loop_conditionals.md

Loops and Conditionals

Python supports for, while, if-elif-else blocks.

If-elif-else

The concept of if-else block is similar to any other language.

x = 10
if x>10:
  print("x is greater than 10")
elif x < 10:
  print("x is less than 10")
else:
  print("x is equal to 10")

print("x is less than 10" if x<10 else "x is greater than equal to 10") # not good handle block statements

NOTE: Python does not support switch statement.

While

While loop continues execution till the condition given in true. If the condition involves variable, it must be updated in each loop statement execution. Otherwise it may result in infinite loop. This execution style is used when the number of times the statement block is to be executed is unknown.

x = 10
while x>=0:
  print(x)
  x-=1

Python does not have do-while loop. However, it can be implemented by using while True: i.e. an infinite loop, and mention the condition for breaking, in the end of the code block. An else block can also be added to while. The else block statements will execute when the while loop execution completes smoothly i.e. is not stopped by a break. NOTE Python supports break and continue like any other high level language.
PS: A continue or break inside a try-except-finally block inside a loop, will still always execute the finally block.

a = 0
b = 2
while a < 3:
    a += 1
    b -= 1
    try:
        a / b
    except ZeroDivisionError:
        print("division by zero")
        break
    finally:
        print("finally executed")
"""OUTPUT
finally executed
division by zero
finally executed
"""

For loop

This is mainly used to iterate through iterables i.e. list, dictionary, set, etc.

d = {"a":1,"b":2,"c":3}
for i in d: # i is the iterator
  print(i, d[i])
'''
a 1
b 2
c 3
'''
arr = [3,2,6]
for item in arr:
  print(item, end=' ')
# 3 2 6
for i, value in enumerate(arr):
  print(i , value)
'''
0 3
1 2
2 6
'''

enumerate() returns index and value in form a tuple.
range() is used to traverse through pre-defined sequences like integers. Atleast one argument is required. range(10) : returns a list wth numbers from 0 to 9.
range(1,10) : returns a list wth numbers from 1 to 9.
range(1,10, 2) : returns a list wth numbers from 1 to 9, with step size 2 i.e. [1,3,5,7,9]
This can be traversed in for loop as follows:

for i in range(1, 101):
  print(i**2)

This will print square of all numbers from 1 to 100.
else can be added to the end of for loop, executed when for is NOT stopped by break, i.e. if for loop had smooth execution till end of condition.

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