Python - loops · The while loop Jan Hnilica Computer modelling 4 3 • there can be a block of statements instead of a single statement • the else statement (in the second example)

Jan Hnilica Computer modelling 4 1

Python - loops

Loops

2 Jan Hnilica Computer modelling 4

• loops allow to repeatedly execute a statement (or a block of statements) as long as a condition remains True

• there are 2 types of loops in the Python language: • the while loop • the for loop

before the loop

statements of the loop

after the loop

condition

True

False

The while loop


• there can be a block of statements instead of a single statement • the else statement (in the second example) is not obligatory

(it is executed when the condition becomes False) • the condition is tested before the first iteration

⇒ if it evaluates False, no iteration is executed

while condition: statement

while condition: statement1 else statement2

x = 1 while x <= 5: print(x) x = x + 1

output: 1 2 3 4 5

The while loop examples


number = 2 while number % 2 == 0: number = int(input("set an odd number: ")) print("thanks, you have set", number)

N = int(input("set a number: ")) x = 1 SUM = 0 while x <= N: SUM = SUM + x x = x + 1 print("sum 1 :", N, "is", SUM)

output: set a number: 25 sum 1 : 25 is 325

• calculation of the sum of 1 : N

• communication with the user until an odd number is obtained:

The for loop


for variable in object: statement

• the object • must be iterable – it must be composed from elements, which are

accessible through indices in square brackets • example of such object is a string (composed from individual characters)

• in individual iterations the elements of the object are copied one by one to the variable; the variable is then accessible in the statement

• the block of statements can be used as well

for variable in object: statement1 else statement2

string = "hello" for i in string: print(i)

output: h e l l o

The function range


range(x)

• returns a sequence of integers, which can be used as an object in the for loop • there are 3 possible ways how to call the function:

returns the sequence from 0 to (x – 1)

for i in range(5): print(i, end = " ")

0 1 2 3 4 ⇒

range(x, y) returns the sequence from x to (y – 1)

for i in range(5, 11): print(i, end = " ")

5 6 7 8 9 10 ⇒

range(x, y, step) returns the sequence from x to (y – 1) with a given step

for i in range(0, 11, 2): print(i, end = " ")

0 2 4 6 8 10 ⇒

Note: the function range returns an object of the class range (we will learn about classes later…)

The for loop examples


N = int(input("set a number: ")) SUM = 0 for i in range(1, N + 1): SUM += i print("sum 1 :", N, "is", SUM)

• calculation of a sum from 1 to N:

n = int(input("set a number: ")) factorial = 1 for i in range(2, n + 1): factorial *= i print("factorial:", factorial)

• calculation of a factorial

Nested loops


• any loop can be nested in a body of another one • the following example prints the square to the screen:

width = 10 length = 5 for i in range(length): # outer for j in range(width): # inner print("*", end = "") print() # new line

• at each iteration of the outer loop all its statements are executed: 1. the full inner loop 2. the print of the new line

• levels of nesting are given by an indentation ⇒ therefore the print of the new line is subordinated to the outer loop

********** ********** ********** ********** **********

⇒

Nested loops


• three levels of nesting: number_of_squares = 3 width = 10 length = 5 for i in range(number_of_squares): for j in range(length): for k in range(width): print("*", end = "") print() # new row print() # a space between squares

********** ********** ********** ********** ********** ********** ********** ********** ********** ********** ********** ********** ********** ********** **********

⇒

The break statement


• allows to immediately exit the loop (regardless of a driving condition) • when the break statement is reached, the program exits the loop, all remaining

statements of the loop are skipped and the program continues with the first statement after the loop

• the break statement is always connected with a condition Example

• the loop works with keyboard input • the input 0 is a signal to finish the loop

while True: # the never-ending loop (can be interrupted only by break) x = int(input()) if x == 0: break # exit the loop else: ... # do something with x how to write such loop without the break?

The continue statement


• allows to immediately finish the actual iteration • when the continue statement is reached, the actual iteration is finished, all

remaining statements of the loop are skipped and the program continues with the condition testing in the top of the loop

• the continue statement is always connected with a condition Example

• the loop reads a number x from the keyboard and prints the value (1/x) • the input -1 is a signal to finish the loop

x = 1 # initial value (in order to get into the loop) while x != -1: x = int(input()) if x == 0: print("division by zero is illegal!!!") continue print(1/x)

Notes to the break and continue statements


• if the loop has the else branch, it is not executed, if the loop is finished by the break statement

• both break and continue statements are quite dangerous and should be used with caution and after some consideration

• both these statements make the loop hard to read and understand, as they cause sudden jumps in the program flow

• often it is better to use the if statement to skip some lines within a body of loop than to use the continue statement

• the break statement can often be replaced by better formulation of a driving condition

The pass statement


• null statement, it means “do nothing” • sometimes it is useful (sometimes it is even necessary)

if condition: ... # do something else pass # do nothing

List


• compound data type, containing an ordered sequence of variables • example: list my_list containing integers 5, 8, 2, 7, 9

my_list 5 8 2 7 9 index 0 1 2 3 4

element my_list[0] my_list[1] my_list[2] my_list[3] my_list[4]

• individual elements are accessible through a name of a list followed by an index in square brackets: list_name[index]

Basic rules: • indices always start from 0 • a list containing N elements has indices from 0 to (N – 1)

