class5.md

Class 5

Array traversal

Traversal by index

int const SIZE = 5;
int array[ SIZE ] = { 1, 2, 3, 4, 5 };

for( int i = 0; i < SIZE; ++i )
	cout << *( array + i ) <<  endl;

The above indexing is written using pointer arithmetic

The only difference is the notation, using * rather than []

Traversal by pointer

int const SIZE = 5;
int array[ SIZE ] = { 1, 2, 3, 4, 5 };

for( int *p = array;
		p < array + SIZE; ++p )
	cout << *p << endl;

1. Walk a pointer across the array elements
2. To get an element, dereference the pointer

C-style strings

Strings in basic C are an array of characters. A sentinel null character marks the end of the string.

The below are equivalent:

char s[ 6 ] = { 'h','e','l','l','o','\0' };

//char s[ ] = "hello";

s is an array of 6 characters, not 5, because there is a null character at the end

A char occupies one byte

Individual characters are encoded in ASCII

sizeof( char ) == 1

char s[ ] = "hello";

const keyword

const T *p;		// T, the object p points to,
			// cannot be changed.
T *const p;		// The pointer p cannot be
			// changed
const T *const p;	// Neither T nor p can be changed

T is the type

Using const keyword, you can tell the compiler that you do not intend ot change either or both:

1. The value of the pointer
2. The value of the object to which the pointer points

More C-strings

const char *p = "hello";

If you declare a pointer to a const char, the compiler puts it somewhere special, and you just get a pointer to it. you're not allowed to change it.

char str[ ] = "hello";
cout << str << endl;

You can print out C-style strings

	C-style strings	C++ strings
Libary header	<cstrings>	<string>
Declaration	char cstr[ ]; char *cstr;	string str;
Length	strlen( cstr );	str.length( );
Copy value	strcpy( cstr1, cstr2 );	str1 = str2;
Indexing	cout << cstr[ i ];	cout << str[ i ];
Concatenate	strcat( cstr1, cstr2 );	str1 += str2
Compare	strcmp( cstr1, cstr2 );	str1 == str2

string to C-style string: char *str = str.c_str( );

C-style string to string: string str = string( cstr );

Comparing strings

C-style strings

There are no built-in string operators. You can compare pointers or what a pointer points at.

To compare a string, you can:

1. Write it yourself
2. Use a library routine, e.g., strcmp(A,B), which returns negative if A < B, 0 if A==B, positive if A > B

Command line arguments

g++ -Wall -Werror -01 -pedantic test.cpp -o test

• g++ is the name of the program to run
• The other "words" are arguments to the g++ program
• The shell (CLI) starts the program and passes arguments

argc	// the number of arguments
argv	// an array of C-style strings

int main( int argc, char **argv ) { ... }

#include <iostream>
using namespace std;

int main( int argc, char **argv, char **envp ) {
	for ( int i = 0; envp[ i ]; i++ )
		cout << envp[ i ] << endl;
}

atoi()

atoi function parses an integer value encoded in a C-style string

Example program

Adds up command line arguments using argv and atoi

#include <iostream>
#include <cstdlib>
using namespace std;

int main( int argc, char **argv ) {
	int sum = 0;
	for ( int i = 1; i < argc; i++ )
		sum += atoi( argv[ i ] );
	cout << sum << endl;
}

Stream input cin and fstream

cin

You can use input redirection to send the contents of a file to cin

string word;
while ( cin >> word )
	cout << "word = " << word << endl;

g++ words.cpp -o words
./words < words.in

sum using cin example

int main( ) {
	int sum = 0;
	string word;
	cout << "Enter numbers to sum" << endl;
	while ( cin >> word && word != "done" )
		sum += stoi( word );
	cout << "sum is " << sum << endl;
}

fstream

string filename = "hello.txt";
ifstream fin;
fin.open( filename );
if ( !fin.is_open() ) {
	cout << "open failed" << endl;
	exit( 1 );
}
string word;
while ( fin >> word )
	cout << "word = " << word << endl;
fin.close( );