Lecture 2

• syntax and semantics
• scalar objects
• simple operations
• expressions, variables and values

• string object type
• branching and conditionals
• indentation
• iteration and loops

• letters, special characters, spaces, digits
• enclose in quotation marks or single quotes

hi = "hello there"

• concatenate strings

name = "Burkay"
greet = hi + name
greet

## 'hello thereBurkay'

greeting = hi + " " + name
greeting

## 'hello there Burkay'

• do some operations on a string as defined in Python docs

silly = hi + " " + name * 3
silly

## 'hello there BurkayBurkayBurkay'

• the length of a string can be computed by len()

name = "Burkay"
len(name)

## 6

surname = "Genc"
fullname = name + " " + surname
len(fullname)

## 11

• You can get a specific character in a string by indexing the string
• An index is a position in a string
• In Python indices start from 0
  • The first character is at index 0
  • The second character is at index 1
  • …

fullname

## 'Burkay Genc'

fullname[0]

## 'B'

fullname[1]

## 'u'

fullname[5]

## 'y'

• How do you print the last character of a string?

• If there are k characters in a string
  • the last character is at index k-1

lastCharacterIndex = len(fullname) - 1
fullname[lastCharacterIndex]

## 'c'

• Python uses negative indices to count from the end
  • So, a simpler solution is :

fullname[-1]

## 'c'

• You can get a subset of characters by slicing
• To slice a string you provide
  • the starting index of the slice (inclusive)
  • separate with a column, :
  • the ending index of the slice (exclusive)

# The string is   : Burkay
# The indices are : 012345

name[2:4]

## 'rk'

• So, str[i:j] gives you characters at indices:
  • i, i+1, i+2, ..., j-1

• You can omit the stating or the ending indices, or both

fullname

## 'Burkay Genc'

fullname[:5]

## 'Burka'

fullname[5:]

## 'y Genc'

fullname[:]

## 'Burkay Genc'

• You can even slice with negative indices:

fullname[-4:]

## 'Genc'

fullname[:-4]

## 'Burkay '

fullname[2:-2]

## 'rkay Ge'

• used to output stuff to console
• keyword is print

x = 1
print(x)

## 1

• print adds a space between parameters. To avoid the space, you should concatenate the parameters yourself.

print("my fav num is", x, ".", "x =", x)

## my fav num is 1 . x = 1

x_str = str(x)
print("my fav num is " + x_str + ". " + "x = " + x_str)

## my fav num is 1. x = 1

• prints whatever is in the quotes
• user types in something and hits enter
• binds that value to a variable

text = input("Type anything... ")
print(5*text)

## Type anything... 5
## 55555

• input gives you a string so you must cast if working with numbers

num = int(input("Type a number... "))
print(5*num)

## Type a number... 5
## 25

• Write a program that will ask your_name and your_age and then print out

your_name is your_age years old.

• For example:

## What is your name? Burkay
## How old are you? 41

## Burkay is 41 years old.

• i and j are variable names
• comparisons below evaluate to a Boolean (true/false)

i > j
i >= j
i < j
i <= j

• equality test, True if i is the same as j

i == j

• inequality test, True if i is not the same as j

i != j

5 > 3

## True

a = 7
b = 5
b >= a

## False

a == b

## False

a == b + 2

## True

• Python is first computing expressions, then checking comparisons

a != b

## True

b = "burkay"
c = "cemal"
d = "davut"
b == c

## False

c < d

## True

b > d

## False

b = "burkay"
B = "Burkay"
B < b

## True

b < B

## False

• Capital letters come first

• a and b are variable names (with Boolean values)
• not negates the boolean value
• and returns True only if both operands are True
• or returns False only if both operands are False

a = 5
b = 7
c = 3
a > b

## False

not (a > b)

## True

(a > b) and (b > a)

## False

(a > b) or (b > a)

## True

(a < c) or ((b > c) and not (a < c))

## True

pset_time = 15
sleep_time = 8
print(sleep_time > pset_time)

## False

derive = True 
drink = False
both = drink and derive
print(both)

## False

• derive obtain (a function or equation) from another by a sequence of logical steps, for example by differentiation.

• We can define branches in the flow of a program using conditions.
• We use the if/else keywords for that:

if <condition>:
   <expression>
   <expression>
   ...
elif <condition>:
   <expression>
   <expression>
   ...
else:
   <expression>
   <expression>
   ...

