Bitwise OR
|
left-to-right
11
Logical AND
&&
left-to-right
Bitwise NOT
~
7
right-to-left
left-to-right
Increment (pre)
++expr
12
Logical OR
||
left-to-right
Decrement (pre)
--expr
13
Conditional
? :
right-to-left
14
Assignment
= += -= *= /=
%= >>= <<=
&= ^= |=
right-to-left
15
Comma
,
left-to-right
Size in bytes
4
->
Increment (post)
Cast
3
Operator
( type )
sizeof
Multiplication
*
Division
/
Modulo
%
Addition
+
Subtraction
-
left-to-right
left-to-right
Highest precedence is Level 1.
Lowest precedence is Level 15.
Use parentheses () to alter the order of evaluation.
11
Hello World Program
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We’ll write and discuss a very simple source
programs
Tongue-in-cheek known as the hello world program
We briefly contrast C and C++, then we cover more
advanced C++ for loops
This famous hello world program does nothing
except to print “Hello” on the standard output device
Hello World in C
#include <stdio.h>
void main( )
{ // main
printf( “Hello World!\n” );
// no return value given; should be 0
} // end main
Hello World in C
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#include provides to the programmer a so-called
library: stdio.h
stdio.h makes available standard input and output
functions, for example printf(), without programmers
having to code them
main() identifies a select C function, always to be
provided in a complete C source program
main() is always the first function to be executed
All work to be done by the main() function is
enclosed between the { and } braces, as has to be
done for every other C function
Hello World in C
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#include directive to include libraries
<stdio.h> specifies library to be included
void reserved keyword, specifies type of function
main reserved keyword, naming a special function
( ) are special symbols, enclosing formal parameters
{ } special symbols, enclosing body of function
// is start of line comment
printf user-defined function name from stdio.h
“Hello World!\n” string literal to be printed
; special symbol, ending statement
Hello World in C++
#include <iostream.h>
int main( void )
{ // main
cout << ”Hello World!” << endl; // endl is \n
return 0;
} // end main
Hello World in C++
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C++ also provides the same common input- and
output-functions of C, but typical C++ IO uses a new
IO format, available in iostream.h
main() again identifies the starting point of execution
Like in C, all work to be done by the C++ main()
function is enclosed between the { and } brackets
Instead of a magic symbol ‘\n’ to identify a “carriagereturn and line-feed” to be output, C++ specifies a
symbolic, fictitious character, named: endl
The << operators define elements to be output
If there are no errors, C++ returns error code 0 to
say: All is OK!
Some Data
#include <stdio.h>
#include <iostream.h>
int main( void )
{ // main
int i;
// we don’t use i!!!
int year = 2014;
// initialized year
float pi = 3.141592; // should be constant
printf( ”year = %d\n”, year );
// old C
cout << “year = “ << year << endl; // C++
return 0;
} // end main
Some Data
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The reserved C keyword int defines a datum, by the
name i, and another by the name year
The former is left uninitialized, the latter receives an
initial value, 2014
Both options are allowed in C
Line “float pi” defines a datum of a different type,
called a floating-point value. This type permits
fractional values, but not all possible values in the
numeric range are representable on a computer
The printf() argument enclosed in a pair of “ is
printed, unless it is a % character, in which data and
format interpretation takes place
Some Data
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Interpretation of data to be printed depends on the
characters after the %
Here the %d means: Find another argument after the
“ pair, and print it as an integer value, to base 10; i.e.
a decimal integer number
The special character sequence \n means: also print
a carriage-return line-feed, as we saw with the hello
world
Quick Excursion to C++
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Now you now know something about arrays and while
loops
Arrays are composite types, AKA aggregate types
Loops perform repetitive actions, so are perfectly suited
to process arrays
We also refer to them as iterations
The for loop is the next construct you learn
For Loop in C++
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Formally, the C++ For Loop defines 3 expressions
More practically, the C++ For Loop defines an iteration
variable, which progresses over a range of values
This is called the iteration space
The first expression defines an iteration variable,
initialized to a defined start value of scalar type; may be
integer or any other scalar type
The second expression tests a boolean condition; if true,
the progression continues; practically the iteration
variable is checked, whether it has reached a final value
The third expression defines a change to the iteration
variable, executed at the end of the loop body
For Loop in C++
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The 3 expressions are enclosed in a pair of ( and )
parens, each separated from the next by a ;
Then comes a statement, which is executed iteratively,
based on the iteration space
It is common to refer to the iteration variable inside the
body of the for loop
Here an example:
#define MAX 10
. . .
for ( int i = 0; i < MAX; i++ ) {
list[ i ] = 0; // best not change
i
} //end for
For Loop in C++
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A for loop executes, as long as the boolean expression 2
yields true
When the expression is false, the for loop ends, and the
operation after the for is executed next
Note that the boolean condition could be false at the
start, in which case the for loop would be empty, i.e.
executed 0 times
Simple For Loop in C++
// declaration of some array
#define TEN 10
char digits[ TEN ];
// loop to initialize digits[]
for( int i = 0; i < TEN; i++ ) {) {
digits[ i ] = ‘0’ + i++;
} //end for
// no compound { and } needed, but is a good habit
More Complex For Loop in C++
// declaration of 2-Dim matrix mat[][]
#define MAX_ROW 50
#define MAX_COL 50
float mat[ MAX_ROW ][ MAX_COL };
// initialize matrix mat[][]
for( int row = 0; row < MAX_ROW; row++ ) {
for( int col = 0; col < MAX_COL; col++ ) {
mat[ row ][ col ] = 0.0;
} //end for
} //end for
// no compound needed, but is a good habit