• strings
• branching - if/elif/else
• indentation
• while loops

• while loops
• for loops
• loop exercises

• the while loop is used to do something as long as a condition holds

while <condition>:
   do_something
   do_something_else
   do_something_more

• You must make sure that the loop terminates
  • Otherwise, you get an infinite loop

• Print the numbers from 1 to a given k (inclusive)

• Print the numbers from 1 to a given k (inclusive)

k = 5
i = 1
while i <= k:
   print(i)
   i = i + 1

## 1
## 2
## 3
## 4
## 5

• Print the odd numbers from 1 to a given k (inclusive)

• Print the odd numbers from 1 to a given k (inclusive)

k = 15
i = 1
while i <= k:
   if i % 2 == 1:
      print(i)
   i = i + 1

## 1
## 3
## 5
## 7
## 9
## 11
## 13
## 15

• Print the odd numbers from 1 to a given k (inclusive)
• Alternative (shorter) solution

k = 15
i = 1
while i <= k:
   print(i)
   i = i + 2

## 1
## 3
## 5
## 7
## 9
## 11
## 13
## 15

• Print all factors of a given number

• Print all factors of a given number

number = 28
i = 1
while i <= number:
   if number % i == 0:
      print(i)
   i = i + 1

## 1
## 2
## 4
## 7
## 14
## 28

• Print whether a given number is prime

• Print whether a given number is prime

number = 28
i = 2
isPrime = True

while i < number and isPrime:
   if number % i == 0:
      isPrime = False
   i = i + 1

if isPrime:
   print(number, "is prime.")
else:
   print(number, "is not prime.")

## 28 is not prime.

• Print whether a given number is prime

number = 29
i = 2
isPrime = True

while i < number and isPrime:
   if number % i == 0:
      isPrime = False
   i = i + 1

if isPrime:
   print(number, "is prime.")
else:
   print(number, "is not prime.")

## 29 is prime.

• The for loop allows iterating in a fixed sequence

for <variable> in <sequence of values>:
   do_something

• each time through the loop, <variable> takes a value from the <sequence of values>
• values are selected according to the given sequence

• The range function provides a sequence of integer values
• range(5) means numbers from 0 (inclusive) to 5 (exclusive)
  • 0, 1, 2, 3, 4
• We can use this with the for loop:

for i in range(5):
   print(i)

## 0
## 1
## 2
## 3
## 4

• Read the code!
  • For i in numbers from 0 to 5, print the value of i.

• You can fine tune range to provide different sequences of numbers
• Normally it has 3 arguments
  • An argument is a parameter input to a function
• range(start, stop, step)
• when we write range(5) it means range(stop = 5)
  • start defaults to 0
  • step defaults to 1

• range(4, 10, 1) means
  • start = 4
  • stop = 10
  • step = 1
• 4, 5, 6, 7, 8, 9

for i in range(4, 10, 1):
   print(i)

## 4
## 5
## 6
## 7
## 8
## 9

• range(2, 8, 2) means
  • start = 2
  • stop = 8
  • step = 2
• 2, 4, 6
• stop is exclusive

for i in range(2, 8, 2):
   print(i)

## 2
## 4
## 6

• range(10, 0, -1) means
  • start = 10
  • stop = 0
  • step = -1
• 10, 9, 8, 7, 6, 5, 4, 3, 2, 1

for i in range(10, 0, -1):
   print(i)

## 10
## 9
## 8
## 7
## 6
## 5
## 4
## 3
## 2
## 1

• The break statement is used to immediately exit a loop
  • while and for loops both work
• Skips remaining expressions in code block
• Exits only innermost loop!

while <condition_1>:       # Loop 1
   while <condition_2>:    # Loop 2
      <expression_a>
      break                # Exits loop 2
      <expression_b>
   <expression_c>

• Stop printing when a number divisible by 7 is found

for i in range(10, 20, 1):
   print(i)
   if i % 7 == 0:
      break

## 10
## 11
## 12
## 13
## 14

• Revisited! Print whether a given number is prime
• We now know the for loop, as well as break
  • Can we use them to produce a shorter algorithm?
  • Faster?
  • Cleaner?
  • More readable?
• A good programmer’s job is not to produce code
  • A good programmer’s job is to produce short, fast, clean, readable code.

