Programming in Python Part I

Dr. Fatma Cemile Serçe

Atılım University2009-2010

The Python Programming Language

High-level language (like C, C++, Perl, Java, etc.)

Interpreted language Python programs executed by an

interpreter Two ways to use the interpreter

command-line mode script mode

The Python Programming Language

In Command-line Mode type Python programs and the interpreter prints the

result

The first line of this example is the command that starts the Python interpreter.

The next two lines are messages from the interpreter The third line starts with >>>, prompt the interpreter uses

to indicate it is ready Type print 1+1 program, and interpreter replied 2

The Python Programming Language In Script Mode

write a program in a file and use the interpreter to execute the contents of the file. The file is called a script

file name ends with “.py”

Ex: use text editor to create a file named “hello.py”.

Then tell interpreter the name of the script$ python hello.py

First Program: “Hello World”

in the Pythonprint "Hello, World!"

in the C#include <stdio.h> int main(void) { printf("hello, world\n"); return 0;

} in the C++#include <iostream.h>void main(){ cout << "Hello, world." << endl;}

in the Javapublic class HelloWorld{ public static void main(String args[]){

System.out.println(“Hello, World!”); }}

Values and Types A value is one of the fundamental things

—like a letter or a number—that a program manipulates Ex:

2 “Hello, World!”

These values are belongs to different types 2: integer “Hello, World!”: string

Values and Types

If you are not sure what type a value has, the interpreter can tell you:

>>> type(’Hello, World!’)

<type ’str’>>>> type(17)

<type ’int’>>>> type(3.2)

<type ’float’> str->String, int->Integer, float->floating-point

Exercise 1

What about values like ’17’ and ’3.2’?

>>> type(’17’)<type ’str’>>>> type(’3.2’)<type ’str’>

They’re strings.

Exercise 2 What is the output?

>>> print 1,000,000

1.000.000 ? NO1 0 0 ? YES

a semantic error: the code runs without producing an error message, but it doesn’t do the “right” thing.

Variables A variable is a name that refers to a

value The assignment statement creates

new variables and gives them values>>> message = ’Hello, World!’>>> n = 17>>> pi = 3.14159

Variables (cont.) The print statement also works with

variables>>> print messageHello, World!>>> print n17>>> print pi3.14159

Variables (cont.) Variables also have types again, we can

ask the interpreter what they are>>> type(message)<type ’str’>>>> type(n)<type ’int’>>>> type(pi)<type ’float’>

The type of a variable is the type of the value it refers to.

Variable names and keywords choose meaningful names both letters and numbers, but begin

with a letter Message and message are different (use

lowercase by convention) use underscore character (_) in names

with multiple words person_name

Variable names and keywords If you give a variable an illegal name, you get a

syntax error:>>> 76tables = ‘seventy six tables’SyntaxError: invalid syntax>>> more$ = 1000000SyntaxError: invalid syntax>>> class = ’COMPE 111’SyntaxError: invalid syntax

76trombones is illegal because it does not begin with a letter.

more$ is illegal because it contains an illegal character, the dollar sign

But what’s wrong with class? It turns out that class is one of the Python keywords.

Variable names and keywords Keywords define the language’s rules

and structure Keywords cannot be used as variable

names Python has twenty-nine keywords:

Statements A statement is an instruction that the Python

interpreter can execute print and assignment

The result of a print statement is a value. Assignment statements don’t produce a result. A script usually contains a sequence of statements. Ex: the script

print 1x = 2print x

produces the output12

Operators and Operands Operators are special symbols that

represent computations like addition and multiplication

The values the operator uses are called operands

20+32 hour-1 hour*60+minute minute/60 5**2 (5+9)*(15-7)

The symbols +, -, and /, and the use of parenthesis for grouping, mean in Pythonwhat they mean in mathematics

The asterisk (*) is the symbol for multiplication

** is the symbol for exponentiation

% modulo

Operators and Operands(cont.) When a variable name appears in the place

of an operand, it is replaced with its value before the operation is performed

Addition, subtraction, multiplication, and exponentiation all do what you expect,

but division!>>> minute = 59>>> minute/600>>> float(minute)/600.938888

The value of minute is 59, and in conventional arithmetic 59 divided by 60 is0.98333, not 0. The reason for the discrepancy is that Python is performing integer division.

use floating-point division

Order of Operations

When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence.

Python follows the same precedence rules for its mathematical operators that mathematics does.

The acronym PEMDAS is a useful way to remember the order of operations:

Order of Operations PEMDAS

Parentheses have the highest precedence 2 * (3-1) is 4, and (1+1)**(5-2) is 8

Exponentiation has the next highest precedence, 2**1+1 is 3 and not 4, and 3*1**3 is 3 and not 27

Multiplication and Division have the same precedence, which is higher than Addition and Subtraction

2*3-1yields 5 rather than 4, and 2/3-1 is -1, not 1 Operators with the same precedence are

evaluated from left to right. 6*100/60 yields 10

Operations on Strings In general, you cannot perform mathematical operations

on strings, even if the strings look like numbers The following are illegal:

message-1 ’Hello’/123 message*’Hello’ ’15’+2

+ operator work with strings. It does concatenation, means joining the two operands by linking them end-to-end

fruit = ’banana’bakedGood = ’ nut bread’print fruit + bakedGood

Output: banana nut bread * operator also works on strings; it performs repetition.

’Fun’*3 is ’FunFunFun’

Warning!

There are limits on where you can use certain expressions.

For example, the left-hand side of an assignment statement has to be a variable name, not an expression.

The following is illegal: minute+1 = hour

Comments Notes to your programs to explain in

natural language what the program is doing, called comments, and they are marked with the # symbol

Everything from the # to the end of the line is ignored—it has no effect on the program

# compute the percentage of the hour that has elapsedpercentage = (minute*100)/60 # caution:integer division