CSCA08 Winter 2018 Week 2: Variables & Functions. Marzieh Ahmadzadeh, Brian Harrington University of Toronto Scarborough

Transcription

1 CSCA08 Winter 2018 Week 2: Variables & Functions Marzieh Ahmadzadeh, Brian Harrington University of Toronto Scarborough

2 Administrative Detail Tutorials and practicals start this week Inverted lecture starts this week Practicals schedule now is online Ex0 due this week 2

3 Variables A variable is a name that refers to a value A variable refers to a value by assignment operator. Each variable has a name and a data type. Names are identifiers that starts with a letter or _ and followed by letters and numbers Cannot use keywords Pyhton is case sensitive Built-in data types: int, float, str, etc. type() From Python s view, a variable is a storage location that holds a value. 3

4 Data Type s Operation Each data type has its own operations +, -, *, /, //, **, % for numbers +, * for string Programming languages care about the type even if you don t need to explicitly specify the type Converting data types is called casting str -> int, int->float, etc. 4

5 Expression & Statements Expressions: Anything that can be resolved into a value Python evaluates the expressions. print( 2+ 2) A combination of values, variables, operators, and calls to functions. Expressions can contain subexpressions. Statements An instruction that can be interpreted and executed by Python. e.g. assignment, if, for, while, import etc. 5

6 Functions A named sequence of codes They are used to make the large programs easy to understand and debug. It provides an efficient use of the sequence of codes When you need that sequence more than once when you want to change the code. By using functions, it is possible to write the sequence of codes once and use it infinitely. 6

7 Function Definition The syntax for a function definition: def name ( parameters): statements {return} def is a reserved word for Python. name is a meaningful name that you choose. parameters is a list of 0 or more variables (i.e. Input to the algorithm). statements is a sequence of instructions (i.e The process part of the algorithm). They should be indented. return is used to return a value. (i.e. output part of the algorithm) What if a function does not have a return statement? 7

8 Function Call To use a function, you should call it. varibale = name(parameters) name is the same name that you have chosen in function definition. parameters number is the same as the number of parameters in function definition. The first parameter s value in function call replaces the first parameter in function definition, the second. Formal parameters vs. actual parameters variable is where the function s return value is resolved into. Pyhton sees a function call as an expression that should be resolved into a value. What is the value of the variable if we don t have return statement in the function definition? 8

9 Flow of Control With functions, flow of control is not sequential any more. When a function is called: Control transfers to the function code. The values given in the function call replaces the parameters in function definition. Function code is executed. Return value is resolved into where the function was called. Control transfers back to the calling code. 9

10 Scope Scope of a variable is the code that can refer to that variable. Global Created outside functions Scope is all over the program Cannot be changed inside a function As long as a program is executing, they are alive Local Created inside a function Scope is inside the function itself Can be changed inside a function As long as a function is executing, they are alive What is the scope of functions formal parameters? 10

11 Memory Model To understand what s going on behind the scene If you understand it, you can understand any program by reading/tracing it. You can find logical errors in your code, when no debugger is available. Trace your code and draw memory model at the same time to see what is happening behind the scene. 11