Strings

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

  hi = "hello there"
  

• concatenate strings

  name = "ajay"
  greeting = hi + " " + name  #"hi ajay"
  silly = name * 3  #"ajayajayajay"
  

Input/Output: print

• used to output stuff to console
• keyword is print

  x = 1
  print(x)  # 1
  x_str = str(x)
  print("my fav num is", x, ".", "x =", x) #my fav num is 1 . x = 1
  print("my fav num is " + x_str + ". " + "x = " + x_str)  #my fav num is 1. x = 1
  

  (notice the difference in spaces).

Input/Output: input

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

text = input("Type anything... ") # say we type "hello"
print(2*text)  # hellohello

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

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

Comparison operators on int, float, string

• i and j are variable names.
• Comparisons below evaluate to a boolean:

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

Logic operators on bools

• not a
• a and b
• a or b

Example:

study_time = 15
sleep_time = 8
good_marks = (study_time > sleep_time)
good_sleep = not (sleep_time < 8)
both = good_marks and good_sleep
print(both)

Control flow - Branching

if <condition>:
  <expression>
  <expression>

Form 2:

if <condition>:
  <expression>
  <expression>
else:
  <expression>
  <expression>

Form 3:

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

<condition> has a value of True or False. Evaluate expressions in that block if <condition> is True.

Indentation

• Matters in Python
• How you denote blocks of code

  x = float(input("Enter a number for x: "))
  y = float(input("Enter a number for y: "))
  if x == y:    #syntax error is use x=y here!
    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!")
  

Control flow: While Loops

Example:

You are in the Lost Forest.
************
************
    Karel
************
************
Go left or right?

Try this with if-else:

if input("You're in the Lost Forest. Go left or right? ") == right:
  if input("You're in the Lost Forest. Go left or right? ") == right:
    if input("You're in the Lost Forest. Go left or right? ") == right:
      if input("You're in the Lost Forest. Go left or right? ") == right:
        if input("You're in the Lost Forest. Go left or right? ") == right:
          .... #goes on forever
        else:
          print("You got out of the Lost Forest!")
      else:
        print("You got out of the Lost Forest!")
    else:
      print("You got out of the Lost Forest!")
  else:
    print("You got out of the Lost Forest!")
else:
  print("You got out of the Lost Forest!")

To do this, we need a new construct:

while <condition>:
  <expression>
  <expression>
  ...

• <condition> evaluates to a boolean.
• If <condition> is True, do all the steps inside the while code block.
• check <condition> again.
• repeat until <condition> is False.

n = input("You're in the Lost Forest. Go left or right? ")
while n == "right":
  n = input("You're in the Lost Forest. Go left or right? ")
print("You got out of the Lost Forest!")

Control flow: while and for loops

# more complicated with while loop
n = 0
while n < 5:
  print(n)
  n = n + 1

Shortcut with for loop:

for n in range(5):
  print(n)

for Loops

for <variable> in range(<some_num>):
  <expression>
  <expression>
  ...

range(start, stop, step). The default values are start=0 and step=1. Loop until value is stop-1.

mysum = 0
for i in range(7, 10):
  mysum += i
print(mysum)
mysum = 0
for i in range(5, 11, 2):
  mysum += i
print(mysum)

break statement

• Immediately exits whatever loop it is in
• skips remaining expressions in code block
• exits only innermost loop!

while <condition_1>:
  while <condition_2>:
    <expression_a>
    break
    <expression_b>
  <expression_c>

Example:

mysum = 0
for i in range(5, 11, 2):
  mysum += i
  if mysum == 5:
    break
    mysum += 1
print(mysum)

for vs. while loops