Python Flow Control & use of functions.pptx

If statements
• Python supports the usual logical conditions from mathematics:
– Equals: a == b
– Not Equals: a != b
– Less than: a < b
– Less than or equal to: a <= b
– Greater than: a > b
– Greater than or equal to: a >= b
• These conditions can be used in several ways, most commonly in "if
statements" and loops.
Example
• If statement:
a = 33
b = 200
if b > a:
print("b is greater than a")
Output:
b is greater than a

Indentation
• Python relies on indentation (whitespace at the beginning
of a line) to define scope in the code. Other programming
languages often use curly-brackets for this purpose.
Example
• If statement, without indentation (will raise an error):
a = 33
b = 200
if b > a:
print("b is greater than a") # you will get an error

Elif statement
• The elif keyword is pythons way of saying "if the previous
conditions were not true, then try this condition".
Example
a = 33
b = 33
if b > a:
elif a == b:
print("a and b are equal") Output:
a and b are equal

Else
• The else keyword catches anything
which isn't caught by the preceding
conditions.
Example
a = 200
b = 33
if b > a:
elif a == b:
print("a and b are equal")
else:
print("a is greater than b")
• You can also have an else without
the elif:
Example
a = 200
b = 33
if b > a:
else:
print("b is not greater than a")
Output:
b is not greater than a
Output:
a is greater than b

Short Hand If
• If you have only one statement to execute, you can put it on the same line as
the if statement.
Example
• One line if statement:
if a > b: print("a is greater than b")
Short Hand If ... Else
• If you have only one statement to execute, one for if, and one for else, you
can put it all on the same line:
Example
• One line if else statement:
a = 2
b = 330
print("A") if a > b else print("B")
This technique is known as Ternary Operators, or Conditional Expressions.
Output:
B

• You can also have multiple else statements on the same line:
Example
• One line if else statement, with 3 conditions:
a = 330
b = 330
print("A") if a > b else print("=") if a == b else print("B")
And
• The and keyword is a logical operator, and is used to combine conditional
statements:
Example
• Test if a is greater than b, AND if c is greater than a:
a = 200
b = 33
c = 500
if a > b and c > a:
print("Both conditions are True")
Output:
=
Output:
Both conditions are True

Or
• The or keyword is a logical operator, and is used to combine conditional statements:
Example
• Test if a is greater than b, OR if a is greater than c:
a = 200
b = 33
c = 500
if a > b or a > c:
print("At least one of the conditions is True")
Nested If
• You can have if statements inside if statements, this is called nested if statements.
Example
x = 41
if x > 10:
print("Above ten,")
if x > 20:
print("and also above 20!")
else:
print("but not above 20.")
Output:
At least one of the conditions is True
Output:
Above ten,
and also above 20!

The pass Statement
• if statements cannot be empty, but if you for some reason
have an if statement with no content, put in the pass
statement to avoid getting an error.
Example
a = 33
b = 200
if b > a:
pass

While Loops
• Python has two primitive loop commands:
– while loops
– for loops
The while Loop
• With the while loop we can execute a set of statements as long as a
condition is true.
Example
• Print i as long as i is less than 6:
i = 1
while i < 6:
print(i)
i += 1
• Note: remember to increment i, or else the loop will continue forever.
Output:
1
2
3
4
5

The break Statement
• With the break statement we can stop the loop even if the
while condition is true:
Example
• Exit the loop when i is 3:
i = 1
while i < 6:
print(i)
if i == 3:
break
i += 1
Output:
1
2
3

The continue Statement
• With the continue statement we can stop the current
iteration, and continue with the next:
Example
• Continue to the next iteration if i is 3:
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
Output:
1
2
4
5
6

The else Statement
• With the else statement we can run a block of code once
when the condition no longer is true:
Example
• Print a message once the condition is false:
i = 1
while i < 6:
print(i)
i += 1
else:
print("i is no longer less than 6")
Output:
1
2
3
4
5
i is no longer less than 6

For Loops
• A for loop is used for iterating over a sequence (that is either a list, a tuple, a
dictionary, a set, or a string).
• This is less like the for keyword in other programming languages, and works
more like an iterator method as found in other object-orientated programming
languages.
• With the for loop we can execute a set of statements, once for each item in a
list, tuple, set etc.
• Even strings are iterable objects, they contain a sequence of characters:
Example
• Loop through the letters in the word "banana":
for x in "banana":
print(x)
Output:
b
a
n
a
n
a

Loop Lists
• You can loop through the list items by using a for loop:
Example
• Print all items in the list, one by one:
thislist = ["apple", "banana", "cherry"]
for x in thislist:
print(x)
Loop Through the Index Numbers
• You can also loop through the list items by referring to their index number.
• Use the range() and len() functions to create a suitable iterable.
Example
• Print all items by referring to their index number:
thislist = ["apple", "banana", "cherry"]
for i in range(len(thislist)):
print(thislist[i])
The iterable created in the example above is [0, 1, 2]
Output:
apple
banana
cherry
Output:
apple
banana
cherry

