Lecture 01c: C++ review Topics: static arrays array / pointer connection dynamic arrays (and pointers) 2D arrays

Overview Arrays are a group of contiguous, homogeneous data. – We can get around both restrictions with [our own] data structures Two types: – statically allocated (the "easier" kind): we know at compile time how many elements will be in the array. – dynamically allocated: we don't Example: reading an image from disk => memory

PartI: Static arrays double enemyDistances[6]; // Use const int or macros? Enemy enemies[6]; // A user-defined class enemyDistances[0] = 15.7; enemyDistances[1] = enemyDistance[2] * 2; cout << enemyDistances[3] << endl;// output? cout << enemyDistances << endl; // output?

Scope We'll talk more about this after functions… Basically scope defines: – where a particular name is valid – when that name "dies" C/C++ use block scope

Scope, cont. All "normal" variables are "destroyed" when control leaves their scope. – This includes statically allocated arrays. int main() { int x; float sizes[4]; cin >> x; if (x > 10) { double x; // Note: "name-masking" // … } }

Part II: Arrays and pointers [Look at the memory diagram for last example]. Now add… double enemyDistances[6]; // Use const int or macros? double d; double * p; enemyDistances[0] = 15.7; p = &d;*p = 17.6; p = &enemyDistances[5];*p = 9.1; p = enemyDistances;*p = 3.3;// Error? cout << p << endl; cout << sizeof(double) << endl; cout << p + 2 << endl; *(p + 2) = 11.2; p[2] = 11.2;// Exactly the same!

Part III: Dynamic allocation Some differences – Memory allocation is from the heap; "normal" allocation (static) is from the stack. – The memory stays allocated until we free it No automatic cleanup when we leave the scope. OK…the compiler / OS probably clean up after the program ends, but… – It must be done through pointers. Note: there are two (slightly different) syntaxes: – array allocation – single allocation

Example fstream fp; int width, height; char * data = NULL; fp.open("my_img.jpg", ios::in | ios::binary); fp.seekg(8, ios_base::beg); // I'm fudging jpeg format... fp.read((char*)&width, 4); // Note the (C-style) cast fp.read((char*)&height, 4); fp.seekg(4, ios_base::cur); data = new char[width * height]; fp.read(data, width * height); // No cast needed fp.close(); //... Use data... // De-allocate data if (data) // Note: NULL = 0 = false delete [] data;

Dynamic allocation (of singular types) Note: you can allocate a single item on the heap – not super useful for "standard" types – Very useful for objects The syntax is a bit different double * single_double = NULL; single_double = new double;// No brackets *single_double = 9.3; cout << *single_double << endl; delete single_double;// No brackets

Part IV: 2D arrays A "normal" array (dynamic and static) are like a 1-dimensional array. You can also represent 2 (or higher) dimensional data Two options: – Create a "flat" array and convert row/col numbers to an offset. – Let the compiler generate these offsets for you.

Examples float static_arr[2][3] = {{4.0f, -6.3f, 2.1f}, {1.9f, 0.0f, -5.8f}}; for (i=0; i<2; i++) { for (j=0; j<3; j++) cout << static_arr[i][j] << " "; cout << endl; } Static array, declared, then initialized float static_arr2[2][3]; static_arr2[0][0] = 4.0f; static_arr2[0][1] = -6.3f; static_arr2[0][2] = 2.1f; static_arr2[1][0] = 1.9f; static_arr2[1][1] = 0.0f; static_arr2[1][2] = -5.8f;

Examples, cont. dynamically allocated [Show memory diagram] float ** dynamic_array; dynamic_array = new float*[2]; dynamic_array[0] = new float[3]; dynamic_array[1] = new float[3]; dynamic_array[0][0] = 4.0f; dynamic_array[0][1] = -6.3f; dynamic_array[0][2] = 2.1f; dynamic_array[1][0] = 1.9f; dynamic_array[1][1] = 0.0f; dynamic_array[1][2] = -5.8f; // Use the same way as the static array before delete [] dynamic_array[0]; delete [] dynamic_array[1]; delete [] dynamic_array;

Examples, cont. Dynamically allocated "flat" array float * dynamic_flat = NULL; int i; dynamic_flat = new float[2 * 3]; dynamic_flat[0 * 3 + 0] = 4.0f; dynamic_flat[0 * 3 + 1] = -6.3f; dynamic_flat[0 * 3 + 2] = 2.1f; dynamic_flat[1 * 3 + 0] = 1.9f; dynamic_flat[1 * 3 + 1] = 0.0f; dynamic_flat[1 * 3 + 2] = -5.8f; for (i=0; i < 3 * 2; i++) { cout << dyamic_flat[i]; if (i > 0 && i % 3 == 0)cout << endl; elsecout << " "; } delete [] dynamic_flat;