Noadswood Science, 2014

 To understand what strings are and how decisions are made Thursday, September 17, 2015

Strings  Python is excellent for using words and sentences within programs: different strings can be joined together, or individual parts of them can be selected and pulled out…  Remember, a string can include letters, numbers, symbols or spaces a = ‘Run! ’ b = ‘Aliens are coming.’

Adding Strings  Adding two numbers together creates a new number, and in the same way two strings can be added together (with one simply joining the other)  The “+” symbol joins one string to the other a = ‘Run! ’< notice the space here after ! b = ‘Aliens are coming.’ c = a + b print(c)

In Between String  New strings can also be added between two strings a = ‘Run! ’ b = ‘Aliens are coming.’ c = b + ‘ Watch out! ‘ + a print(c)  Consider where your spaces go!

String Length  Python allows you to identify the length of a string using the “len()” function  It will count all the characters (including spaces) to give the total number of characters in a string a = ‘Run! ’ b = ‘Aliens are coming.’ print(len(b))  The answer should be 18 (there are 18 characters in string “b”)

String Escape  Strings can go in single or double quotes – it is important that strings start and end with the same type of quote  Write the following string print(‘It’s a lovely day today.’)  This will cause an error – in order to avoid this Python has a function known as escaping – within the string if an apostrophe is needed type “\” before it such as the code below print(‘It\’s a lovely day today.’)

Numbering Characters  Each character in a string is allocated a number according to its position – this position number can be used to look at individual letters or symbols, or to pull them out of a string…  When counting the positions, Python starts at 0 (the second character is 1, the third is 2 etc…) FLAMINGO

Numbering Characters  The position number is known as the “index” and can be used to pull out a particular letter from a string FLAMINGO a = ‘FLAMINGO’ print(a[3])< the character is position 3 from variable “a”

Slicing  Two indexes can be used to pull out a part of the string (slice it) – the letter in the last position isn’t included FLAMINGO a = ‘FLAMINGO’ print(a[3:7])< a slice from index 3 to 6 in variable “a”

Start to End  If you leave off the start or end index, Python will automatically use the first or last character of the string FLAMINGO a = ‘FLAMINGO’ print(a[:3]) print(a[3:])

Input  The “input()” function is used to accept input from the keyboard into a program – it waits until a user finishes typing and presses the return or Enter key name = input (‘Enter your name: ‘) print(‘Hello’, name)

Output  The “print()” function is used to display characters in the shell window – it can be used to show a combination of text and variables a = ‘David’ b = ‘is’ c = 31< no quote marks as this is an integer print(a, b, c) print(‘Goodbye’, a)

Separating Strings  Strings can be separated in a variety of ways, including using the separator command (“sep”)  Strings can be separated using spaces  Strings can be separated using hyphens “-” (or other characters such as “+” or “*”)  Strings can also be separated by printing on a new line

Separating Strings a = ‘David’ b = ‘is’ c = 31 print(a, b, c)< separates using a space print(a, b, c, sep=‘-’)< separates using a hyphen print(a, b, c, sep=‘\n’)< separates onto a new line

Ending Output  There are several ways to signal the end of an output of a “print” function a = ‘Hurray!’ print(a, ‘.’)" print(a, end=‘.’)< “end=‘.’” ensures no space is printed print(end='\n')< new line for n in range(3):< repeat x3 print(a, end=‘ ‘)< output all on one line using a space print(a, end=‘\n\n\n\n’)< “\n” starts a new line print('4 new lines')

Decisions  Programs make decisions about what to do by comparing variables, numbers and strings using Boolean expressions (True or False)  Logical operators are used to compare variables against numbers, strings or other variables – the result is either True or False ==(equals operator) !=(not equal operator) <(less than operator) >(more than operator) <=(less than or equal to operator) >=(more than or equal to operator) *Remember, Python uses the equals sign (=) to assign a value to a variable

Decisions  Write a Python code for a variable (toys) which equals the integer of 10  Print off the results for equal, not equal, less than, more than, less than or equal to and more than or equal to operators toys = 10 print(toys == 1) print(toys != 1) print(toys < 1) print(toys > 1) print(toys <= 1) print(toys >= 1)

Not, Or, And  The not logical operator (not) reverses the answer (from True to False and vice versa)  The or logical operator (or) checks if the outcome is one or another  The and logical operator (and) checks if the outcome is one thing and another toys = 10 print(not toys == 1) print(toys == 1 or toys == 10) print(toys == 1 and toys == 10)

In  The in operator (in) can be used to see whether one string is inside another string (e.g. is a letter in a word / is a word in a sentence) print(‘a’ in ‘abc’) print(‘d’ in ‘abc’) print(‘Mr Crowley’ in ‘Computer Science course with Mr Crowley’) print(‘Mr Hewitt’ in ‘Computer Science course with Mr Crowley’)

Compare Strings  Two strings can be compared using the equal (==) or not equal (!=) operators  Strings have to match exactly to get a True output dog = 'Woof woof' print(dog == 'Woof woof')(True – matches) print(dog == 'woof woof')(False – no capital) print(dog == 'Woof woof ')(False – extra space)

Rachel’s Birthday Today?  Rachel’s birthday is the 9 th July – write a program using logical operators to check if it is Rachel’s birthday today…  Write a second program to check if it is not Rachel’s birthday today  Write a third program to check if it is Rachel’s birthday or New Year or Christmas today import datetime(Python function to find the date) i = datetime.datetime.now()(variable i for time now) %i.day(today’s day) %i.month(today’s month) %s (needed for string formation)

Rachel’s Birthday import datetime i = datetime.datetime.now() day = 9(Rachel’s birthday day) month = 7(Rachel’s birthday month) print((“Rachel’s birthday day is”), day) print((“Rachel’s birthday month is”), month) print(‘Current day is %s’ %i.day) print(‘Current month is %s’ %i.month) print((“Today is Rachel’s birthday:”), day == i.day and month == i.month)

Not Rachel’s Birthday import datetime i = datetime.datetime.now() day = 9 month = 7 print((“Rachel’s birthday day is”), day) print((“Rachel’s birthday month is”), month) print(‘Current day is %s’ %i.day) print(‘Current month is %s’ %i.month) print((“Today is not Rachel’s birthday:”), day != i.day and month != i.month)

Rachel’s Birthday or Xmas or New Year import datetime i = datetime.datetime.now() day = 9 month = 7 Xmas_day = 25 Xmas_month = 12 NY_day = 1 NY_month = 1 print((“Rachel’s birthday day is”), day) print((“Rachel’s birthday month is”), month) print((“Xmas day is”), day) print((“Xmas month is”), month) print((“New Years day is”), day) print((“New Years month is”), month) print(‘Current day is %s’ %i.day) print(‘Current month is %s’ %i.month) print(("Today is either Rachel's birthday, or Xmas or New Year:"), day == i.day and month == i.month \ or Xmas_day == i.day and Xmas_month == i.month or NY_day == i.day and NY_month == i.month) Use this character (\) to code over multiple lines

Rachel’s Birthday or Xmas or New Year