Review

Calvin (Deutschbein)

W6Wed: 01 Oct

Announcements

• Midterm on Friday.
  • Practice Exams posted
  • There is a practice exam on Canvas where the exam will be hosted.
• Sections this week on Midterm review.

Today

• Review
  • Problem 1
    • Numerical Arithmetic
    • Booleans
    • String Slices
  • Problem 2: Program Tracing
  • Problem 3
    • For
    • If
    • Print
    • Return
  • Problem 4: Strings

Numerical Arithmetic

• Recall PEMDAS:
  • Parenthesis ()
  • Exponents **
  • Multiplication/Division * / //
  • Addition/Subtract + -

Mult > Add

• Calculations are not left-to-right. 1 + 2 * 3 # 7
• The multiplication occurs prior to the addition, same as: 2 * 3 # 6 1 + 6 # 7

Mult/Div Together

• The order here does not matter. 5 * 6 // 3 # 10 (5 * 6) // 3 # 10 5 * (6 // 3) # 10
• So they occur the same.

Add/Sub Together

• The order here does not matter. 1 + 2 - 3 # 0 (1 + 2) - 3 # 0 1 + (2 - 3) # 0
• So they occur the same.

Mult > Add

• Calculations are not left-to-right. 1 + 2 * 3 # 7
• Order does matter here: (1 + 2) * 3 # 9 1 + (2 * 3) # 7
• Multiplication first.

Exp > Mult

• Exponentiation beats out multiplication. 2 ** 1 * 3 # 6 (2 ** 1) * 3 # 6 2 ** (1 * 3) # 8

Example

• Just break things down incrementally. 3 + 3 * 3 ** 3 % 3
• Exponents first. 3 + 3 * 27 % 3 # 3 ** 3 is 27
• Multiply, divide, mod, right-to-left 3 + 3 * 27 % 3 # 3 ** 3 is 27
• Multiply, divide, mod, right-to-left 3 + 81 % 3 # 3 * 27 is 81
• Multiply, divide, mod, right-to-left 3 + 0 # 81 % 3 is 0 (81 is a multiple of 3)
• Add, subtract right-to-left 3 # 3 + 0 is 3

Today

• Review
  • Problem 1
    • ✓ Numerical Arithmetic
    • Booleans
    • String Slices
  • Problem 2: Program Tracing
  • Problem 3
    • For
    • If
    • Print
    • Return
  • Problem 4: Strings

Numerical Arithmetic

• Recall not / and / or
• Recall "truthiness"

not

• Changes True or False to the other. print(not True) print(not False)
• Just flips it. False True

and

• True if both sides are True, otherwise False

  print(True and True) print(True and False) print(False and True) print(False and False)

  True False False False

or

• True if either side is True, otherwise False

  print(True or True) print(True or False) print(False or True) print(False or False)

  True True True False

not > and

• Do any "not" before "and"

  print((not True) and False) print(not (True and False)) print(not True and False)

  False True False

and / or

Comparisons

• Usually we get booleans from comparisons.

  print(1 == 0) print(1 != 0) print(1 >= 0) print(1 <= 0) print('a' in 'aieou') print('a' == 'aieou'[0]0

  False True True False True True

Truthiness

• Zero and "" are like False

  print(bool(0)) print(bool(1)) print(bool("")) print(bool(" ")) print(bool(.5))

  False True False True True

Today

• Review
  • Problem 1
    • ✓ Numerical Arithmetic
    • ✓ Booleans
    • String Slices
  • Problem 2: Program Tracing
  • Problem 3
    • For
    • If
    • Print
    • Return
  • Problem 4: Strings

Indices

• Strings (words) have indices:
  • Each character (letter) has number corresponding to its location
  • Numbers begin with zero and increase by one
  • The numbers are all integers (whole numbers). s = "hi there" print(s[0]) print(s[1]) print(s[2]) print(s[3])
  • We see: h i t

Negative Indices

• Strings (words) have indices:
  • Each character (letter) has number corresponding to its location
  • Negative values begin with -1 at the end of the string. s = "hi there" print(s[-1]) print(s[-2]) print(s[-3]) print(s[-4])
  • We see: e r e h

