MESSAGE
| DATE | 2010-02-28 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics - Aggregate Data Types
|
Explicit Cast:
For a variety of reasons, one might need to cast data intentionally.
There are two styles to do this, the older C style and the newer C++
standard. First the new style.
The kinds of New Style Casting:
static_cast, dynamic_cast, const_cast, and reinterpret_cast. Syntax for
these casts follows the following conventions:
int int_variable = static_cast(char_variable);
cast_name(variable) in the general form. I'm not going to yet
explain the differences at this point, but will come back to it soon
enough. I will say that the result is to forcefully convert the data
from one type to another, in the case above, from a char to a int.
In the C style, parenthesis are used to make the cast:
char letter;
int var = (int) letter;
casts the value of letter to an int.
Aggregate Data Types:
Most of the action involving your program will involve more than a single
indepent integer, char or float. Groups of data types together creates
most of the useful. C and C++ gives multiple tools for handling these
agregate data types. The key element is the C style array. An array's
syntax is declared, defined and assigned like the elementry data types,
and looks like this, using the square bracket operator:
char mystraing[]; // Declares an array of chars without dimension
char mystring[100]; //Declares an array of chars with 100 chars within
//it
char * mystring[]; //Declares an array of pointers to chars similar to
//the paramenter of main char * argv[];
One can assign and declare your array with a single statement. When
doing so, C and C++ has several syntax tools to help you create many
necessecary subtle data contructions that you need for your programming.
The comments below outlines these examples and behaviors.
char mystring[] = "This is our first string"; //Declares a char array of
//27 chars which is terminated with a null value
char mystring[] = {'a','b','c','d','e'}; //Creates an array of 5 chars.
int matrix[100] = {1.2,3,4,5}; //This creates an array of 100
//integers filling the first 5 locations with 1,2,3,4,5
//and then adds 0's or NULLS to the remaining 95 indexed
//locations
int matrix[100] = {'1'.'2','3','4','5'}; //This creates an array of 100
//integers where the equivilent of the short
//intergers which represent the ascii values for
//the characters '1' and '2' etc, and then fills
//the rest of the array with zeros. It is
//similar to the next statement (but not
//exactly)
char matrix[100] = "12345"; // This example creates a string literal
//"12345" which ends in a null, and then
//pads the rest of the array with nulls. The
//result is the same as above, but via a
//different mechanism because all string
//literals end in null. The above examples
//has implicit promotion from char to integer
//types. This example must be a char type,
//otherwise the the compiler will not accept
//the assignment. Furthmore, only the care
//type will print a string when asked. The
//top example needs an explicit cast. See
//and thry this example for a demonstration.
#include
using namespace std;
int main(int argc, char * argv[]){
unsigned short int matrix[100] = {'1','2','3','4','5'};
char matrix2[100] = "12345";
cout << "First Martix "<< matrix << endl;
cout << "Second Matrix " << matrix2 << endl;
for(int i=0;i<5;i++){
cout << matrix[i] << endl;
}
for(int i=0;i<5;i++){
cout << static_cast(matrix[i]) << endl;
}
return 0;
}
ruben-at-www2:~/cplus> g++ -Wall test.cc -o test.bin
You have mail in /var/mail/ruben
ruben-at-www2:~/cplus>
ruben-at-www2:~/cplus> ./test.bin
First Martix 0xbfc98c3c
Second Matrix 12345
49
50
51
52
53
1
2
3
4
5
ruben-at-www2:~/cplus>
Notice that the second matrix prints a seemingly random number. That
number is actually the memory address that matrix points at. It acts
like a pointer in the context of cout. The for loop itself will be
looked at more closely when we discuss flow control operators.
We can not mix data types in an array. An array is defined by as a
single data type only.
Arrays are indexed starting with zero. You have to know the size of
your arrays, otherwise you can walk past the end of them into the
undefined sections of your memory. Usually this will cause a
segmentation fault, but not always. Arrrays have syntax that allow them
to be converted to pointers. Pointers is the next section, after we
look at arrays, and we wil look closely at pointers and arrays at soon.
Arrays can have two dimensions like this:
float matrix[4][7];
That declares an array of 4 columns of nine rows (c before r),
for example, we can initialize such an array like this:
float matrix[4][7] = {
{ 2.11, 2.22, 2.33, 2.44, 2.55, 2.66, 2.77 },
{ 3.11, 3.33, 3.33, 3.44, 3.55, 3.66, 3.77 },
{ 4.11, 4.44, 4.33, 4.44, 4.55, 4.66, 4.77 },
{ 5.11, 5.55, 5.33, 5.44, 5.55, 5.66, 5.77 }
};
or you can drop in inside curly braces and the compiler will do the
rest..
float matrix[4][7] = {
2.11, 2.22. 2.33, 2.44, 2.55, 2.66, 2.77 ,
3.11, 3.33. 3.33, 3.44, 3.55, 3.66, 3.77,
4.11, 4.44. 4.33, 4.44, 4.55, 4.66, 4.77 ,
5.11, 5.55. 5.33, 5.44, 5.55, 5.66, 5.77
};
Although we stupid humans conceptualize this as columns and rows, in RAM
this is stored as a single linear block of memory.
There are alot of minefields with two dimensional arrays, and this
program shows some of them:
#include
using namespace std;
int main(int argc, char * argv[]){
unsigned short int matrix[100] = {'1','2','3','4','5'};
char matrix2[1000] = "12345";
float dmatrix[4][7] = {
{ 2.11, 2.22, 2.33, 2.44, 2.55, 2.66, 2.77 },
{ 3.11, 3.33, 3.33, 3.44, 3.55, 3.66, 3.77 },
{ 4.11, 4.44, 4.33, 4.44, 4.55, 4.66, 4.77 },
{ 5.11, 5.55, 5.33, 5.44, 5.55, 5.66, 5.77 }
};
float * track;
cout << "First Martix "<< matrix << endl;
cout << "Second Matrix " << matrix2 << endl;
for(int i=0;i<5;i++){
cout << matrix[i] << endl;
}
for(int i=0;i<5;i++){
cout << static_cast(matrix[i]) << endl;
}
for(int i=0;i<100;i++){
cout << &matrix[i] << endl;
}
for(int i=0;i<5;i++){
cout << "STRING " << reinterpret_cast(&matrix2[i]) << endl;
}
track = *dmatrix;
float * last = &dmatrix[3][6];
for(int count = 0; track <= last; track++){
cout << "Position ==>" << count++ << "\tMemory Location==>"< |
|
