Noadswood Science, 2014

To understand what strings are and how decisions are made

Wednesday, April 19, 2023

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 + bprint(c)

In Between String

New strings can also be added between two strings

a = ‘Run! ’b = ‘Aliens are coming.’c = b + ‘ Watch out! ‘ + aprint(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…)

F L A M I N G O

0 1 2 3 4 5 6 7

Numbering Characters

The position number is known as the “index” and can be used to pull out a particular letter from a string

F L A M I N G O

0 1 2 3 4 5 6 7

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

F L A M I N G O

0 1 2 3 4 5 6 7

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

F L A M I N G O

0 1 2 3 4 5 6 7

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

integerprint(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 = 31print(a, b, c) < separates using a spaceprint(a, b, c, sep=‘-’) < separates using a hyphenprint(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 printedprint(end='\n') < new linefor 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 lineprint('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 = 10print(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 = 10print(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 9th 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 datetimei = 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 datetimei = datetime.datetime.now()day = 9month = 7print((“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 datetimei = datetime.datetime.now()day = 9month = 7Xmas_day = 25Xmas_month = 12NY_day = 1NY_month = 1print((“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