Python Programming

Lecture 3 Iteration

3.1 Iteration: while

The while statement


n = 5
while n > 0:
    print(n)
    n = n - 1

print('Blastoff!')



5
4
3
2
1
Blastoff!

  1. Evaluate the condition, yielding True or False.

  2. If the condition is false, exit the while statement and continue execution at the next statement.

  3. If the condition is true, execute the body and then go back to step 1.

    • This type of flow is called a loop because the third step loops back around to the top.

    • We call each time we execute the body of the loop an iteration.

  • The body of the loop should change the value of one or more variables so that eventually the condition becomes false and the loop terminates.

  • If the loop repeats forever, it results in an infinite loop.


n = 10
while True:
    print(n)
    n = n - 1
print('Done!')

  • What will happen if you run this?

  • Ctrl+C to terminate it.

  • Finishing iterations with break


while True:
    line = input('Please enter:')
    if line == 'done':
        break
    print(line)
print('Done!')

  • Finishing iterations with continue


while True:
    line = input('Please enter:')
    if line == '#':
        continue
    if line == 'done':
        break
    print(line)
print('Done!')

Example 1: Even and Odd


numbers = [12, 37, 5, 42, 8, 3]
even = []
odd = []
while len(numbers) > 0:
    x = numbers.pop()
    if x % 2 == 0:
        even.append(x)
    else:
        odd.append(x)
print(even)
print(odd)


[8, 42, 12]
[3, 5, 37]
Example 2: BMI Calculator v2.0

print("BMI指数计算器\n")
while True:
    try: 
        inp_1 = input('请输入您的体重(kg):\n')
        weight = float(inp_1)
        break 
    except: 
        print('请输入数字')
while True:
    try: 
        inp_2 = input('请输入您的身高(cm):\n')
        height = float(inp_2) 
        break
    except: 
        print('请输入数字')

Example 3: Hot Potato Game

  • To begin, let's consider the children's grame Hot potato. In this game children line up in circle and pass an item from neighbor to neighbor as fast as they can. At a certain point in the game, the action is stopped and the child who has the item (the potato) is removed from the circle. Play continues until only one child is left.
  • This game is a modern-day equivalent of the famous Josephus problem. 

name_list = ['a', 'b', 'c', 'd', 'e', 'f']
i = 1
while len(name_list) > 1:
    if i % 7 == 0:
        y = name_list.pop()
    else:
        y = name_list.pop()
        name_list.insert(0, y)
        # print(name_list)
    i = i + 1
print(name_list)
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Exercise

Change-Making Problem（找零问题）

  • 输入一个金额，用最少的硬币数找零（如25分、10分、5分、1分）。用贪心算法解决：
    • 从最大面额开始;
    • 在不超过目标金额的前提下，尽可能多地使用这种硬币;
    • 减去已找出的金额；
    • 对剩余金额重复上述步骤；
    • 当剩余金额为 0 时，算法结束。
    
coins = [25, 10, 5, 1]   # 面额从大到小
amount = 68              # 要找的金额
i = 0                    # 当前硬币索引
used = []                # 记录已使用的硬币

# 在这里添加你的代码

print("使用的硬币列表：", used)

    
使用的硬币列表： [25, 25, 10, 5, 1, 1, 1]

3.2 Iteration: for

Definite loops using for

  • for statement works on the lists.


friends = ['Joseph', 'Glenn', 'Sally']
for friend in friends: 
    print('Happy New Year:', friend)
print('Done!')


Happy New Year: Joseph
Happy New Year: Glenn
Happy New Year: Sally
Done!

  • We refer to the while loop as an indefinite loop because it continues executing until a specific condition becomes False, whereas the for loop iterates over a known set of items, running as many times as there are items in the set.

  • In particular, friend is the iteration variable for the for loop. The variable friend changes for each iteration of the loop and controls when the for loop completes. The iteration variable steps successively through the three strings stored in the friends variable.

  • The indentation errors are common.

  • Python uses indentation to determine when one line of code is connected to the line above it. Some languages require the "end" statement.

