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 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 is used to determine the execution of code based on the evaluation of a Boolean expression.
- If the
ifstatement expression evaluates to
True, then the indented code following the statement is executed.
- If the expression evaluates to
Falsethen the indented code following the
ifstatement is skipped and the program executes the next line of code which is indented at the same level as the
# 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 provides alternate code to execute if the expression in an
if statement evaluates to
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
# 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
In Python, a
for loop can be used to perform an action a specific number of times in a row.
range() function can be used to create a list that can be used to specify the number of iterations in a
# Print the numbers 0, 1, 2: for i in range(3): print(i) # Print "WARNING" 3 times: for i in range(3): print("WARNING")
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
C keys together on your keyboard.
In Python, a
while loop will repeatedly execute a code block as long as a condition evaluates to
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)
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:
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!
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)