• Revisited! Print whether a given number is prime

number = 29

for i in range(2, number):     # step defaults to 1
   if number % i == 0:
      print(number, "is not prime.")
      break

if i == (number - 1):
   print(number, "is prime.")

## 29 is prime.

• Can you be faster?
  • Do you really need range(2, number)?

• We can also provide a sequence literally

for i in [1, 3, 9, 24]:
   print(i)

## 1
## 3
## 9
## 24

for i in ["a", "b", "c", "d"]:
   print(i)

## a
## b
## c
## d

for i in ["a", 1, "c", True]:
   print(type(i))

## <class 'str'>
## <class 'int'>
## <class 'str'>
## <class 'bool'>

• Print a string in reverse.

• Print a string in reverse.

name = "burkay genc"
for i in range(len(name) - 1, -1, -1):
   print(name[i], end = "")

## cneg yakrub

• Check if a substring exists in a string
  • If so, return its position

• Check if a substring exists in a string
  • If so, return its position

s = "Check if a substring exists in a string"
ss = "st"

for i in range(len(s) - len(ss)):
   if s[i:(i + len(ss))] == ss:
      print(i)

## 14
## 24
## 33

• Given a random sequence of numbers, find the value of the maximum number in the sequence

sequence = [4, 7, 1, 0, -8, 12, -3, 1, 4, 11, 9, -2, 10]

• Given a random sequence of numbers, find the value of the maximum number in the sequence

sequence = [4, 7, 1, 0, -8, 12, -3, 1, 4, 11, 9, -2, 10]
max = sequence[0]

for i in sequence:
   if i > max:
      max = i
print(max)

## 12

• Given a random sequence of numbers, find the position of the maximum number in the sequence

sequence = [4, 7, 1, 0, -8, 12, -3, 1, 4, 11, 9, -2, 10]

• Given a random sequence of numbers, find the position of the maximum number in the sequence

sequence = [4, 7, 1, 0, -8, 12, -3, 1, 4, 11, 9, -2, 10]
max = 0

for i in range(len(sequence)):
   if sequence[i] > sequence[max]:
      max = i
print(max)

## 5

• Fibonacci sequence
  • 1, 1, 2, 3, 5, 8, 13, 21, ...
  • \(F_i = F_{i-1} + F_{i-2}\)
• Print the first k Fibonacci numbers

• Print the first k Fibonacci numbers

k = 15
F = 1
F_1 = 0
F_2 = 0
i = 1

while i <= k:
   print(F)
   F_2 = F_1
   F_1 = F
   F = F_1 + F_2
   i = i + 1

## 1
## 1
## 2
## 3
## 5
## 8
## 13
## 21
## 34
## 55
## 89
## 144
## 233
## 377
## 610

• Make a game of guess
  • The user tries to guess a number stored in the program
  • If the user’s guess is higher than the stored value, output Guess is too high, and ask for a new guess
  • If the user’s guess is lower than the stored value, output Guess is too low, and ask for a new guess
  • If the guess is succesfull, output ‘You Win!!!’ and quit the program.

• Write a program that asks for a word from the user
• Then, the program checks whether the word is a palindrome
• A palindrome is a string that is equal to its reverse
  • kayak
  • kelek
  • abcdcba

• Write a program that asks for a string from the user and then asks for a single character
• Then, the program returns the number of occurances of that character in the given string
• For example,
  • Given the string Korkma sönmez bu şafaklarda yüzen al sancak and the character k, the program should output 3
  • K and k are assumed to be different!