Slices

• Slices take part of a string, based on indices.
  • They begin at some index, and continue up to some index.

    s = "hi there" print(s[0:4]) print(s[2:6]) print(s[-4:-2]) print(s[1:-1])

    hi t the he i ther

Slices

• Slices take part of a string, based on indices.
  • If the first index is ommitted, it is regard as zero.

    s = "hi there" print(s[:2]) print(s[0:2]) print(s[:4]) print(s[0:4])

    hi hi hi t hi t

Slices

• Slices take part of a string, based on indices.
  • If the last part is omitted, it is regard as the length of the string.

    s = "hi there" print(len(s)) print(s[6]) print(s[6:]) print(s[6:8]) print(s[8])

    8 r re re Traceback (most recent call last): File "...", line 6, in print(s[8]) ~^^^ IndexError: string index out of range

  • NOTE: This does not correspond to a character!

Slices

• Slices take part of a string, based on indices.
  • There may be a third value, a step size.

    s = "hi there" print(s[:4]) print(s[0:4:1]) print(s[:4:1]) print(s[:4:2]) print(s[:6:2])

    hi t hi t hi t h h h

  • Step of '2' gives every other letter.

Slices

• Slices take part of a string, based on indices.
  • There may be a third value, a step size.
  • We can omit start and stop indices to take the whole list.

    s = "hi there" print(s[::1]) print(s[::2]) print(s[::3]) print(s[::5]) print(s[5])

    hi there h hr htr he e

  • Basically, take indices that are multiples of the step size.

Slices

• Slices take part of a string, based on indices.
  • There may be a third value, a step size.
  • We can omit start and stop indices to take the whole list.
  • Negative steps are permitted.

    s = "hi there" print(s[::-1]) print(s[::-2]) print(s[::-4]) print(s[-4])

    ereht ih eeti et h

  • This is much weirder.
  • In practice: basically only -1 is ever used, to reverse things.

Today

• Review
  • Problem 1
    • ✓ Numerical Arithmetic
    • ✓ Booleans
    • ✓ String Slices
  • Problem 2: Program Tracing
  • Problem 3
    • For
    • If
    • Print
    • Return
  • Problem 4: Strings

Tracing

• Take your time. def enigma(s, i): return s[:3:-3*i] def mystery(w): s = "" for i in range(1,3): s += enigma(w, i) return s print(mystery("abcdefgh"))
• Start at the beginning.

Tracing

• Take your time. def enigma(s, i): return s[:3:-3*i] def mystery(w): s = "" for i in range(1,3): s += enigma(w, i) return s print(mystery("abcdefgh")) # start here
• "def" lines must be called, one is called at the end.

Tracing

• We can imagine the code isn't in a def, like so: def enigma(s, i): return s[:3:-3*i] w = "abcdefgh" # was def mystery(w): s = "" for i in range(1,3): s += enigma(w, i) print(s) # was return s
• Same as having the code block of mystery run then print the return value.

Tracing

• We can do the same with "enigma"
• But we have to be careful!
• "enigma s" and "mystery w" are the same
• "enigma s" and "mystery s" are totally different. def enigma(s, i): return s[:3:-3*i] w = "abcdefgh" s = "" for i in range(1,3): s += w[:3:-3*i] # was enigma(w, i) print(s)
• Same as having the code block of mystery run then print the return value.

Tracing

• Simplify. w = "abcdefgh" s = "" for i in range(1,3): s += w[:3:-3*i] print(s)
• This is all that is left.