  • Always indent the line after the for statement in a loop.


friends = ['Joseph', 'Glenn', 'Sally']
for x in friends: 
print('Happy New Year:', x)
print('Done!')


IndentationError: expected an indented block


message = "Hello Python world!"
    print(message)


IndentationError: unexpected indent

  • Forgetting to indent additional lines (logical error)


friends = ['Joseph', 'Glenn', 'Sally']
for x in friends: 
    print('Happy New Year:', x)
print('Looking forward to seeing you,', x)


Happy New Year: Joseph
Happy New Year: Glenn
Happy New Year: Sally
Looking forward to seeing you, Sally


friends = ['Joseph', 'Glenn', 'Sally']
for x in friends: 
    print('Happy New Year:', x)
    print('Looking forward to seeing you,', x)


Happy New Year: Joseph
Looking forward to seeing you, Joseph
Happy New Year: Glenn
Looking forward to seeing you, Glenn
Happy New Year: Sally
Looking forward to seeing you, Sally

range function

  • The range function generates a sequence of integers.
  • It often used to control the number of iterations in a for loop.
    • 
for value in range(1,4): 
    print(value)

    
1
2
3

    
>>> list(range(1,6)) 
[1, 2, 3, 4, 5]
>>> list(range(6))
[0, 1, 2, 3, 4, 5]
>>> list(range(1, 10, 2))
[1, 3, 5, 7, 9]
>>> list(range(10, 1, -2))
[10, 8, 6, 4, 2]
  • Example: generate a list of square numbers 
    • 
squares = []
for value in range(1,11):
    square = value**2
    squares.append(square)
print(squares)

    
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Replicate the functionality of list functions using for loops.

sum(): how to calculate 1 + 2 + 3 +...+ 100?


total = 0
for value in range(1, 101):
    total = value + total  # total+=value
print(total)

len()


a_list = [3, 41, 12, 9, 74, 15]
count = 0
for x in a_list:
    count = count + 1
print('Count: ', count)

max(), min()


a_list = [3, 41, 12, 9, 74, 15]
largest = a_list[0]
for x in a_list:
    if x > largest :
         largest = x
print('Largest:', largest)

Nested Loops


x = []
for i in range(1,4):
    x.append([])
    for j in range(1,4):
        x[i-1].append(1)
print(x)


[[1, 1, 1], [1, 1, 1], [1, 1, 1]]
Video Thumbnail 

x = []
for i in range(1,4):
    x.append([])
    for j in range(1,i+1):
        x[i-1].append(1)
print(x)






[[1], [1, 1], [1, 1, 1]]


for i in range(1,4):
    for j in range(1,i+1):
        print(i, end=" ")
    print()


1 
2 2 
3 3 3
  • The default value of end is \n meaning that after the print statement it will print a new line. So simply stated end is what you want to be printed after the print statement has been executed

for i in range(1,4):
    for j in range(i):
        print(i,end="+")
    print()


1+
2+2+
3+3+3+

3.3 Money Tracker (记账本) v1.0

Objective: require users to enter expenses one by one, and report the total expenses


# Step 1: Create a list containing lists
print("简易记账本（九月）")
Sept=[]
for date in range(30):
    Sept.append([])

# Step 2: Require users to enter the date
day=int(input("请问输入几号的开销?\n"))


# Step 3: Enter the expenses one by one
print("请输入每一笔开销，结束请输入0：\n") 
while True:
    each = float(input("请输入金额：\n"))
    if each == 0:
        break
    else: 
        Sept[day-1].append(each)
        print("记录成功\n")
  • Multiple days: replace the code in Step 2 (Nested loops)

while True:
    day=int(input("请问输入几号的开销？结束请输入0:\n"))
    if day == 0:
        break
    else:
        # all the programs in Step 3
  • indentation: select programs, tab (or space)
  • cancel indentation: select programs, shift + tab

# Step 4: Calculate the total
total=0
for each_day in Sept:
    total += sum(each_day)

print("本月支出汇总报告")
print("总支出：", total)

Summary

  • Iteration
    • Reading: Python for Everybody Chapter 5