CS 161Introduction to Programming

and Problem Solving

Chapter Hello World Program in C++

And Lexical Rules

Herbert G. Mayer, PSUStatus 10/18/2014

Syllabus Lexical Rules Identifiers Keywords Literals Scopes Numeric Limits, from <limits.h> Hello World in C and C++ Some Data For Loops in C++

Lexical Rules

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

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

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 const double float int short struct unsignedbreak continue else for long signed switch voidcase default enum goto register sizeof typedef volatilechar do extern if return static union 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 dynamic_cast namespace reinterpret_cast trybool explicit new static_cast typeidcatch false operator template typenameclass friend private this usingconst_cast inline public throw virtualdelete mutable protected true wchar_t

Lexical Rules: Literals 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 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. Upper- and 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 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 // for PI+3.142592 // for PI with redundant sign-3.142592 // negative PI 0.3142592E1 // PI scaled by factor * 1031.42592E-1 // PI scaled by 10-1

Numeric Limits, from <limits.h>

12

Lexical Rules: Special Symbols as Operators

L Description Operator Associativity L Description Operator Associativity

1

Function call ( )

left-to-right

5Bitwise left shift <<

left-to-rightArray subscript [ ] Bitwise right shift >>

Struct member .

6

Less than <

left-to-rightStruct dereference -> Greater than >

Increment (post) expr++ LT or equal to <=

Decrement (post) expr-- GT or equal to >=

2

Indirection *

right-to-left

7Equal to ==

left-to-rightReference (addr) & Not equal to !=

Unary plus + 8 Bitwise AND & left-to-right

Unary minus - 9 Bitwise XOR ^ left-to-right

Logical negation ! 10 Bitwise OR | left-to-right

Bitwise NOT ~ 11 Logical AND && left-to-right

Increment (pre) ++expr 12 Logical OR || left-to-right

Decrement (pre) --expr 13 Conditional ? : right-to-left

Cast ( type )14 Assignment

= += -= *= /= %= >>= <<=&= ^= |=

right-to-leftSize in bytes sizeof

3

Multiplication *

left-to-right

15 Comma , left-to-right

Division /Highest precedence is Level 1.Lowest precedence is Level 15.

Use parentheses () to alter the order of evaluation.

Modulo %

4Addition +

left-to-rightSubtraction -

Hello World Program

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

#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

#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++

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 “carriage-

return 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

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

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++

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 arraysWe also refer to them as iterations

The for loop is the next construct you learn

For Loop in C++ 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++ 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++ 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 10char 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 50float 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