Creation of a list


• manually, using the square brackets my_list = [] # an empty list my_list = [4, 5, 7, 1, 3] # a list containing integers my_list = [4, "hello", 2.133] # a list composed from different data types

• using the list function my_list = list() # empty list

• the list function can take one argument – an iterable object (the elements of a new list are created from the elements of the object)

my_list = list(range(10)) # list containing integers 0 – 9 my_list = list("hello") # list containing the characters of the string

Working with lists


• accessing elements from the end using negative indices

my_list = list("abc") print(my_list[-1], my_list[-2])

⇒ c b

• individual elements of a list are mutable (contrary to the string data type)

my_list = list("abc") my_list[0] = "A" print(my_list)

⇒ ['A', 'b', 'c']

• a list can be printed using the print function

my_list = list("abc") print(my_list)

⇒ ['a', 'b', 'c']

• a whole list can be multiplied by * operator (it returns a new list variable)

my_list = list("abc") new_list = my_list * 2 print(new_list)

⇒ ['a', 'b', 'c', 'a', 'b', 'c']

Working with lists


• a slice from the list • my_list[x:y] returns a list containing the elements from x to (y – 1) • my_list[x:y:z] returns a list containing the elements from x to y with a step z

my_list = list(range(10)) print(my_list) my_list_2 = my_list[0:3] print(my_list_2) my_list_3 = my_list[0:10:2] print(my_list_3)

⇒ [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [0, 1, 2] [0, 2, 4, 6, 8]

• any of indices in a slice can be omitted

my_list_2 = my_list[::3] print(my_list_2)

⇒ [0, 3, 6, 9]

each third from the beginning to the end

Functions working with lists


• the functions are called using a syntax: function(list) • these functions can work even with other data structures

len(my_list) # returns a number of elements min(my_list) # returns a minimum value max(my_list) # returns a maximum value sum(my_list) # returns a sum of elements sorted(my_list) # return sorted copy of list (does not modify the original) all(my_list) # returns True if all of elements evaluates to True any(my_list) # returns True if at least one of elements evaluates to True

List methods


• in general - methods are the functions of a particular class • the methods of the class list are called using a syntax: list.method() • some of the list methods:

my_list.append(x)

• appends a variable x to the end of the list my_list.pop()

• returns the last element of a list (the element is removed from the list) my_list.pop(i)

• removes and returns i-th element my_list.sort()

• sorts the elements my_list.clear()

• removes all elements

(you will learn about classes later)

List methods


# code ml = list() for i in range(10, 0, -1): ml.append(i) print("original:", ml) ml.sort() print("sorted:", ml) for i in range(len(ml)): print("pop:", ml.pop())

# output original: [10, 9, 8, 7, 6, 5, 4, 3, 2, 1] sorted: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] pop: 10 pop: 9 pop: 8 pop: 7 pop: 6 pop: 5 pop: 4 pop: 3 pop: 2 pop: 1

Passing through list elements


ml = list(range(5)) for i in ml: print(i)

• typically the for loop is used

• the elements can be modified in this way

0 1 2 3 4

⇒

ml = [8, 4, 3] print(ml) for i in range(len(ml)): ml[i] = ml[i] + 1 print(ml)

⇒ [8, 4, 3] [9, 5, 4]

Passing through list elements


ml = list(range(5)) index = 0 while index < len(ml): print(ml[index]) index += 1

• the while loop can be used as well

0 1 2 3 4

⇒

Deletion elements


ml = list(range(5)) print(ml) del ml[1] # the same as ml.pop(1) print(ml)

• function del my_list[i] deletes the i-th element

[0, 1, 2, 3, 4] [0, 2, 3, 4] ⇒

• the slice can be deleted as well

ml = list(range(5)) print(ml) del ml[0:3] # deletes the first two elements print(ml) [0, 1, 2, 3, 4]

[3, 4] ⇒

2D list (matrix)


• = list of lists (list, whose elements are lists)

Example: matrix 3 × 4: 1 2 7 5

9 6 3 8

3 4 2 5

is in Python represented:

1 2 7 5

9 6 3 8

3 4 2 5

# manual creation matrix = [[1, 2, 7, 5], [9, 6, 3, 8], [3, 4, 2, 5]]

matrix[0][0] matrix[0][3]

matrix[2][3]

matrix[0]

matrix[1]

matrix[2]

Creation of a matrix of size m×n


• creation of a matrix m×n filled with zeros

# solution 1 matrix = [] for i in range(m): temp = [] # temporary list for j in range(n): temp.append(0) matrix.append(temp) # solution 2 matrix = [] for i in range(m): matrix.append([0] * n) # use of multiplied list

m

n

Creation of a matrix of size m×n


This solution does not work:

# solution 3 matrix = [[0] * n] * m

m

n

Reason: [0] * n is only a reference to a list, therefore the matrix now contains three references to the same list (if you set matrix[0, 0] = 1, also matrix[1, 0] and matrix[2, 0] will be 1)

Passing through a matrix


• incrementing of all elements by 1

for i in range(m): for j in range(n): matrix[i][j] += 1

m

n

• sum of individual elements

s = 0 for i in range(m): for j in range(n): s += matrix[i][j]

Conclusion


• more information about the list methods can be obtained by typing

help(list)

• a complete enumeration of all functions of the class list can be obtained by

dir(list)

• note: the functions help and dir provide information also about other classes…

Documents

Python - loops · The while loop Jan Hnilica Computer modelling 4 3 • there can be a block of statements instead of a single statement • the else statement (in the second example)