elif
Statement
The Python elif
statement allows for continued checks to be performed after an initial if
statement. An elif
statement differs from the else
statement because another expression is provided to be checked, just as with the initial if
statement.
If the expression is True
, the indented code following the elif
is executed. If the expression evaluates to False
, the code can continue to an optional else
statement. Multiple elif
statements can be used following an initial if
to perform a series of checks. Once an elif
expression evaluates to True
, no further elif
statements are executed.
# elif Statement
pet_type = "fish"
if pet_type == "dog":
print("You have a dog.")
elif pet_type == "cat":
print("You have a cat.")
elif pet_type == "fish":
# this is performed
print("You have a fish")
else:
print("Not sure!")
if
Statement
The Python if
statement is used to determine the execution of code based on the evaluation of a Boolean expression.
- If the
if
statement expression evaluates toTrue
, then the indented code following the statement is executed. - If the expression evaluates to
False
then the indented code following theif
statement is skipped and the program executes the next line of code which is indented at the same level as theif
statement.
# if Statement
test_value = 100
if test_value > 1:
# Expression evaluates to True
print("This code is executed!")
if test_value > 1000:
# Expression evaluates to False
print("This code is NOT executed!")
print("Program continues at this point.")
else
Statement
The Python else
statement provides alternate code to execute if the expression in an if
statement evaluates to False
.
The indented code for the if
statement is executed if the expression evaluates to True
. The indented code immediately following the else
is executed only if the expression evaluates to False
. To mark the end of the else
block, the code must be unindented to the same level as the starting if
line.
# else Statement
test_value = 50
if test_value < 1:
print("Value is < 1")
else:
print("Value is >= 1")
test_string = "VALID"
if test_string == "NOT_VALID":
print("String equals NOT_VALID")
else:
print("String equals something else!")
Python Loops with range()
.
In Python, a for
loop can be used to perform an action a specific number of times in a row.
The range()
function can be used to create a list that can be used to specify the number of iterations in a for
loop.
# Print the numbers 0, 1, 2:
for i in range(3):
print(i)
# Print "WARNING" 3 times:
for i in range(3):
print("WARNING")
Infinite Loop
An infinite loop is a loop that never terminates. Infinite loops result when the conditions of the loop prevent it from terminating. This could be due to a typo in the conditional statement within the loop or incorrect logic. To interrupt a Python program that is running forever, press the Ctrl
and C
keys together on your keyboard.
Python while
Loops
In Python, a while
loop will repeatedly execute a code block as long as a condition evaluates to True
.
The condition of a while
loop is always checked first before the block of code runs. If the condition is not met initially, then the code block will never run.
# This loop will only run 1 time
hungry = True
while hungry:
print("Time to eat!")
hungry = False
# This loop will run 5 times
i = 1
while i < 6:
print(i)
i = i + 1
Python Nested Loops
In Python, loops can be nested inside other loops. Nested loops can be used to access items of lists which are inside other lists. The item selected from the outer loop can be used as the list for the inner loop to iterate over.
groups = [["Jobs", "Gates"], ["Newton", "Euclid"], ["Einstein", "Feynman"]]
# This outer loop will iterate over each list in the groups list
for group in groups:
# This inner loop will go through each name in each list
for name in group:
print(name)
break
Keyword
In a loop, the break
keyword escapes the loop, regardless of the iteration number. Once break
executes, the program will continue to execute after the loop.
In this example, the output would be:
0
254
2
Negative number detected!
numbers = [0, 254, 2, -1, 3]
for num in numbers:
if (num < 0):
print("Negative number detected!")
break
print(num)
# 0
# 254
# 2
# Negative number detected!
The Python continue
Keyword
In Python, the continue
keyword is used inside a loop to skip the remaining code inside the loop code block and begin the next loop iteration.
big_number_list = [1, 2, -1, 4, -5, 5, 2, -9]
# Print only positive numbers:
for i in big_number_list:
if i < 0:
continue
print(i)