3. How to Control Flows - if-else, loops; Something more

• ==    equal to
• !=    not equal to
• >    greater than
• >=    greater than or equal to
• <    less than
• <=    less than or equal to
• Let's try the next example with the console.

  age = 21
  age == 40
  age < 40
  age <= 41
  age > 40
  age >= 40
  

• not
• and
• or
• Do we remember the truth tables?
• Let's try the next example with the concole.

  age = 21
  age <= 16 or age > 35  # What value?
  (age > 16) and (age <= 35)  # ???
  not age > 35  # ???
  

• Comparison operators have higher precedences than boolean operators.

if condition:
    statement or statements    # There should be an indentation before a statement or indentaitons before statements.

age = 21
if age < 35:
    print("Young")
    print(age)

row = ???(input("Enter the row number: ")  # Integer 0, 1, or 2 is expected.
if row < 0 ??? row > 2:
    print("Wrong row number")
else
    print("Correct row number")

age = 40
if age < 35:
    print("Young")
    print(age)
else:
    print("Not young anymore")

age = 40
if age < 35:
    print("Young")
print(age)

age = input("Age: ")
if age >= 20:
    if age >= 60:
        print("Well aged");
    else:
        print("Adult")
else:
    print("Teenager, or")
    print("Kid")

age = input("Age: ")
name = ???("Name: ")

if name == "John" and age == 20:
    print("John Doe")
else:
    if name == 'John' ??? age == 25:
        print("John Gilbert")
    else:
        print("Who are you?")
        
if name == "John" and age == 20:
    print("John Doe")
elif name == 'John' ??? age == 25:
    print("John Gilbert")
else???
    print("Who are you?")

• Let's think about the agorithm that reads numbers and finds the average. Any good idear?

  Repeat 20 times the followings:
      Read a number
      Add the number to a variable
  Divide the variable by 20
  Print the result
  

• The repeatition of a block is called a loop.
• A question is how to repeat the loop 20 times. Any good idea? Here is a more detail algorithm.

  count = 0
  Repeat the followings while count < 20:
      Read a number
      Add the number to a variable
      Increase count by 1
  Divide the variable by 20
  Print the result
  

• How to read a number and add the number to a variable? How to increase the counter variable in the above algorithm?

  count = 0
  Repeat the followings while count < 20:
      number = Read a number
      sum = sum + number
      count = count + 1
  Divide the variable by 20
  Print the result
  

• while loop statement

  while condition:
      ...
  

• Let's write the correct code for the above algorithm to find the average.

  count = ???
  sum = ???
  while count < 20:
      number = input("Enter a number: ")
      sum = sum + number
      count = count + 1
  average = sum / 20
  print("Average = " + average);
  

• Let's try the above code. Anthing wrong? How to fix?

• Can we write an algorithm to solve the above problem?

  While the user wants to keep playing, repeat the followings:
      Print the value;
      Increase the value;
  

• What to do with "While the user wants to keep playing"?

  While answer is 'yes', repeat the followings:
      Print the value
      Increase the value
      answer = input("Continue? ")
  

• Let's write the code for the above algorithm.

  ????
  while answer == 'yes':
      print(value)
      value = value + 1
      answer = input("Continue? ")
  

• Let's try the above code. Anthing wrong? How to fix?

• Let's write an algorithm to solve the above problem.

  Repeat the followings forever:    # forever? how?
      If the user enters a negative value:    # how?
          break the loop    # how?
      Add the value into sum    # how?
  Print the sum
  

• Let's write the code for the above algorithm. We need to know how to break out a loop when the input is negative.

  ????
  while True:  # called infinite loop
      value = input("Enter a value: ")
      If value < 0:
          break    # the break statement to exit out of a loop
      sum = sum + value
  print("The sum is " + sum)
  

• Let's try the above code. Anthing wrong? How to fix?

• Let's write an algorithm to solve the above problem. We need to think there can be short password phrases.

  Repeat the followings forever:    # how?
      password <- Read a password phrase
      If the length of password < 5:
          Skip to the next password    # how?
      Print password
      count = count + 1
      if count >= 5:
          Stop the loop    # how?
  

• Let's write the code for the above algorithm. You need to know how to skip the loop to the next iteration.

  ????
  while True:
      password = input("Enter a password: ")
      if len(password) < 5:
          print("Too short password phrase")
          continue    # the continue statement to the next iteration of a loop
      print(password)
      count = count + 1
      if count >= 5:
          break    # the break statement to exit out of a loop
  

• Let's try the above code. Anthing wrong? How to fix?

• Ininite loop
• If the user enters the correct row value, then the control needs to break out of the loop.
• Let's write the code for the above algorithm.

  ????  # infinite loop
      row = int(input("Enter row number: "))
      if ????
          ????
  ????  # infinite loop
      column = int(input("Enter column number: "))
      if ????
          ????
  print(row)
  print(column)
  

for i in range(10):
    print(i)

• Algorithm?

  Repeat the followings with i from 1 upto 1000:
      sum = sum + i
  Print sum
  

• How to do with "Repeat the followings with i from 1 upto 1000"?

  for i in range(1001):    # 0, 1, 2, ..., 1000
  

• Let's write the code for the above algorithm.

  for i in range(1001):
      sum = sum + i
  print(sum)
  

• Let's try the above code. Anthing wrong? How to fix?

strww = 'What a wonderful world!'
print('|', end='')
for c in strww:  # for each character in strww
    print(c, end='')  # end='' makes the next character be printed in the same line.
print('|')
# another idea using string concatenation
prtstr = ''
prtstr = prtstr + '|'  # string concatenation
for c in strww:  # for each character in strww
    prtstr = prtstr + c
prtstr = prtstr + '|'
print(prtstr)

range(end)    # 0, ..., end-1
range(start, end)    # start, start+1, ..., end-1
range(start, end, step)    # start, start+step, ...

range(10)    # 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
range(3, 10)    # 3, 4, 5, 6, 7, 8, 9
range(0, 10, 2)    # 0, 2, 4, 6, 8

for i in range(10, -5, -2):    # 10, 8, 6, 4, 2, 0, -2, -4
    print(i)

????
for i in range(????):
    ????
print(sum)

import random
for i in range(5):
    r = random.randint(0, 19)    # .randint() - a random integer in [0, 19] (or [0, 20))
    print(r)

import random
for random in range(5):
    r = random.randint(10, 100-1)
    print(r)

import sys, os, random

from random import *
for i in range(5):
    r = randint(-10, 10)    # randint() is defined in random.
    print(r)
for i in range(5):
    r = random()    # random() generates a random floats in [0, 1).
    print(r)
for i in range(5):
    r = uniform(-5, 2.5)    # uniform(a, b) generates a random floats in [a, b).
    print(r)

import random
for i in range(1000):
    r = random.randint(0, 20-1)
    if r == 0 or r == 1:
        exit()
    print(r)