Python Basics: Statements Expressions Loops Strings Functions
Program • A program is a sequence of instructions or statements. • To run a program is to: • create the sequence of instructions according to your design and the language rules • turn that program into the binary commands the processor understands • give the binary code to the OS, so it can give it to the processor • OS tells the processor to run the program • when finished (or it dies :-), OS cleans up.
Example Code Listing # 1. prompt user for the radius, # 2. apply the area formula # 3. print the results import math radiusString = input("Enter the radius of your circle:") radiusFloat = float(radiusString) circumference = 2 * math.pi * radiusFloat area = math.pi * radiusFloat * radiusFloat print() print("The cirumference of your circle is:",circumference,\ ", and the area is:",area)
Getting Input The function: input(“Give me a value”) • prints “Give me a value” on the screen and waits until the user types something (anything), ending with [Enter] key • Warning! Input() returns a string (sequence of characters), no matter what is given. (‘1’ is not the same as 1, different types) Convert from string to integer • Python requires that you must convert a sequence of characters to an integer • Once converted, we can do math on the integers
Import of Math • One thing we did was to import the math module with import math • This brought in python statements to support math (try it in the python window) • We precede all operations of math with math.xxx • math.pi, for example, is pi. math.pow(x,y) raises x to the yth power.
Assignment Statement The = sign is the assignment symbol • The value on the right is associated with the variable name on the left • A variable is a named location that can store values (information). A variable is somewhat likea file, but is in memory not on the HD. • = Does not stand for equality here! • What “assignment” means is: • evaluate all the “stuff” on the rhs(right-hand-side) of the = and take the resulting value and associate it with the name on the lhs (left-h-s)
Printing Output myVar = 12 print(‘My var has a value of:’,myVar) • print() function takes a list of elements to print, separated by commas • if the element is a string, prints it as is • if the element is a variable, prints the value associated with the variable • after printing, moves on to a new line of output
Syntax • Lexical components. • A Python program is (like a hierarchy):. • A module (perhaps more than one) • A module is just a file of python commands • Each module has python statements • Statements may have expressions • Statements are commands in Python. • They perform some action, often called a side effect, but do not return any values • Expressions perform some operation and return a value
Side Effects and Returns • Make sure you understand the difference. What is the difference between a side effect and a return? • 1 + 2 returns a value (it’s an expression). You can “catch” the return value. However, nothing else changed as a result • print “hello” doesn’t return anything, but something else - the side effect - did happen. Something printed!
Whitespace • white space are characters that don’t print (blanks, tabs, carriage returns etc. • For the most part, you can place “white space” (spaces) anywhere in your program • use it to make a program more readable • However, python is sensitive to end of line stuff. To make a line continue, use the \ print “this is a test”, \ “ of continuation” prints this is a test of continuation
Python Tokens Keywords: You are prevented from using them in a variable name Reserved operators in Python (expressions):
Python Punctuators • Python punctuation/delimiters ($ and ? not allowed).
Operators • Integer • addition and subtraction: +, - • multiplication: * • division • quotient: // • remainder: % • Floating point • add, subtract, multiply, divide: +, -, *, /
Loops: Repeating Statements Draw Square: Repeat the following steps 4 times: • Draw a line • Turn left from turtle import * forward(100) left(90) forward(100) left(90) forward(100) left(90) forward(100) from turtle import * forcount in range(4): forward(100) left(90)
While and For Statements from turtle import * for count in range(4): forward(100) left(90) • The for statementis useful for iteration, moving through all the elements of data structure, one at a time. • The while statement is the more general repetition construct. It repeats a set of statements while some condition is True. from turtle import * count=1 while count<=4: forward(100) left(90) count=count+1
Range Function • The range function generates a sequence of integers • range(5) => [0, 1, 2, 3, 4] • assumed to start at 0 • goes up to, but does not include, the provided number argument. • range(3,10) => [3, 4, 5, 6, 7, 8, 9] • first argument is the number to begin with • second argument is the end limit (not included!)
Iterating Through the Sequence for num in range(1,5): print(num) • range generates the sequence [1, 2, 3, 4] • for loop assigns num each of the values in the sequence, one at a time in sequence • prints each number (one number per line) • list(range(-5,5)) # in shell to show range
Sequence of Characters • We’ve talked about strings being a sequence of characters. • A string is indicated between ‘ ‘ or “ “ • The exact sequence of characters is maintained, including spaces. • Does NOT include multiple lines • Use backslash \ for line continuation
And Then There is “““ ””” • Triple quotes preserve both vertical and horizontal formatting of the string • Allow you to type tables, paragraphs, whatever and preserve the formatting (like <pre> tag in html) “““this is a test of multiple lines”””
Strings Can use single or double quotes: • S = “spam” • s = ‘spam’ Just don’t mix them! • myStr = ‘hi mom” ERROR Inserting an apostrophe: • A = “knight’s” # mix up the quotes • B = ‘knight\’s’ #escape single quote
The Index • Because the elements of a string are a sequence, we can associate each element with an index, a location in the sequence: • positive values count up from the left, beginning with index 0
Accessing an Element • A particular element of the string is accessed by the index of the element surrounded by square brackets [ ] helloStr = ‘Hello World’ print helloStr[1] => prints ‘e’ print helloStr[-1] => prints ‘d’ print helloStr[11] => ERROR
Basic String Operations • + is concatenation newStr = ‘spam’ + ‘-’ + ‘spam-’ print newStr spam-spam- • * is repeat, the number is how many times newStr * 3 spam-spam-spam-spam-spam-spam-
String Function: len • The lenfunction takes as an argument a string and returns an integer, the length of a string. myStr = ‘Hello World’ len(myStr) 11 # space counts
Example • A methodrepresents a special program (function) that is applied in the context of a particular object. • upper is the name of a string method. It generates a new string of all upper case characters of the string it was called with. myStr = ‘Python Rules!’ myStr.upper() ‘PYTHON RULES!’ • The string myStrcalled the upper() method, indicated by the dot between them.
Functions • From mathematics we know that functions perform some operation and return one value. • Why to use them? • Support divide-and-conquer strategy • Abstraction of an operation • Reuse: once written, use again • Sharing: if tested, others can use • Security: if well tested, then secure for reuse • Simplify code: more readable
Python Invocation • Consider a function which converts temps. in Celsius to temperatures in Fahrenheit: • Formula: F = C * 1.8 + 32.0 • Math: f(C) = C*1.8 + 32.0 • Python def celsius2Fahrenheit (C): return C*1.8 + 32.0 Terminology: “C” is an argument to the function
Return Statement • Functions can have input (also called arguments) and output (optional) • The return statement indicates the value that is returned by the function. • The return statement is optional (a function can return nothing). If no return, the function may be called a procedure.
from turtle import * def draw_square (size): for i in range (4): forward (size) right(90) draw_square(25) draw_square(125) draw_square(75) draw_square(55)
