Python for beginners in Tieto Czech: Conditionals, Loops

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Conditionals, Loops

Import statement

Typical import:

import something

from somemodule import somefunction

from somemodule import somefunction, anotherfunction

from somemodule import *

from somemodule import somefunction as different_name

Boolean values

False:

False    None    0    ""    ()    []    {}

True:

True    1   "Some string"   (1, 2)  [1, 2]  {1: 'One'}

if-elif-else

Most common decision construction is if-elif-else.

if condition1:
    body1
elif condition2:
    body2
elif condifion3:
    body3
.
.
.
elif condition(n-1)
    body(n-1)
else:
    body(n)

Examples:

num = int(input('Enter a number: '))
if num > 0:
    print('The number is positive')
elif num < 0:
    print('The number is negative')
else:
    print('The number is zero')

Example with pass:

if x < 5:
    pass
elif x > 5:
    pass
else:
    x = 5

Nested block example:

name = input('What is your name? ')
if name.endswith('Gumby'):
    if name.startswith('Mr.'):
        print('Hello, Mr. Gumby')
    elif name.startswith('Mrs.'):
        print('Hello, Mrs. Gumby')
    else:
        print('Hello, Gumby')
else:
    print('Hello, stranger')

Comparison Operators

Expression	Description
x == y	x equals y
x < y	x is less than y
x > y	x is greater than y
x >= y	x is greater than or equal to y
x <= y	x is less than or equal to y
x != y	x is not equal to y
x is y	x and y are the same object
x is not y	x and y are different objects
x in y	x is a member of the container (e.g., sequence) y
x not in y	x is not a member of the container (e.g., sequence ) y

is: The identity operator

>>> x = y = [1, 2, 3]
>>> z = [1, 2, 3]
>>> x == y
True
>>> x == z
True
>>> x is y
True
>>> x is z
False

Loops

while loops

while condition:
    body
else: # optional
    code continuation

Example:

x = 1
while x <= 100:
    print(x)
    x += 1

for loops

for variable in sequence:
    body
else: # optional
    code continuation

Example:

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
for number in numbers:
    print(number)

range() function

Function range can be used in combination with len and for loop like this:

>>> x = [1, 2, 3, -7, 5, 10, -5, 4]
>>> for i in range(len(x)):
...  if x[i] < 0:
...    print("Negative number on index ", i)
...
Negative number on index  3
Negative number on index  6

>>> range(10)
range(0, 10)

The following program writes out the numbers from 1 to 100:

for number in range(1,101):
    print(number)

Iterating over dictionaries:

d = {'x': 1, 'y': 2, 'z': 3}
for key, value in d.items():
    print(key, 'corresponds to', value)

Breaking out of loop

break

To end (break out of) a loop, you use break.

>>> x = 1
>>>
>>> while True:
...   x += 1
...   if x == 10:
...     break
...   else:
...     print("Value is ", x)
...
Value is  2
Value is  3
Value is  4
Value is  5
Value is  6
Value is  7
Value is  8
Value is  9

continue

It causes the current iteration to end and to “jump” to the beginning of the next.

>>> x = 1
>>>
>>> while True:
...   x += 1
...   if x < 10:
...     continue
...   else:
...     print("x has the value ", x)
...     break
...
x has the value  10

Toto je stránka lekce z kurzu, který probíhá nebo proběhl naživo s instruktorem.

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