Types, Operators, Variables

Types, Operators, Variables#

Last recitation:#

  • Administration

  • Install Anaconda and Pycharm

  • Simple coding with Pycharm

Agenda#

Types

  • int, float, bool, str

Operators

  • + - * / ** // % ...?!@#

Variables

  • Assignment

Boolean statements

Types and conversions#

c=float(137)
print(c, type(c))

137.0 <class 'float'>

c=int(3.17)
print(c, type(c))

3 <class 'int'>

c=float("3.17")
print(c, type(c))

3.17 <class 'float'>

c=int("3.17")
print(c, type(c))

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[5], line 1
----> 1 c=int("3.17")
      2 print(c, type(c))

ValueError: invalid literal for int() with base 10: '3.17'

c=int(float("3.17"))
print(c, type(c))

3 <class 'int'>

Always: order of actions inner parentheses \(\rightarrow\) outer parantheses#

c=False
print(type(c))

<class 'bool'>

c=bool(3.17)
print(c, type(c))

True <class 'bool'>

c=bool(1)
print(c, type(c))

True <class 'bool'>

c=bool(0)
print(c, type(c))

False <class 'bool'>

Operators#

print(13+7)

20

print(13-7)

6

print(13*7)

91

print(13/7)

1.8571428571428572

13//7

1

13%7

6

13**7

62748517

Slide 13-14

Variables and assignments#

alt_text

  • First evaluate the expression

  • Then assigns the value to the variable

A variable name is a sequence of letters, digits and underscores (‘_’), starting with a letter

n=1
print(n)
n=n+2
print(n)
n=n+4
print(n)

1
3
7

Strings#

  • Ordered collection of characters.

  • First index is zero (0)

H

e

l

l

o

0

1

2

3

4

-5

-4

-3

-2

-1

  • Strings can be enclosed in either ‘single quotes’ or “double quotes”.

print('If you spend all day shuffling words around, you can make anything sound bad')

If you spend all day shuffling words around, you can make anything sound bad

print('I'll tell you how I feel about school... It's a waste of time. Bunch of people runnin’ around... got a guy up front says, ‘Two plus two,’ and the people in the back say, 'Four'...')

  Cell In[20], line 1
    print('I'll tell you how I feel about school... It's a waste of time. Bunch of people runnin’ around... got a guy up front says, ‘Two plus two,’ and the people in the back say, 'Four'...')
          ^
SyntaxError: invalid syntax. Perhaps you forgot a comma?

print('I\'ll tell you how I feel about school... It\'s a waste of time. Bunch of people runnin\' around... got a guy up front says, \'Two plus two,\' and the people in the back say, ‘Four’...')

I'll tell you how I feel about school... It's a waste of time. Bunch of people runnin' around... got a guy up front says, 'Two plus two,' and the people in the back say, ‘Four’...

  • Individual characters are accessed using brackets notation “[]” with the positional index

my_string="c-137"
print(my_string[0].upper())
print(my_string[-2])
print(my_string[4])
print(my_string[5])

C
3
7

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[22], line 5
      1 my_string="c-137"
      2 print(my_string[0].upper())
      3 print(my_string[-2])
      4 print(my_string[4])
----> 5 print(my_string[5])

IndexError: string index out of range

  • len() is used for getting the length of a string

my_string="c-137"
print(len(my_string))

5

String slicing#

  • syntax: some_str[x:y:z]

  • Returns a new string based on characters from some_str

  • from position x (including) until position y (not including) with steps of size (and direction) z

## "Input" values
some_str="Wubba Lubba Dub Dub"
x=1
y=10
z=2

# slice from x to y in step of size 1
print(some_str[x:y])

ubba Lubb

# slice from x to end in step of size z
print(some_str[x::z] )

ub ub u u

# slice from beginning to y with step of size z
print(some_str[:y:z] )

WbaLb

(3): string slicing#

## "Input" values
some_str="Wubba Lubba Dub Dub"
x=1
y=10
z=2

print(some_str[y:x:-z] )

abLab

Comparisons / boolean statementsm#

Comparison operators:#

==

equal

Note: twice ‘=‘, this is not an assignment!

!=

not equal

>

greater than

<

less than

>=

grater than or equal

<=

less than or equal

Comparisons are evaluated to Boolean (bool) values#

# Simple inequality (numeric)
c=137<138
print(c)

True

# Simple inequality (lexicography)
c='rick' <'morty'
print(c)

False

Multiple comparisons#

c=136<137 and 137<135
print(c)

False

# Can also mix it up:
c=6<7 or 'morty'<'rick'
print(c)

True

Operations on mixed variables#

c=140 

print(c-137)

3

print("c"-137) 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[35], line 1
----> 1 print("c"-137)

TypeError: unsupported operand type(s) for -: 'str' and 'int'

print("c-"+137)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[36], line 1
----> 1 print("c-"+137)

TypeError: can only concatenate str (not "int") to str

print("c-"+"137")

c-137

print(c*2)

280

print("c"*2)

cc

print("c-"*2+137)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[40], line 1
----> 1 print("c-"*2+137)

TypeError: can only concatenate str (not "int") to str

print(c**2)

19600

print("c"**2)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[42], line 1
----> 1 print("c"**2)

TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'

Self learning#

Exercise 1#

  • Return the number resulting from reversing the digit of a given four-digit number

  • No str functions

Example: 1370 –> 731 (0731)

num=1370

num1=(num%10)*1000 
num2=(num%100//10)*100 
num3=(num%1000//100)*10 
num4=((num)//1000)*1

print(num1,num2,num3,num4)
print(num1+num2+num3+num4)

0 700 30 1
731

Exercise 2#

Write a code that receives a string s with only lower letters, and prints a similar string, but with the middle letter in an upper case format.#

e.g.,
efghi \(\rightarrow\) efGhi
efghij \(\rightarrow\) efgHij

## "Input" values
s = 'limerick'

# Find string’s length
n = len(s)
print(n)

8

# Find string’s middle index
middle = n / 2 
print(middle)

4.0

# Find string’s middle letter
middle_letter = s[middle]

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[47], line 2
      1 # Find string’s middle letter
----> 2 middle_letter = s[middle]

TypeError: string indices must be integers, not 'float'

# A float cannot serve as an index! use integer division instead
middle = n // 2
print(middle)
middle_letter = s[middle]

4

# Converts middle letter to an upper case 
U_middle_letter = middle_letter.upper()

# Slice the string before and after the capitalized character and merge it with the revised character
new = s[0:middle] + U_middle_letter + s[middle+1:]
print(new)

limeRick