• We can also create multiple branches using elif keyword:

if <condition>:
   <expression>
   <expression>
   ...
elif <condition>:
   <expression>
   <expression>
   ...
elif <condition>:
   <expression>
   <expression>
   ...
else:
   <expression>
   <expression>
   ...

a = 5
b = 3
if (a < b):
   print("a is less than b.")
else: 
   print("a is greater than b.")

## a is greater than b.

• Returning the maximum of two numbers

a = 7
b = 3

if a > b:
   print(a)
else:
   print(b)

## 7

• What if they are equal?

a = 7
b = 3

if a > b:
   print(a)
elif a < b:
   print(b)
else:
   print("they are equal")

## 7

• matters in Python
• how you denote blocks of code

# block level 1
do_something()
if True:
   # block level 2
   do_something()
   if True: 
      # block level 3
      do_something()
   # block level 2
   do_something()
# block level 1
do_something()

if False:
   print("This shouldn't print.")
print("But this should.")

## But this should.

vs.

if False:
   print("This shouldn't print.")
   print("And this shouldn't print as well.")

• We can nest if statements to create complex branching
• Nesting is done via indentation

x = float(input("Enter a number for x: "))
y = float(input("Enter a number for y: "))
if x == y:
   print("x and y are equal")
   if y != 0:
      print("therefore, x / y is", x/y)
elif x < y:
   print("x is smaller")
else:
   print("y is smaller")
print("thanks!")

• Exercise
  • Execute this code in Python and test with different inputs.
  • Find a pair of values for x and y for each possible output.

• Be careful not to confuse = and ==
• = is assignment
  • righthand side value is assigned to lefthand side variable
• == is a check for equality
  • returns True if lefthand side is equal to righthand side

x = 5
if x == 5:
   print("This will be accepted and printed.")

## This will be accepted and printed.

if x = 5:
   print("This won't be accepted by Python.")

## Error: invalid syntax (<string>, line 1)

• Ask the user for a number and then print that many X’s

numberOfXs = int(input("Enter a number:"))

if numberOfXs == 1:
   print("X")
elif numberOfXs == 2:
   print("XX")
elif numberOfXs == 3:
   print("XXX")
elif numberOfXs == 4:
   print("XXXX")
# ...
# where to stop???

• Can we be smarter than this?
  • Yes!

• A better solution is:

numberOfXs = int(input("Enter a number:"))

print(numberOfXs * 'X')

• We use a property of Python to solve this.
  • We can multiply strings by numbers to repeat them
• But will this always work?
  • No!

• Ask for a number and print numbers from 1 to that number

aNumber = int(input("Enter a number:"))

if aNumber == 1:
   print("1")
elif aNumber == 2:
   print("1 2")
elif aNumber == 3:
   print("1 2 3")
elif aNumber == 4:
   print("1 2 3 4")
elif aNumber == 5:
   print("1 2 3 4 5")
## Where to stop???

• What to do now?

• When we want a program to do something many times following a certain structure we use iteration
• Examples
  • For each student in this class, ask his name
  • Water every flower in this room
  • For each number from 1 to 10, compute square of that number
  • Given k, compute k!
• All of these require a repetitive task to be done many times
• Python allows repetitive tasks to be easily coded by looping

• to do something repetitively until we hit a termination condition we use the while loop

while <condition>:
   do_something

• Python will do_something again and again until <condition> becomes False
  • Beware! If <condition> never becomes False, then the program executes forever!

a = 0
while a < 5:
   print(a)
   a = a + 1   # Don't forget this!

## 0
## 1
## 2
## 3
## 4

• You can iterate both ways:

a = 10
while a > 0:
   print(a)
   a = a - 1

## 10
## 9
## 8
## 7
## 6
## 5
## 4
## 3
## 2
## 1

• Compute the factorial of a given number

number = 6
factorial = 1  # starting with 1 is important!

while number > 1:
   factorial = factorial * number
   number = number - 1
   
print(factorial)

## 720

OR

number = 6
factorial = 1  # starting with 1 is important!
i = 1

while i <= number:  # why <= and not < ???
   factorial = factorial * i
   i = i + 1
   
print(factorial)

## 720

• Find all numbers perfectly dividing a given number