## Before session 5

### Loops and iterations

Watch the lecture videos:

*Slides** in English*

*Textbook** in English *

### Quiz

Solve the quiz on loops and iterations in Moodle.

### Examples

Loops (*while* and *for*) allow to execute statements in program many times.

#### Example 1

In many cases, it is possible to solve the problem using either a *while* or a *for* loop. The following two programs calculate the sum of all numbers from 1 to 9.

The first program uses *for* loop:

print("Finding the sum with for loop") sum1 = 0 for i in [1, 2, 3, 4, 5, 6, 7, 8, 9]: sum1 = sum1 + i print("Sum so far", sum1) print("Sum of numbers 1-9 is", sum1)

The second program does the same with the help of *while* loop.

print("Finding the sum with while loop") sum2 = 0 a = 1 while a < 10: sum2 = sum2 + a print("Sum so far", sum2) a += 1 print("Sum of numbers 1-9 is", sum2)

#### Example 2

The following program contains a loop which repeatedly prompts the user for positive integers and outputs the biggest of them at the end. The program continues asking until the user enters 'done'. If the user enters something that is not an integer, the program processes the conversion error using *try* and *except*, prints out a message and skips to the next number. If the user enters an integer but it is not positive, then the program finds this out by an *if* statement.

def maximum(a, b): if a > b: return a else: return b m = 0 while True: line = input("Enter positive integer or 'done': ") if line == "done": break try: x = int(line) if x > 0: m = max(m, x) else: print("Your integer should be positive") except: print("Please enter an integer") if m > 0: print("Maximum is", m)

### Exercises

#### 1. Product

Write a program that prompts the user for a positive integer *n* and prints out the value of the expression

{$$2\cdot\frac21\cdot\frac23\cdot\frac43\cdot\frac45\cdot\ldots\cdot\frac{2n}{2n-1}\cdot\frac{2n}{2n+1}.$$}

Test your program with an increasing set of inputs. Think yourself: what value does this product approach to, as *n* increases?

#### 2. Number of days

Write a function *number_of_days* with one integer parameter, which represents the number of month; the functions returns the number of days in that month. Numbers of months are 1, 2, ..., 12, and let's assume that it is not a leap year. If the number is some other integer, then the program should return âˆ’1.

>>number_of_days(9) 30 >>number_of_days(2) 28

Test the function with different arguments.

Write a program that repeatedly asks the user for number of month and prints out the number of days in that month, until the user enters 'done'. The program should perform all necessary input checks and call the function *number_of_days* only if the number of month is an integer in the range 1, ..., 12.

Enter number of month or 'done':3This month has 31 days Enter number of month or 'done':4This month has 30 days Enter number of month or 'done':fifthPlease enter a valid number Enter number of month or 'done':-1Number of month must be in the range 1-12 Enter number of month or 'done':100Number of month must be in the range 1-12 Enter number of month or 'done':10This month has 31 days Enter number of month or 'done':done

*Optional modification.* Implement the function *number_of_days* in such a way that it returns 28 or 29 for February, depending on whether the current year in the system is ordinary year or leap year.

#### 3. Dragons, snakes, and dinosaurs

After the creation of the world three kinds of animals lived on the earth â€“ dragons, snakes, and dinosaurs. Each dragon eats one snake as a breakfast, each snake eats one dinosaur as a lunch, and each dinosaur eats one dragon as a supper. (If there is not enough food for some animal, it stays without.)

Write a program that asks the user for the initial numbers of dragons, snakes and dinosaurs, and prints out the day of the last meal and what kinds on animals and how many of them will be left after that.

Number of dragons:3Number of snakes:5Number of dinosaurs:9The last meal will be on day 5 There will be 2 snakes left

### Submit your solutions

Submit your solutions of exercises 1, 2, 3 in Moodle under the names *home1.py*, *home2.py*, *home3.py*, respectively.