University:
Boston University
Course:
CAS CS 501 | Computer Science Practicum
Academic year:

2022
Views:

130

Pages:

24
Author:

Zoie Hudson

Python as a Calculator A Note about Errors What happens when you type: 1 z 2 " Hello " + 1 Python as a Calculator A Note about Errors - Output 1 2 3 4 5 6 7 8 >>> z Traceback ( most recent call last ): File " < console > " , line 1 , in < module > NameError : name ’z ’ is not defined >>> " Hello " + 1 Traceback ( most recent call last ): File " < console > " , line 1 , in < module > TypeError : cannot concatenate ’ str ’ and ’ int ’ objects Python as a Calculator A Note about Errors - Output 1 2 3 4 5 6 7 8 >>> z Traceback ( most recent call last ): File " < console > " , line 1 , in < module > NameError : name ’z ’ is not defined >>> " Hello " + 1 Traceback ( most recent call last ): File " < console > " , line 1 , in < module > TypeError : cannot concatenate ’ str ’ and ’ int ’ objects A traceback occurs: TypeError is the error that occurs cannot concatenate ’str’ and ’int’ objects is the ‘helpful’ message and every thing from Traceback to the error tells you where it happened Python as a Calculator Strings 3 Single or Double quotes are OK: 1 print ’ Hello ’ 2 print " Hello " But be careful to escape extra quotes: 1 print ’I ’m a happy camper ’ 2 print " I ’m a happy camper " 3 print ’I \ ’ m a happy camper ’ And you can multiply strings by an integer: 1 h = " Happy " 2 b = " Birthday " 3 print ( h + b ) * 10 Python as a Calculator Strings 3 - Output 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 >>> print ’ Hello ’ Hello >>> print " Hello " Hello >>> print ’I ’m a happy camper ’ File " < console >" , line 1 print ’I ’m a happy camper ’ ^ SyntaxError : invalid syntax >>> print ’I \ ’ m a happy camper ’ I ’m a happy camper >>> print " I ’m a happy camper " I ’m a happy camper >>> h = " Happy " >>> b = " Birthday " >>> print ( h + b ) * 10 HappyBirthdayHappyBirthdayHappyBirthdayHappyBirthdayH Python is a Scripting Language From interactive prompt to... The Python prompt is great for quick tasks: math, short bits of code, and testing. For bigger projects, it’s easier to store code in a file. One such example can be found in examples\nobel.py Python is a Scripting Language Script Practice: Nobel Laureates 1 2 Exit python: exit() then hit Enter The >>> is now replaced by % ; This is the Unix Prompt. Go to examples directory with this command: % cd examples 3 4 Run the script by typing: % python nobel.py Open ’nobel.py’ (command: % gedit nobel.py & ) and answer these questions: Python is a Scripting Language Script Practice: Nobel Laureates 1 2 Exit python: exit() then hit Enter The >>> is now replaced by % ; This is the Unix Prompt. Go to examples directory with this command: % cd examples 3 4 1 2 3 Run the script by typing: % python nobel.py Open ’nobel.py’ (command: % gedit nobel.py & ) and answer these questions: How do you comment code in Python? How do you print a newline? How do you print a multi-line string so that whitespace is preserved? Python is a Scripting Language Booleans 1 A Boolean type is a type with two values: True/False. Try typing the following: 1 2 3 4 True type ( True ) False type ( False ) Python is a Scripting Language 1 2 3 4 5 6 7 8 >>> True True >>> type ( True ) < type ’ bool ’ > >>> False False >>> type ( False ) < type ’ bool ’ > Python is a Scripting Language Booleans 2a You can also compare values, to see if they’re equal: 1 0 == 0 2 0 == 1 Python is a Scripting Language Booleans 2b You can also compare values, to see if they’re equal: 1 2 3 4 >>> 0 == 0 True >>> 0 == 1 False == (equal equal) is for equality test = (equal) is for assignment Be careful! This can lead to bugs! Python is a Scripting Language Booleans 3 You can do other comparisons: != means not equal 1 " a " != " a " 2 " a " != " A " Others are just like math class: 1 2 3 4 1 > 0 2 >= 3 -1 < 0 .5 <= 1 Python is a Scripting Language Booleans 3 - Output 1 2 3 4 5 6 7 8 9 10 11 12 >>> " a " != " a " False >>> " a " != " A " True >>> 1 > 0 True >>> 2 >= 3 False >>> -1 < 0 True >>> .5 <= 1 True Python is a Scripting Language Booleans 4 You can see if something is in something else: 1 " H " in " Hello " 2 " X " in " Hello " or not: 1 " a " not in " abcde " 2 " Perl " not in " Python Tutorial " Python is a Scripting Language Booleans 4 - Output 1 2 3 4 5 6 7 8 >>> " H " in " Hello " True >>> " X " in " Hello " False >>> " a " not in " abcde " False >>> " Perl " not in " Python Tutorial " True Python is a Scripting Language Flow Control 1 You can use Booleans to decide if some code should be executed: 1 if 6 > 5: 2 print " Six is greater than five ! " This is a multi-line piece of code: 1 2 3 4 5 6 if 6 > 5: Enter 4 spaces print "Six is greater than five!" Enter Enter again... Python is a Scripting Language Flow Control 2 The ”...” is a special prompt; Python realizes this is a code block. Final enter is to signify the end of code block. Python is a Scripting Language Flow Control 2 The ”...” is a special prompt; Python realizes this is a code block. Final enter is to signify the end of code block. 1 2 3 4 >>> if 6 > 5: ... print " Six is greater than five ! " ... Six is greater than five ! What’s going on here? if looks for a Boolean, and if it is true, the code block is executed. Python is a Scripting Language Flow Control 2 The ”...” is a special prompt; Python realizes this is a code block. Final enter is to signify the end of code block. 1 2 3 4 >>> if 6 > 5: ... print " Six is greater than five ! " ... Six is greater than five ! What’s going on here? if looks for a Boolean, and if it is true, the code block is executed. 6 > 5 is True so the next line is executed. Python is a Scripting Language Flow Control 3 Now what will happened? 1 if 0 > 2: 2 print " Zero is greater than two ! " 3 if " banana " in " bananarama " : 4 print " I miss the 80 s . " Python is a Scripting Language Flow Control 3 - Output 1 2 3 4 5 6 7 >>> if 0 > 2: ... print " Zero is greater than two ! " ... >>> if " banana " in " bananarama " : ... print " I miss the 80 s " ... I miss the 80 s Python is a Scripting Language Indentation, what’s up with that? If you’ve programmed in other languages, this indentation thing might seem weird. Python prides itself as an easy-to-read language, and indentation makes it easy to read code blocks. So Python requires indentation over if/end-if, begin-block/end-block organization. Python is a Scripting Language Indentation - example 1 2 3 4 5 6 7 8 9 # this looks like other languages , # but I use a comment to organize if 1 == 1: print " Everything is going to be OK ! " if 10 < 0: print " or is it ? " # end if print " Inside first code block ! " # end if Don’t use #end confusing... if, just keep it in your mind if it gets

Python for Non-programmers - A Gentle Introduction 2

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