2.4. If statements
An if statement is a conditional statement that runs or skips code based on whether a condition is True or False.
2.4.1 True or False
In order to work with if statements in Python, we need to recall our Boolean data type. A Boolean data type is a data type that has one of two possible values (usually denoted true and false), intended to represent the two truth values of logic and Boolean algebra. In Python, the two possible values are True and False. True is represented as 1 and False is represented as 0. It is important to note that True and False are not the same as 1 and 0.
Let us see some examples:
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| # True or False statements with comparisons
print(5==5)
print(5!=5)
print(5>5)
print(5<5)
print(5>=5)
print(5<=5)
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| True
False
False
False
True
True
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| # True or False statements with boolean operators
print(5==5 and 5!=5)
print(5==5 or 5!=5)
print(not 5==5)
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| print(bool(0))
print(bool(1))
print(bool(''))
print(bool('hello'))
print(bool(' '))
print(bool(None))
a = 0
print(bool(a))
b = 2
print(bool(b))
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| False
True
False
True
True
False
False
True
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| Condition | True | False |
|---|
| 5 == 5 | True | False |
| 5 != 5 | False | True |
| 5 > 5 | False | False |
| 5 < 5 | False | False |
| 5 >= 5 | True | False |
| 5 <= 5 | True | False |
| A | B | NOT (A and B) |
|---|
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 0 |
| A | B | NOT (A or B) |
|---|
| 0 | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
NOT AND is equivalent to NOT(A and B) == (NOT A) or (NOT B)NOT OR is equivalent to NOT(A or B) == NOT(A) and NOT(B)
2.4.2 if statement
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| # if statement
a = 5
if a == 5:
print('a is 5')
else:
print('a is not 5')
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| # if else statement
a = 4
if a == 5:
print('a is 5')
else:
print('a is not 5')
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If we need to check more than one condition, we can use elif keyword. It is used to indicate the next condition if the previous condition is false. The general structure of an if statement with elif is as follows:
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| if condition1:
# code to be executed if condition1 is True
elif condition2:
# code to be executed if condition1 is False and condition2 is True
else:
# code to be executed if condition1 and condition2 are False
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| # example if elif else
a = 5
if a == 5:
print('a is 5') # condition is true
elif a == 4: # condition is not true
print('a is 4')
else: # condition is not true
print('a is not 5 or 4')
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2.4.3 Nested if statements
- if statement can be nested inside another if statement. This means that an if statement can be inside another if statement.
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| # nested if statement
a = 5
b = 10
if a == 5:
if b == 10:
print('a is 5 and b is 10')
else:
print('a is 5 but b is not 10')
else:
print('a is not 5')
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2.4.4 Match Statements
Match Statement
Let us first see an example in if-elif-else statement that we will then convert into match statement.
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| # example with if elif else
user_input = int(input('Enter a number: '))
if user_input == 1:
print('You entered 1')
elif user_input == 2:
print('You entered 2')
else:
print('You did not enter 1 or 2')
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| You did not enter 1 or 2
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| # same code with match
user_input = int(input('Enter a number: '))
match user_input:
case 1:
print('You entered 1')
case 2:
print('You entered 2')
case _:
print('You did not enter 1 or 2')
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Using Match with conditionals
- We can use match statement with conditionals.
- we will use
_ in the match statement to indicate the case with certain condition. - we also use
| to indicate OR condition. - we also use
& to indicate AND condition. - we also use
not to indicate NOT condition.
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| # example with if elif else
user_input = int(input('Enter a number: '))
if user_input == 1 | 2:
print('You entered 1 or 2')
else:
print('You did not enter 1 or 2')
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| # same code with match
user_input = int(input('Enter a number: '))
match user_input:
case 1 | 2:
print('You entered 1 or 2')
case _:
print('You did not enter 1 or 2')
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| # example with if elif else
user_input = int(input('Enter a number: '))
if user_input == 0:
print('You entered 0')
elif user_input > 0:
print('You entered a positive number')
elif user_input < 0:
print('You entered a negative number')
else:
print('You did not enter a number')
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| # same code with match
user_input = int(input('Enter a number: '))
match user_input:
case 0:
print('You entered 0')
case _ if user_input > 0:
print('You entered a positive number')
case _ if user_input < 0:
print('You entered a negative number')
case _:
print('You did not enter a number')
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Let use show now another beautiful example.
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| # example with match
def process_data(data):
match data:
case str():
return data.upper()
case list() as l if len(l) > 2:
return l[0], l[-1]
case int() | float():
return data * 2
case _:
return "Unknown data type"
print(process_data('hello'))
print(process_data([1, 2, 3, 4, 5]))
print(process_data(5))
print(process_data(5.5))
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| HELLO
(1, 5)
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11.0
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| # rewritten with if elif else
def process_data(data):
if isinstance(data, str):
return data.upper()
elif isinstance(data, list) and len(data) > 2:
return data[0], data[-1]
elif isinstance(data, (int, float)):
return data * 2
else:
return "Unknown data type"
print(process_data('hello'))
print(process_data([1, 2, 3, 4, 5]))
print(process_data(5))
print(process_data(5.5))
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| HELLO
(1, 5)
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11.0
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2.5 Loops
- A loop is a sequence of instructions that is repeated until a specific condition is met.
