MESSAGE
| DATE | 2010-03-04 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics
|
>
> 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==>"< |
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