CS 31 Discussion, Week 2

1. CS 31 Discussion, Week 2 Faisal Alquaddoomi, faisal@cs.ucla.edu Office Hours: BH 2432, MW 4:30-6:30pm,F 12:30-1:30pm (today)

2. What’s been covered so far? • Program Structure • Reading input (cin) and producing output (cout) • Variables • Expressions • Types (int, double, string) • Operators (arithmetic, boolean) • Control Structures (if, for, while, etc.)

3. Program Structure • Program consists of the following in order: • Includes, e.g. “#include <iostream>” • Global statements, e.g. “using namespace std;” • Functions • Most important one now: int main() { } • As a matter of good practice, main() should always end with “return 0;” even if it’s not strictly required

4. Input and Output • Output: cout << “Hello, world!” << endl; • What is endl? • Input: cin >> someVariable; • Depending on the “type” of someVariable, cin will read differently • Why is there a problem with reading a number followed by a string?

5. Variables • Places to put a value • But what’s a “value”? We’ll get to this with expressions and types • The rule of thumb is that a variable must be “declared” before it is used • Declaration: “intmyVariable;” • Use: “myVariable = 32;” • Otherwise, the compiler produces the familiar “undefined identifier” message

6. Expressions • An expression is a “value” mentioned before • Can be as simple as 32, or as complicated as 46+(12/3)*8 • “Resolving” an expression means figuring out what its actual value is when it appears in the program • “intmyVariable = 80*1000;” • When the above appears, 80*1000 is computed and stored into myVariable • Variables can appear in expressions • Resolving the expression uses the value of the variable at the time of resolution

7. Types • The expressions referred to earlier have “types”, which are kinds of values • Sample expressions with their types: • Integer (int): 50+12*3 • Decimal (double): 3.5 + 0.7 • String: “Hello!” • Variables also have types, e.g. “intmyVar;” is an integer • A variable can only store expressions of its type • In an expression, a variable confers its type to the expression • cout and cin are intelligent enough to deal with all the basic types

8. Operators • Two kinds of operators (for now): • Arithmetic (+, -, *, /) • Used for computing mathematical expressions • Boolean (>, <, >=, <=, ==, !=) • Used for performing comparisons • Expressions can be a single value, but are usually composed of many values strung together by operators • (5+3) < 5 • (5+3) and 5 are expressions on their own, but ‘<‘ strings them together into a larger expression

9. Operator Precedence • Just like in math, operators are not simply computed left to right • Ex: 5+3*2, the 3*2 occurs first, then 5+6 • Precedence (lowest to highest): • (+,-),(*,/),(>,<,>=,<=,==,!=) • Operators take “operands” and produce a value • The final output value of the operator depends on the types of its operands • The type of an expression is the value produced by the operator with the highest precedence • (5+3)*0.25 is 2.0, which is a double since the final multiplication had a double as one of its operands

10. Control Structures • Aside from just printing, reading, and storing, we need some way to produce different results for different inputs (beyond just expressions) • If: “if something is true, do this (else do this)” • We also need a way to repeat something an unknown number of times • While and for are used for this • Will discuss these with examples