Loops

• While loop
  • Repeatedly echo input() until q

  • q is a “sentinel value”

    str = input()
    while str != 'q':
     print('You said: ', str)
     str = input()
    

• Get numbers until sentinel is seen to compute average (from book)

  values_sum = 0
  num_values = 0
    
  curr_value = int(input())
       
  while curr_value > 0: # Get values until 0 (or less)
      values_sum += curr_value
      num_values += 1
      curr_value = int(input())
  print(f'Average: {values_sum / num_values:.0f}\n')
  

• Print growth of savings in an account (from book)

  initial_savings = 10000
  interest_rate = 0.05
    
  print(f'Initial savings of ${initial_savings}')
  print(f'at {interest_rate*100:.0f}% yearly interest.\n')
    
  years = int(input('Enter years: '))
  print()
    
  savings = initial_savings
  i = 1  # Loop variable
  while i <= years:  # Loop condition
      print(f' Savings at beginning of year {i}: ${savings:.2f}')
      savings = savings + (savings*interest_rate)
      i = i + 1  # Increment loop variable
    
  print('\n')
  

• How to add additional principal once per year?

• Quick intro to a list
  • Note: Avoid using list as a variable name.

  lst = ['a', 'b', 'c', 'd', 'e']
  print(lst[0])
  print(lst[3])
  lst[4] = 'changed'
  print(lst)
  

• Use for loop to sum over a list.
• Use for to loop through a string and count ‘e’s
• inClass basic loops:
  • guessing game
  • count_evens
  • max_int
  • contains_negative
  • is_prime
  • num_primes

Range

• Write tests for strictly-increasing (In-Class 6)

• Try to do strictly-increasing with standard for-each loop.

• When you loop over part of a list or string, you need to specify the indices you want.
• In this case, you often need to know the length of the item:

• Use built-in function len to get the length

  lst = ['a', 'b', 'c', 'd', 'e']
  print(len(lst))
  

• range
  • end value not included.
  • can add a step
  • step can be negative (to count down)
  • What happens if step is negative but range is not range(5, 10, -1)

  • range is better than while since range won’t let you accidentally write an infinite loop

    for index in range(3, 5):
       print(f"value at {index}: {lst[index]}")
    

  • use list(range(...)) to demonstrate what range is created.
• Show how to do is_strictly_increasing with range
• Look at is_equal on in-Class 6.

Break and Continue

• break immediately ends a loop:

  for val in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]:
     if val > 5:
        break
      print(val)
  

• Show how to do is_strictly_increasing with break

• Continue skips the rest of the loop

  for val in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]:
     if val % 3 == 0
        continue
     print(val)
  

Loop else

• If you put an else on a loop, that block runs only if the loop runs to completion.

  for val in [1, 3, 5, 9, 7, 11, 221, 339, 446]:
    if val % 2 == 0:
      print(f"{val} is the first even number on the list")
      break
  else:
    print("There were no even numbers on the list")
  

• When you want both the index and the value, use enumerate

  origins = [4, 8, 10]
  
  for (index, value) in enumerate(origins):
    print(f'Element {index}: {value}')
  

• Do count_successors for homework