Tracing

• We can expand the for loop. w = "abcdefgh" s = "" s += w[:3:-3*1] s += w[:3:-3*2] print(s)

Tracing

• Do some multiplications. w = "abcdefgh" s = "" s += w[:3:-3] s += w[:3:-6] print(s)

Tracing

• These indices:
  • Read backwards
  • End at position 3 (the fourth letter, 'd')
• So we remove those four letters and the ending indices. w = "efgh" s = "" s += w[::-3] s += w[::-6] print(s)

Tracing

• Perhaps easier with the string right there. s = "" s += "efgh"[::-3] s += "efgh"[::-6] print(s)

Tracing

• Perhaps easier with the string right there. s = "" s += "efgh"[::-3] s += "efgh"[::-6] print(s)
• For this one: "efgh"[::-3]
  • Start at the end with 'h'
  • Go BACKWARD g -> f -> e
  • This is 'he'

Tracing

• Perhaps easier with the string right there. s = "" s += "he" s += "efgh"[::-6] print(s)
• For this one: "efgh"[::-6]
  • Start at the end with 'h'
  • There aren't 6 letters, so it's just 'h'.

Tracing

• Perhaps easier with the string right there. s = "" s += "he" s += "h" print(s)
• For this one: "efgh"[::-6]
  • Start at the end with 'h'
  • There aren't 6 letters, so it's just 'h'.

Tracing

• We can combine the += into a big string add. s = "" + "he" + "h" print(s)

Tracing

• We do the string add... s = "heh" print(s)

Tracing

• When reviewing, we can check in Python: def enigma(s, i): return s[:3:-3*i] def mystery(w): s = "" for i in range(1,3): s += enigma(w, i) return s print(mystery("abcdefgh"))
• That leaves us with: print("heh")

Today

• Review
  • Problem 1
    • ✓ Numerical Arithmetic
    • ✓ Booleans
    • ✓ String Slices
  • ✓ Problem 2: Program Tracing
  • Problem 3
    • For
    • If
    • Print
    • Return
  • Problem 4: Strings

for / if / print / return

• From zero up to n:
  • Print "even" or "odd" for each number, and
  • Return the sum of the even numbers.
  def even_sum(n):

for / if / print / return

• From zero up to n:
  • Print "even" or "odd" for each number, and
  • Return the sum of the even numbers.
  def even_sum(n): for i in range(n): # from zero up to n

for / if / print / return

• From zero up to n:
  • Print "even" or "odd" for each number, and
  • Return the sum of the even numbers.
  def even_sum(n): for i in range(n): # from zero up to n print("even") # print even

for / if / print / return

• From zero up to n:
  • Print "even" or "odd" for each number, and
  • Return the sum of the even numbers.
  def even_sum(n): for i in range(n): # from zero up to n if # something here print("even") # print even else: print("odd") # or odd
• Even numbers are divisible by two, so...

for / if / print / return

• From zero up to n:
  • Print "even" or "odd" for each number, and
  • Return the sum of the even numbers.
  def even_sum(n): for i in range(n): # from zero up to n if i % 2 == 0: print("even") # print even else: print("odd") # or odd

for / if / print / return

• From zero up to n:
  • Print "even" or "odd" for each number, and
  • Return the sum of the even numbers.
  def even_sum(n): result = 0 # return sum for i in range(n): # from zero up to n if i % 2 == 0: print("even") # print even else: print("odd") # or odd return result # return sum

for / if / print / return

• From zero up to n:
  • Print "even" or "odd" for each number, and
  • Return the sum of the even numbers.
  def even_sum(n): result = 0 # return sum for i in range(n): # from zero up to n if i % 2 == 0: result += i # sum of evens print("even") # print even else: print("odd") # or odd return result # return sum

for / if / print / return

• From zero up to n:
  • Print "even" or "odd" for each number, and
  • Return the sum of the even numbers.
  result = even_sum(7) print("Now result...") print(result)
• We'll see... even odd even odd even odd even Now result... 12

Today

• Review
  • Problem 1
    • ✓ Numerical Arithmetic
    • ✓ Booleans
    • ✓ String Slices
  • ✓ Problem 2: Program Tracing
  • ✓ Problem 3
    • ✓ For
    • ✓ If
    • ✓ Print
    • ✓ Return
  • Problem 4: Strings

Announcements

• Midterm on Friday.
  • Practice Exams posted
  • There is a practice exams as PDFs on the course webpage.
• Sections this week on Midterm review.