List Comprehension
• List comprehension offers a shorter syntax when you want to create a new list based on the
values of an existing list.
• Example: Based on a list of fruits, you want a new list, containing only the fruits with the
letter "a" in the name.
• Without list comprehension you will have to write a for statement with a conditional test
inside:
Example
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []
for x in fruits:
if "a" in x:
newlist.append(x)
print(newlist)
With list comprehension you can do all that with only one line of code:
Example
newlist = [x for x in fruits if "a" in x]
print(newlist)
Output:
['apple', 'banana', 'mango']
Output:
['apple', 'banana', 'mango']

The Syntax
newlist = [expression for item in iterable if condition == True]
• The return value is a new list, leaving the old list unchanged.
Condition
• The condition is like a filter that only accepts the items that valuate to True.
Example
• Only accept items that are not "apple":
newlist = [x for x in fruits if x != "apple"]
print(newlist)
The condition if x != "apple" will return True for all elements other than "apple",
making the new list contain all fruits except "apple".
• The condition is optional and can be omitted:
Example
• With no if statement:
newlist = [x for x in fruits]
Output:
['banana', 'cherry', 'kiwi', 'mango']

Iterable
• The iterable can be any iterable object, like a list, tuple, set etc.
Example
• You can use the range() function to create an iterable:
newlist = [x for x in range(10)]
print(newlist)
• Same example, but with a condition:
Example
• Accept only numbers lower than 5:
newlist = [x for x in range(10) if x < 5]
print(newlist)
Expression
• The expression is the current item in the iteration, but it is also the outcome,
which you can manipulate before it ends up like a list item in the new list:
Output:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Output:
[0, 1, 2, 3, 4]

Example
• Set the values in the new list to upper case:
newlist = [x.upper() for x in fruits]
print(newlist)
• You can set the outcome to whatever you like:
• Example
• Set all values in the new list to 'hello':
newlist = ['hello' for x in fruits]
print(newlist)
The expression can also contain conditions, not like a filter, but as a way to manipulate the outcome:
Example
• Return "orange" instead of "banana":
newlist = [x if x != "banana" else "orange" for x in fruits]
print(newlist)
The expression in the example above says:
• "Return the item if it is not banana, if it is banana return orange".
Output:
['APPLE', 'BANANA', 'CHERRY', 'KIWI', 'MANGO']
Output:
['hello', 'hello', 'hello', 'hello', 'hello']
Output:
['apple', 'orange', 'cherry', 'kiwi', 'mango']

Else in For Loop
• The else keyword in a for loop
specifies a block of code to be
executed when the loop is finished:
Example
• Print all numbers from 0 to 5, and
print a message when the loop has
ended:
for x in range(6):
print(x)
else:
print("Finally finished!")
• Note: The else block will NOT be
executed if the loop is stopped by a
break statement.
Example
• Break the loop when x is 3, and see
what happens with the else block:
for x in range(6):
if x == 3: break
print(x)
else:
print("Finally finished!")
Output:
0
1
2
3
4
5
Finally finished!
Output:
0
1
2

Nested Loops
• A nested loop is a loop inside a loop.
• The "inner loop" will be executed one time for each iteration of the "outer loop":
Example
• Print each adjective for every fruit:
adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]
for x in adj:
for y in fruits:
print(x, y)
The pass Statement
• for loops cannot be empty, but if you for some reason have a for loop with no
content, put in the pass statement to avoid getting an error.
Example
for x in [0, 1, 2]:
pass
Output:
red apple
red banana
red cherry
big apple
big banana
big cherry
tasty apple
tasty banana
tasty cherry

Loop Tuples
• You can loop through the tuple items by using a for loop.
Example
• Iterate through the items and print the values:
thistuple = ("apple", "banana", "cherry")
for x in thistuple:
print(x)
Output:
apple
banana
cherry

Loop Sets
• You can loop through the set items by using a for loop:
Example
• Loop through the set, and print the values:
thisset = {"apple", "banana", "cherry"}
for x in thisset:
print(x)
Output:
cherry
banana
apple

Loop Dictionaries
• You can loop through a dictionary by
using a for loop.
• When looping through a dictionary, the
return value are the keys of the
dictionary, but there are methods to
return the values as well.
Example
• Print all key names in the dictionary,
one by one:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
for x in thisdict:
print(x)
Example
• Print all values in the dictionary, one
by one:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
for x in thisdict:
print(thisdict[x])
Output:
brand
model
year
Output:
Ford
Mustang
1964

Example
• You can also use the values()
method to return values of a
dictionary:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
for x in thisdict.values():
print(x)
Example
• You can use the keys()
method to return the keys of a
dictionary:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
for x in thisdict.keys():
print(x)
Output:
Ford
Mustang
1964
Output:
brand
model
year

Example
• Loop through both keys and values, by using the items() method:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
for x, y in thisdict.items():
print(x, y)
Output:
brand Ford
model Mustang
year 1964

Python Flow Control & use of functions.pptx

More Related Content

Similar to Python Flow Control & use of functions.pptx

Recently uploaded

Python Flow Control & use of functions.pptx