- We have different types of loops in Python:
2.5.1 for loop
For Loop
- A for loop is used to iterate over a sequence (list, tuple, string, dictionary, set, etc.) and execute a block of code for each element in the sequence.
- The general structure of a for loop is as follows:
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| for item in sequence:
# code to be executed for each item in the sequence
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- there’s a special sequence called
range that can be used to generate a sequence of numbers. The general structure of a range is as follows:
Range Function
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| range(start, stop, step)
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start is the starting number of the sequence (optional, default is 0)stop is the ending number of the sequence (mandatory, exclusive)step is the difference between each number in the sequence (optional, default is 1)
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| # basic for loop with range
for i in range(5):
print(i) # 0, 1, 2, 3, 4
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| # for loop with start and end range
for i in range(2, 5):
print(i) # 2, 3, 4
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| # for loop with start, end and step
for i in range(2, 5, 2):
print(i) # 2, 4
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| # we can go backwards
for i in range(5, 2, -1):
print(i) # 5, 4, 3
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We will get back to for loops later, when we learn about other sequences.
2.5.2 while loop
While Loop
- A while loop is used to execute a block of code until a certain condition is met.
- The general structure of a while loop is as follows:
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| while condition:
# code to be executed
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| # while loop
i = 0
while i < 5:
print(i) # 0, 1, 2, 3, 4
i += 1 # increments i by 1. it corresponds to i = i + 1
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This example is quite similar to a for loop. In fact, every for loop can be transformed into a while loop. However, there are some cases where a while loop is more appropriate than a for loop. For example, when we don’t know the number of iterations in advance.
General rule of thumb:
- when the number of iterations is known, that is when we need to loop throug a defined sequence,use a for loop
- when the number of iterations is unknown, use a while loop
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| # while loop without knowing the number of iterations
user_input = ''
while not user_input:
user_input = input('Enter something: ') # if empty string, while loop continues
print(user_input)
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Many times we need to count the number of iterations that a while loop performs. This can be done using a counter variable. It is usually initialized to 0 and then incremented by 1 each time the loop iterates.
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| count = 0
user_input = ''
while not user_input:
user_input = input('Enter something: ')
count += 1
print(f'You entered {user_input} after {count} times')
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2.5.3 Continue, Break and Pass
- The
continue statement is used to skip the rest of the current iteration of a loop. - The
break statement is used to exit a loop. - The
pass statement is used to do nothing at all.
2.5.3.1 continue
continue
- The
continue statement is used to skip the rest of the current iteration of a loop.
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| # continue in a for loop
# skips the rest of the code in the current iteration
for i in range(5):
if i == 2: # if i == 2, it will go to the next iteration
continue
print(i) # 0, 1, 3, 4
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| # continue in a while loop
# skips the rest of the code in the current iteration
i = 0
while i < 5:
if i == 2: # if i == 2, it will go to the next iteration
i += 1
continue
print(i) # 0, 1, 3, 4
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2.5.3.2 break
break
- The
break statement is used to immediately exit a loop.
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| # break in a for loop
# terminates the loop
for i in range(5):
if i == 2: # if i == 2, it will terminate the loop
break
print(i) # 0, 1
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| # break in a while loop
# terminates the loop
i = 0
while i < 5:
if i == 2: # if i == 2, it will terminate the loop
break
print(i) # 0, 1
i += 1
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2.5.3.3 pass
pass
- The
pass statement is used to do nothing at all. - It is used when you don’t want to do anything in a particular iteration of a loop.
- We use it as a temporary placeholder when we don’t want to do anything at the moment in a particular iteration.
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| # pass in a for loop
# terminates the loop
for i in range(5):
pass
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| # pass in a while loop
i = 0
while i < 5:
pass
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Sometimes we need to combine different conditions in a loop to skip, break, or do nothing, depending on the singe condition.
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| # example using break, continue, pass in one loop
for i in range (10):
if i % 2 == 0: # skips even numbers
continue
elif i == 7: # exits the loop if i == 3
break
print(i)
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2.6. Random Module
https://docs.python.org/3/library/random.html
- The
random library in Python is used to generate random numbers. The randint() function from this library is often used when an integer needs to be randomly selected within a certain range. - The
randint() function takes two arguments: the start and the end of the range (both inclusive). It returns a random integer within this range.
Here’s how you can use it:
In this example, random.randint(1, 10) will generate a random integer between 1 and 10 (1 and 10 included).
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| import random
random_number = random.randint(1, 10)
print(random_number)
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| import random
# generate a random number
random_number = random.randint(1, 10)
print(random_number) # 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
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Here’s another example:
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| import random
for _ in range(5):
print(random.randint(1, 100))
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In this example, we generate and print five random integers between 1 and 100.
The random library is not just limited to the randint() function. There are also other functions available that provide more flexibility and control over the randomization process. For example, the random() function returns a random floating point number in the range [0.0, 1.0). Here’s an example:
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| import random
random_float = random.random()
print(random_float)
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In this code, random.random() generates a random float between 0 (inclusive) and 1 (exclusive).