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CS 31 Discussion, Week 6

CS 31 Discussion, Week 6. Faisal Alquaddoomi, faisal@cs.ucla.edu Office Hours: BH 2432, W 4:30-6:30pm, F 12:30-1:30pm. Notes and Agenda. Slides are now online: http://cs.ucla.edu/~faisal/ Arrays Recap C-Strings 2D Arrays Memory and the Stack (optional, on board). Arrays Review.

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CS 31 Discussion, Week 6

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  1. CS 31 Discussion, Week 6 Faisal Alquaddoomi, faisal@cs.ucla.edu Office Hours: BH 2432, W 4:30-6:30pm,F 12:30-1:30pm

  2. Notes and Agenda • Slides are now online:http://cs.ucla.edu/~faisal/ • Arrays Recap • C-Strings • 2D Arrays • Memory and the Stack • (optional, on board)

  3. Arrays Review • Arrays are useful for dealing with large numbers of values • Create an array like so, specifying the type, name and size: intmyArray[200]; • You can also statically allocate arrays, with or without setting the size (it’s set automatically if you leave it out): intmyOtherArray[] = { 1, 3, 5, 7, 9, 11 }; • Array slots are numbed 0 to length-1, e.g. intsomeArray[10]; // size 10, slots 0-9

  4. Arrays Review #2 • Arrays can be passed to functions • They’re always passed by reference • Can prevent modification using “const” modifier • Syntax for taking an array as a parameter:void printAll(constint list[], intlen){ for (inti = 0; i < len; i++) {cout << “#” << (i+1);cout << “: “ << list[i] << endl; } } • Don’t go past the end of an array or before the beginning!

  5. C-Strings • A “C-String” is just an array of characters! For instance:char text[10]; // 10 characters • As we’ve mentioned, arrays can be of any type, including chars • Convenient for representing text when the string class wasn’t around • Can be used with cout, can be assigned to strings, etc. (as we’ll describe) • But remember that we have trouble with array sizes • e.g. we have to pass them around all the time and be sure not to run off the end or else…

  6. Nul-Termination • The solution to the array size problem is nul-termination • By convention, a special element that indicates the end of the text in a C-String is a literal 0 • (not the character ‘0’, which is 48) • Referred to in the literature as a ‘nul-terminator’ • the ‘nul’ part because it’s 0, and the ‘terminator’ part because it ends the string • You may also see the 0 represented as ‘\0’, which is an escape code that also has the literal value 0 • Chosen to allow the length of a C-String to be determined by just examining the elements • Unlike your project, we don’t have to pass around the length of C-Strings (at least if they’re well-formed) • Also implies that a 0 cannot occur within the text itself • Also, if the nul-terminator is missing, things will go badly in your life (more on this later)

  7. Creating a C-String • Just like any other array, but with some shortcuts (that we’ll see later) • Creating a C-String with “Hi!” in it, the hard way:char str[4]; // size 4, slots 0-3str[0] = ‘H’;str[1] = ‘i’;str[2] = ‘!’;str[3] = 0; // the nul-terminator

  8. Creating a C-String, Cont’d • Marginally easier way:char str[4] = {‘H’, ‘i’, ‘!’, ‘\0’}; • Even easier:char str[4] = “Hi!”; • The above automatically inserts the nul-terminator for us • which is why the char array is still of length 4 • …although it could have been any size >= 4 • All of the literal strings you’ve seen so far have been C-Strings all along

  9. C-Strings as Parameters and Arguments • Taking a C-String as a parameter: void takesCStr(char str[]) { /* etc */ } • You can use ‘const’ before the ‘char’ in order to prevent it from being modified • Passing a C-String as an argument: char myStr[] = “Test”; takeCStr(myStr); • Largely the same as arrays (since C-strings are arrays)

  10. Operations with C-Strings • Unlike regular strings, you can’t do much with C-Strings without help from some library functions • You can do a few operations given this array:char str[] = “Hi!”; • Print it:cout << str << endl; • Read it:cin >> str; // ‘str’ must be big enough • Get a character out of it, or change a character:char p = str[2]; // p == ‘!’str[0] = ‘P’; // str is now “Pi!” • Aside from the printing/reading, pretty much things you’d do with normal arrays…

