During the course of solving client requirements, comes across situations where group of some data needs to be processed against a defined set of instructions.
Loops help in resolving situations where a piece of code needs to be executed against a set of data repetitedly or till certain condition is met or un-met. Or, you use to process a large quantity of data, such as lines of a file or records of a database that must be processed by the same code block.
Python provides two constructs to help in these situations.
forwhileLets start with the for loops.
It is one of the most often used construct in Python. It can accept not only accept static sequences, but also sequences generated by iterators (Structures which allow iterations, i.e. sequentially access to collection of elements). It runs the code block against known iterations of dataset.
The syntax for for is as follows:
Syntax:
for <reference> in <sequence>:
<code block>
continue
break
else:
<code block>During the execution of a for loop, the reference points to an element in the sequence. At each iteration, the reference is updated, in order for the for code block to process the corresponding element.
The clause break stops the loop and continue passes it to the next iteration. The code inside the else is executed at the end of the loop, except if the loop has been interrupted by break.
Example:
In [15]:
for x in "Manish Gupta":
print(x, end="^~", flush=True)
In the above example, "Manish Gupta" is a sequence of characters and for loop traverse that sequence of characters. Also you will note that we are ending the print statement with space instead of new line using the option end=.
Similarly in the below example we are going to use the range function to generate the sequence of numbers starting from 30 and ending with 5 with difference of 5.
The function range(m, n, p), is very useful in loops, as it returns a list of integers starting at m through smaller than n in steps of length p, which can be used as the order for the loop.
We can also define the start, stop and step size as range(start,stop,step size). step size defaults to 1 if not provided.
We can generate a sequence of numbers using range() function. range(10) will generate numbers from 0 to 9 (10 numbers).
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().
In [1]:
# Output: range(0, 10)
print(range(10))
# Output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(list(range(10)))
# Output: [2, 3, 4, 5, 6, 7]
print(list(range(2, 8)))
# Output: [2, 5, 8, 11, 14, 17]
print(list(range(2, 20, 3)))
print(list(range(20, 2, -3)))
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print(dir(range(10)))
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# Sum 0 to 99
s = 0
for x in range(30, 1, -5):
print(x)
s = s + x
print("sum of 30 to 1 with steps -5 is", s)
In [2]:
# Sum 0 to 99
s = 0
for x in range(30, 1, -5):
print(x)
s += x
print("sum of 30 to 1 with steps -5 is", s)
We can also have nested for loops as shown in the below example
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for x in range(1, 6):
for y in range(1, x+1):
print(x, y)
NOTE: Please avoid below case
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for x in range(1, 6):
for x in range(1, x+1):
print(x, x)
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cols = ["Red", "Green", "Yellow", "White"]
for color in cols:
print(color)
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cols = ["Red", "Green", "Yellow", "White"]
for color in cols:
print(color)
else:
print(" ~~~~ Done ~~~~")
we can also have conditions where multiple values are returned every iteration.
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# Advance code, to be used after learning slicing. Please use instead the other code
# for x in "Manish Gupta"[::-1]:
# print(x, end=" ")
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reverse_text = ""
for char in "Manish Gupta":
reverse_text = char + reverse_text
print(reverse_text)
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x_test = [[1,2],[3,4],[5,6]]
for x in x_test:
print(x)
a = x[0]
b = x[1]
print (a, b)
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x_test = [[1,2],[3,4],[5,6], [7,8,9]]
for x in x_test:
print(x)
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x_test = [[1, 2],[3, 4],[5, 6], [7, 8]]
for x, y in x_test:
print(x, y)
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x_test = [[1, 2],[3, 4],[5, 6], [7, 8, 9]]
try:
for x, y in x_test:
print(x, y)
except Exception as e:
print(e)
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x_test = [[1,2],[3,4],[5,6], [7,8,9]]
for x in x_test:
print(x)
a = x[0]
b = x[1]
print (a, b)
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color = {"c2": "Red", "c1": "Green", "c3": "Orange"}
for value in color.values():
print(value)
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color = {"c1": "Red", "c2": "Green", "c3": "Orange"}
for col in color:
print(col, color[col])
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color = {"c1": "Red", "c2": "Green", "c3": "Orange"}
for value in color.values():
if(value=="Green"):
break
print(value)
else:
print("Done")
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color = {"c1": "Red", "c2": "Green", "c3": "Orange"}
for value in color.values():
if(value=="Green"):
continue
print(value)
else:
print("Done")
Executes a block of code in response to a condition.
