CS 31 Discussion, Week 5

1. CS 31 Discussion, Week 5 Faisal Alquaddoomi, faisal@cs.ucla.edu Office Hours: BH 2432, MW 4:30-6:30pm,F 12:00-1:00pm (today)

2. Reflections on Project 3 • Get started early • Develop incrementally and with tests • write a test case for a function before you’ve implemented it so that you know when it works • Functions will save you time and effort • “rule of three”: if you repeat the same large block of code more than twice, it should probably be in a function • Passing by reference is useful

3. Common Errors in Project 3 • Misunderstandings about how functions are defined • Functions cannot be nested inside of other functions • Functions that are not main() must be called from main() or they won’t run at all • Trouble with loops • For loops are good for stepping over items one at a time, not so great for stepping over an unknown number of items • Use while loops when you don’t know how far you’re going to be stepping forward • Strings • Before you attempt to access a character in a string, you must be sure that the character exists • if (k < text.size()) /* do something with text[k]… */

4. How do we store lots of data? • So far we’ve stored data in variables, either as input from the user, hardcoded values, etc. • int x = 32; // ‘x’ is hardcoded as 32 • One variable, one value • Today we’re going to learn how to store lots of data in a structure called an array

5. The Problem • Let’s say you want to store ten values • One variable, one value so far… • Could create variables int x1; int x2; int x3; etc. • This is clearly cumbersome, but it’s the best we can do so far • We also can’t iterate over these; we’d have to write an expression to deal with them one at a time • You could also stick them in a string • but then they’d be text and you’d have to extract them when you want to use them (also burdensome)

6. The Solution • An array is a collection of variables that all have the same type and are declared at the same time • Declaring one looks like this: intvals[10]; • You can set and read from the array like any other variable • e.g. vals[0] = 12; • We’ll go into more depth on that in a bit…

7. Declaring Arrays • intvals[10]; • The above declares an array of typeint and of size10 • Arrays can be any type we’ve learned so far • ints, doubles, strings, booleans… • All of the entries in the array are of the same type • The number in the brackets is the number of slots and must be a positive integer • That is, > 0 and a whole number

8. Using Arrays • You can put a value into an array (after it’s been declared) like so:vals[3] = 12; • The number in the brackets is the slot into which you’re storing • The [3] is called the subscript operator; it’s used to set and retrieve values from an array • Like strings, the slot numbers start at 0 and go up to length-1 • In the above, we’re storing to slot #4 out of 10 • It’s illegal to access a slot number past the maximum (your program will crash) • You can also get a value out of an array like so: cout << vals[3] << endl;

9. Types and Declaration Variations • Static allocation: intmyArray[5] = { 5, 3, 6, 7, 9 }; • Length can also be implied: intmyArray[] = { 8, 8, 8, 9 }; • Arrays of strings: string cities[] = { “Glendale”, “Pasadena”, “West Covina” }; • As usual, you can have newlines anywhere you like

10. Array Pitfalls • There is no “size()” function for arrays • You just have to remember how large they are • Or better yet, use a constant that you can refer to later • You can’t print an entire array using coutcout << myArray << endl; // this won’t work • Again, attempting to access a slot past the end of the array is the most common (and most disastrous) mistake people make with arrays

11. Arrays and Functions • Just like any other variable, you can declare arrays as parameters to functions • Which implies that you can pass them as arguments • Unlike other parameters, arrays are always passed by reference • Changes to the array that you pass in the function will be reflected in the caller’s array • If you want to avoid this, use the “const” modifier • Let’s look at an example…

12. Summary • Arrays are useful for dealing with large numbers of values • Arrays can be created like so; size must be > 0 and an integer: intmyArray[200]; • Arrays can be set to default values: intmyOtherArray[] = { 1, 3, 5, 7, 9, 11 }; • Array slots are set and retrieved via the subscript (square brackets) operator: • Set: myArray[3] = 12; • Retrieve: for (inti = 0; i < 200; i++)cout << myArray[i] << endl;

13. Summary #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!