  11. Things You Can’t do With C-Strings • Compare two C-strings:char a[] = “Hello!”; char b[] = {‘H’, ‘e’, ‘l’, ‘l’, ‘o’, ‘!’, ‘\0’}; if (a == b) { /* nope */ } • Concatenate two C-Strings:cout << a + b << endl; // nope • Assign one C-String to another:a = b; // doesn’t work • Check the size of a C-String:a.size(); // won’t work, sadly • Thankfully, there are easy ways to do these things…

  12. Library Functions for Strings • #include <cstring> in these key functions:

  13. A Brief Notation Note • You may occasionally see a character array (or any array) taken as a parameter like so:intstrlen(const char *str); • That star means “pointer” in C/C++, but this won’t make any sense if you’re not familiar with memory and addressing • You can simply ignore this and pass an array when you see something that asks for a “char *” (or any other type, e.g. “int *”) for now… • I intend to discuss pointers at the end of this lecture, time permitting and interest withstanding

  14. 2D Arrays • The arrays we discussed previously are technically called ‘1D arrays’ • Have a single ‘dimension’, e.g. myArray[10] has one dimension from 0 to 9 • A 2D array is exactly the same as a 1D array in terms of use, except there are two dimensions • Used for making tables, grids, matrices, etc. • Project 3 used one to hold all the dots that appear on the screen

  15. Creating and Accessing a 2D Array • Similar to a 1D array, creating a 2D array:intmyArray[10][10]; • // ^ has 10*10 == 100 slots • Accessing and setting an element:int p = myArray[3][7]; // row 3, col 7myArray[9][9] = 9000; • // ^ sets the last row, col • By convention, the first dimension is the row and the second is the column

  16. Statically Allocating a 2D Array • Pretty similar to 1D arrays:intmultTable[4][4] = { { 1, 2, 3, 4 }, { 2, 4, 6, 8 }, { 3, 6, 9, 12 }, { 4, 8, 12, 16 } }; // multTable[(3-1)][(2-1)] == 6 • Unlike 1D arrays, we can only leave out the first dimension if we like; the second is required:multTable[][3] = { { 1, 2, 3 } };

  17. 2D Arrays as Params, Arguments • Take a 2D array as an argument like so: void myTwoDeeFunc(char input[][5]) { /* etc*/ } • Note that the second dimension must be specified (like in static allocation of 2D arrays earlier) • There’s a funny reason for this, which you’ll learn if you stick around for memory and the stack (optional) • Passing a 2D array is the same as a 1D array:char stuff[3][5];myTwoDeeFunc(stuff);

  18. Processing an Entire 2D Array • Typically uses an outer ‘for’ to go through the rows (#1)… • …And an inner ‘for’ to go through the columns for each row (#2)intmyArray[10][15];for (inti = 0; i < 10; i++) { // #1 for (int j = 0; j < 15; j++) { // #2myArray[i][j] = 0; // row i, col j }}// at the end, all slots are 0

  19. Recap • C-Strings are just char arrays, with some convenient functions for manipulating them • End in a ‘nul-terminator’ that signifies the end of the string • Leaving off the nul-terminator is disastrous • C-Strings have special manipulation functions in the <cstring> library, e.g. #include <cstring> • Whenever you use “Hello!” in your code, you’re using a C-String • (but thankfully std::string is good at converting from C-Strings to std::strings when needed)

  20. Recap, Part 2 • 2D arrays are like 1D arrays, except they have an extra dimension • by convention, the first dimension is the row, the second is the column • declare:intmyArray[5][10]; // 5 rows, 10 columns • access/change:myArray[2][3] = 888; // ^ item at row 2, col 3 is now 888 • 2D arrays can be statically allocated, but the second dimension (columns) must be specified • Similarly, when writing a function that takes a 2D array as a parameter, the second dimension must also be specified:void myFunc(char inArray[][5]) { /* body of function here… */ }

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