Syntax:
while <condition>:
<code block>
continue/break/pass
else:
<code block>
The code block inside the while loop is repeated while the loop condition is evaluated as true.
Example:
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# Sum 0 to 99
s = 0
x = 1
while x < 100:
s = s + x
x = x + 1
else:
print("!!! Hurry Hurry !!!")
print(x)
print ("Sum of 0 to 99", s)
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# Sum 0 to 99
s = 0
x = 1
while x < 100:
s += x
x += 1
else:
print("!!! Hurry Hurry !!!")
print(x)
print ("Sum of 0 to 99", s)
NOTE: Bad Code Below
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while x < 0:
print("Hello")
else:
print("Sorry")
In [17]:
# while x > 0:
# print("Hello")
# else:
# print("Sorry")
NOTE: Please try to avoid code similar to above commented code
NOTE: The while loop is appropriate when there is no way to determine how many iterations will occur and there is a sequence to follow.
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s = 0
x = 100
while x < 100:
s = s + x
x = x + 1
else:
print("x is already equal or greater than 100")
print(s)
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x = 1;
s = 0
while (x < 10):
s = s + x
x = x + 1
if (x == 5):
break
else:
print('The sum of first 9 integers : ',s)
print('The sum of', x, 'numbers is :',s)
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while 10 != int(input('Enter a passkeyid: ')):
print("Wrong Passkey"),
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while int(input('Enter a passkeyid: ')) != 10:
print("Wrong Passkey"),
else:
print("!!! Welcome to the world of Magic !!!")
The break statement is used to exit a for or a while loop. The purpose of this statement is to end the execution of the loop (for or while) immediately and the program control goes to the statement after the last statement of the loop. If there is an optional else statement in while or for loop it skips the optional clause also
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num_sum = 0
count = 0
for x in range(1, 9):
print(x)
num_sum = num_sum + x
count = count + 1
if count == 5:
break
print("Sum of first ",count,"integers is : ", num_sum)
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for x in range(8):
if (x == 3 or x==6):
print("\tSkipping:", x)
continue
print("This should never print")
else:
print(x)
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for x in [1, 10, 4]:
if x == 10:
continue
print("Hello", x)
else:
print("processing completed without issues.")
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print("-" * 20)
for x in [1, 10, 4]:
if x == 10:
break
print("Hello", x)
else:
print("processing completed without issues.")
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def meets_condition(x):
return x==20
data = [10, 20, 33, 42, 44]
for x in data:
if meets_condition(x):
break
else:
print("No one met the condition")
print("lets end it")
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def meets_condition(x):
return x==21
data = [10, 20, 33, 42, 44]
for x in data:
if meets_condition(x):
break
else:
print("No one met the condition")
print("lets end it")
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print(list(range(2, 4)))
print(4%2)
n-> 2:
x <- []
n -> 3:
x -> 2
3%2
Prime number
n -> 4
x -> [2, 3]
4%2
n -> 5:
x -> [2, 3, 4]
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for n in [2, 3, 4, 5, 6, 7, 8, 9]:
for x in range(2, n):
if n % x == 0:
print(n, 'equals', x, '*', n/x)
break
else:
# loop fell through without finding a factor
print(n, 'is a prime number')
NOTE: When used with a loop, the else clause has more in common with the else clause of a try statement than it does that of if statements: a try statement’s else clause runs when no exception occurs, and a loop’s else clause runs when no break occurs. For more on the try statement and exceptions, see Handling Exceptions.
In [4]:
a = [1, 2, 3, 4, 5, 6, 7, 8]
b = [2, 6]
c , d = [], []
for x in a:
(c, d)[x in b].append(x)
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print(c, d)