Python Indentation:
- Programming languages like C, C++, Java use braces { } to define a block of code.
- However, Python uses indentation to define the code block.
- Indentation basically means, use of whitespaces.
- Generally 4 whitespaces (4 times space bar) are used for indentation & it is preferred over tabs.
- Tabs also can be used instead of whitespaces. However, make sure that it’s not mixture of tabs and whitespaces. Use any one of it.
- The amount of indentation is up to user, but it must be consistent throughout the code block.
Example:
print(“Hello”)
- Indentation in Python makes the code look neat and clean.
- Above code can also be written on single line as
if True: print(‘Hello’)
- This results into Python programs that look similar and consistent.
- Incorrect indentation will result into IndentationError.
Python Flow Control using : If… statement
- Decision making is required when we want to execute a piece of code only if a certain condition is true.
- The if…elif…else statement is used in Python for decision making.
- Syntax:
if testexpression:
statement(s)
- In Python, the body of the if statement is indicated by the indentation. Body starts with an indentation and the first unindented line marks the end.
- Python interprets non-zero values as True. None and 0 are interpreted as False.
# If the number is positive, print an appropriate message
num = 3
if num > 0:
print(num, “is a positive number.”)
Python Flow Control: If… else
- Syntax:
Body of if
else:
Body of else
- Example:
num = 3
if num >= 0:
print(“Positive or Zero”)
else:
print(“Negative number”)
Python Flow Control: If…elif… else
- Syntax:
if testexpression:
Body of if
elif test expression:
Body of elif
else:
Body of else
- Example:
num = 3
if num > 0:
print(“Positive number”)
elif num == 0:
print(“Zero”)
else:
print(“Negative number”)
Python Nested if statements:
- Indentation is the only way to figure out the level of nesting.
- This can get confusing, so should be avoided if we can.
- Example:
num = int(input(“Enter a number: “))
if num >= 0:
if num == 0:
print(“Zero”)
else:
else:
Python for Loop:
- The for loop in Python is used to iterate over a sequence (list, tuple, string) or other iterable objects.
- Iterating over a sequence is called traversal.
- Syntax of for Loop
for var in sequence:
Body of for
- Here, var is the variable that takes the value of the item inside the sequence on each iteration.
- Loop continues until we reach the last item in the sequence.
- The body of for loop is separated from the rest of the code using indentation.
- Example:
# Program to find the sum of all numbers stored in a list
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
add = 0
for x in numbers:
add = add+var
print(“The sum is”, add).
# Output will be 48
for loop with else:
- A for loop can have an optional else block as well.
- The else part is executed if the items in the sequence used in for loop exhausts.
- Example:
Here, the for loop prints items of the list until the loop exhausts. When the for loop exhausts, it executes the block of code in the else and prints
digits = [0, 1, 5]
for i in digits:
print(i)
else:
print(“No items left.”)
Output:
0
1
5
How for loop actually works?
for element in iterable:
iter_obj = iter(iterable) # create an iterator object from iterable
while True: # infinite loop
try:
element = next(iter_obj) # do something with element
StopIteration: # if StopIteration is raised, break from loop
break
- So internally, the for loop creates an iterator object, iter_obj by calling iter()on the iterable.
- Ironically, this for loop is actually an infinite while loop.
- Inside the loop, it calls next() to get the next element and executes the body of the for loop with this value.
- After all the items exhaust, StopIteration is raised which is internally caught and the loop ends.
- Note that any other kind of exception will pass through
Python while Loop:
The while loop in Python is used to iterate over a block of code as long as the test expression (condition) is true. We generally use this loop when we don’t know beforehand, the number of times to iterate.
while testexpression:
- Body starts with indentation and the first unindented line marks the end.
- Python interprets any non-zero value as True. None and 0 are interpreted as False.
Example:
# Program to add numbers upto n numbers
n = int(input(“Enter number: “))
add = 0
i = 1
while i <= n:
add = add + I
Enter number: 10
The sum is 55
while loop with else:
- Same as that of for loop, we can have an optional else block with while loop as well.
- The else part is executed if the condition in the while loop evaluates to False.
Example:
counter = 0
while counter <= 3:
print(“we are inside the loop”)
counter = counter + 1
else:
print(“We are now in else statement”)
Output:
we are inside the loop
we are inside the loop
We are now in else statement
The range() function:
- We can generate a sequence of numbers using range() function.
- The range(10) will generate numbers from 0 to 9 (10 numbers).
- We can also define the start, stop and step size as range(start,stop,step size). step size defaults to 1 if not provided.
- This function does not store all the values in memory, it would be inefficient. So it remembers the start, stop, step size and generates the next number on the go.
- To force this function to output all the items, we can use the function list().
Example:
print(range(10))
print(list(range(10)))
print(list(range(2, 8))) # Starting from 2 till (8-1)
print(list(range(2, 20, 3))) # Start from 2 to (20-1) stepping 3 at a time
- We can use the range() function in for loops to iterate through a sequence of numbers
- It can be combined with the len() function to iterate though a sequence using indexing.
Example: In below example, we have three strings elements in a list. That basically means we do not have, how many iterations. So we count length of list (3 elements) and use it as range number.
sport = [‘cricket’, ‘chess’, ‘carrom’]
for i in range(len(sport)): # iterate over the list using index
print(“I do like”, sport[i])
Output:
I do like cricket
I do like chess
Python break Statement:
- Loops iterate over a block of code until test expression is false, but sometimes we might want to terminate the current iteration or even the whole loop without checking test expression.
- The break statement is used in these cases.
- The break statement terminates the loop containing it.
- Control of the program flows to the statement immediately after the body of the loop.
- If break statement is inside a nested loop (loop inside another loop), break will terminate the innermost loop.
Example:
Python continue statement:
- The continue statement is used to skip the rest of the code inside a loop for the current iteration only.
- Loop does not terminate but continues on with the next iteration.
Python pass statement:
- In Python programming, pass is a null statement.
- The difference between a comment and pass statement in Python is that, while the interpreter ignores a comment entirely, pass is not ignored.
- However, nothing happens when pass is executed. It results into no operation (NOP).
Example:
- We can do the same thing in an empty function or class as well.
def function(args):
pass
class example():
pass