Lecture 03 Iteration in Python, Lecture notes of Design

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Lecture 03

Iteration in Python

based in part on notes from the CS-for-All curriculum developed at Harvey Mudd College

Last Time (lecture 02)

• Conditional Statements and Flow of Control
  • if
  • if-else
  • if-elif-else
    • True/False Blocks (require indentation)
• Variable Scope
  • Local
  • Global
• Memory: Frames and the Stack
  • Tracing global, local, and printed output
  • Functions calling Functions

Review

Default Index/Slicing Values

s == s[:]

s[:] = s[::]

s[::] = s[0:len(s):1]

s[len(s):] == ‘’

s=‘01234’ # len(s) == 5

s[2:] == s[2:5:1]

s[:3] == s[0:3:1]

s[::2] == s[0:5:2]

s[:4:3] == s[0:4:3]

s[1::2] == s[1:5:2]

Lecture 03 Goals

• Introduce Test Driven Design (TDD)
• Iteration
  • Definite vs. Indefinite looping - for loops
  • Element-based vs. Index-based
• List comprehensions
  • Generative vs. Manipulative
  • Uniform vs. Conditional

Test Driven Design Example

Write a function gap(x,y) that returns the distance between the numbers x and y? Use if statements and not a function like abs or max.

1. Think clearly about how each function should work
   • Inputs (what are arguments) Two numbers, x and y
   • Outputs (what should be returned) The distance between x and y, i.e. |x-y|

• Special cases If x==y, must return 0
• Usual cases x > y or x < y

Test Driven Design Example

Write a function gap(x,y) that returns the distance between the numbers x and y? Use if statements and not a function like abs or max.

1. Develop a function signature (def + docstring)

def gap(x,y):

’’’Returns distance between two input numbers.’’’

NOTE: The doc string should explain what the function does (and how to use it, i.e. inputs, outputs) but NOT how it does it.

Improving Tests

1. Add/improve tests as needed
   • Creating student accounts for CS department machines
   • The code was tested and it worked, but it failed to account for cases where there were two sections of the class on CAB (CS 4) - Edge case- a case that will rarely happen, but your program should still be able to handle it
   • For CS logins, add test to make sure it works for class with two sections

Test Driven Design

Now code/test your function, design will be informed by tests that need to pass. def gap_test(): assert gap(10,10)==0, 'x==y test failed' assert gap(1, 10)==9, 'xy test failed’

def gap(x, y): # Fill in after first set of tests! ''' Returns the distance between two input numbers.''' if x > y: return x – y else: return y – x

gap_test() As you proceed keep testing,

1. Add/improve tests as needed 11

• A loop is a sequence of instructions to be repeated
• Definite and Indefinite
  • Definite: repeat exactly X times
  • Indefinite: repeat until some condition changes

Iteration: Loops

This is Bijou. Bijou is demonstrating the following iteration examples:

for every front paw paw = paw + frilly blue glove while sun == shining shed_more_fur()

Definite Loops

based in part on notes from the CS-for-All curriculum developed at Harvey Mudd College

for Loops (cont.)

for i in [1, 2, 3]: print('Warning') print(i)

• General syntax:
• In this case, our sequence is a sequence of values , but it could be any sequence (i.e. for word in list_of_words)
• For each value in the sequence:
• the value is assigned to the variable
• all statements in the body of the loop are executed using that value
• Once all values in the sequence have been processed, the program continues with the first statement after the loop.

for in :

Executing a for Loop

for in :

execute statement after the loop

yes

no

does the more values?

assign the next value in the sequence to variable

execute the statements in the body

sequence have

for i in [1, 2, 3]: print('Warning') print(i)

Executing Our Earlier Example

(with one extra statement) for i in [ 1 , 2, 3]: print('Warning') print(i) print('That's all.')

no

does more values?

assign the next value in the sequence to i

[1, 2, 3] have more? i output/action yes 1 yes

print('That's all.')

print('Warning') print(i)

Executing Our Earlier Example

(with one extra statement) for i in [1, 2, 3]: print('Warning') print(i) print('That's all.')

yes

no

does more values?

assign the next value in the sequence to i

[1, 2, 3] have more? i output/action yes 1 Warning 1

print('That's all.')

print('Warning') print(i)