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Loops in Python

    A LOOP is a control structure, which on fulfilling of certain condition, repeats a block of codes. When that condition becomes False, or break statement is used inside the LOOP, then that loop stops executing the block of codes. There are two types of LOOPs − (a) Definite/Finite, and (b) Indef­inite or infinite loops.

    Unlike PHP and JavaScript, there are only two type of loops in Python − (1) while and (2) for. The while loop behaves like general loop in which you have to declare (i) initial­ization, (ii) condit­ional expres­sion, and (iii) iteration. However, the for loop works only on collection of items or characters (sequences), and thus, it behaves like foreach loop in other program­ming languages, such as PHP and JavaScript.

while Loop

    Following two examples are typical illust­ration of finite while loop, where you have to (i) initialize the numeric data-type variable, (ii) place condition after the while keyword, and (iii) after the : (colon) symbol, in next line, inside the block of codes, iterate (either increment or decrement) the numeric data-type variable. When the condition becomes False, the while loop exits out of the block.

num = 0                   # initial­ization
while num < 10:
  print( num )          # print() function automat­ically outputs the result in new line
  num = num + 1     # iteration
print( "Outside while loop: ", num )

num = 0                   # initial­ization
while num > -10:
  print( num )
  num -= 1                 # iteration
print( "Outside while loop: ", num )

    There is no ++ or -- operators in Python, therefore, you should use either arithe­matic operators, or arithe­matic assign­ment operators. You can copy and save both of above examples in a file with .py extension, for example, new.py in your project folder, and then run/execute the it (for Windows, use python or py, and for Mac/Linux, you can use python or python3.10 keyword instead of py). The output of both examples (when they are executed together), are as follows:

C:\xampp\htdocs\python2023>py new.py
0
1
2
3
4
5
6
7
8
9
Outside while loop: 10
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
Outside while loop: -10

    There are many ways in which you can do operations, such as finding squares ** 2, squre-root ** (1/2), cubes ** 3, or cube-root ** (1/3), etc. of each numbers.

num = 0
while num < 10:
  num += 1
  print( num ** 2 )   # squares of each number will be displayed
print( "Outside while loop: ", num )

    Many program­mers also use else keyword after the while block of codes, to execute/run some statements (within indent­ation), when the condition in while loop is evaluated as False. In WEB Develop­ement, and other applica­tions, you may encounter such situations, and therefore you can also use else keyword after the while block. For example:

num = 0
while num > 10:
  num += 1
  print( num ** (1/2) )   # square-roots of each number will be displayed
else:
  print( "The condition is False, therefore, num is ", num )
print( "The statement in this line, has no connec­tion with while loop." )

    When you replace the > (greater than) operator with < (less than), the above example will output square-roots of each number from 1 to 10 (see below results when both > and < operators are used one-by-one). The else keyword thence also, outputs the result, when the condition is evaluated as False, at the end of the program. This is different from if else or if elif else block of codes, where only one set of statements (after either if, elif, or else), is the output (result).

C:\xampp\htdocs\python2023>py new.py
The condition is False, therefore, num is 0
The statement in this line, has no connec­tion with while loop.

C:\xampp\htdocs\python2023>py new.py
1.0
1.4142135623730951
1.7320508075688772
2.0
2.23606797749979
2.449489742783178
2.6457513110645907
2.8284271247461903
3.0
3.1622776601683795
The condition is False, therefore, num is 10
The statement in this line, has no connec­tion with while loop.

Exhaustive Loop

    In the following example, the condition is evaluated as True, and a NESTED if block is inserted to break the loop when condition will be equal to 3. In this example, the else block (one or more lines of codes) will not be executed. The statement below the if condition is indented four blank spaces. When you comment the if condition, and the indented statement (break) below it, the while loop will be exhausted completely.

num = 0
while num < 10:
  num += 1
  print( num ** (1/3) )   # cube-roots of each number will be displayed
  if num == 3:
    break
else:
  print( "The condition is False, therefore, num is ", num )
print( "The loop is broken by the break keyword." )

Infinite while Loop

Caution: In each of above examples, the while loop has been iterated either by increasing or decreasing the variable name, num initial­ized before it. You can see that in every block of codes below the while condition, the statement num += 1 or num -= 1 has been placed before or after other statements.

    When either you remove the iteration statement, that is, num += 1 or num -= 1, or in certain situations, the iteration statement is placed so that it cannot be executed; then the while loop will beocme infinite loop. When you run/execute such program, the computer depending on speed of the Processor (CPU) and memory size of RAM, will output the result indefinitely.

    You have to press Ctrl C or Cmd C keys simultan­eously to terminate/stop the program. You can also place a iteration statement (that is, num += 1) just after nested if condition and (always above the continue statement) to avoid the infinite loop.

num = 0
while num < 10:
  if num == 3:
    print( num )   # This statement will be displayed indefin­itely.
    continue       # continue will skip all codes below it inside the block.
  num += 1
  print( num ** 3 )   # Cube of each number
else:
  print( "The condition is False, therefore, num is ", num )
print( "The loop is skipped by the continue keyword." )   # Press Ctrl C or Cmd C keys simultan­eously.

Practical Uses of Indefinite Loops

    In real world WEB Applica­tions, the user (public) wants to do certain task repeatedly, and the programmer doesn't know, how many times, the codes should be executed. In such situations, indefinite or infinite loops are very useful, for example:

while True:
  num = int( input( "Please enter a number between 1 and 10. Thereafter, press 'Enter' or 'Return' key: " ) )
  if num == 5:
    break
print( "You played the game. Bye!" )

    In the above example, the while condition is always evaluated as True. Hence, it is a infinite loop, in which the public (user) has been asked to input certain number. Remember that the input() function always returns a string data-type. Therefore, to compare it with any numeric value, you must have to type-cast the returned value with int() or float() in-built functions. When the user will enter 5 and press Enter or Return key, the infinite while loop shall be terminated.

for Loop

    The for loop only works on sequence data-types, such as string, list, tuple, etc. In the following example, the string 'Hello' is a sequence of five letters, and the for loop outputs each lettter in new line.

string = 'Hello'
for item in string:
  print( item )

C:\xampp\htdocs\python2023>py new.py
H
e
l
l
o

    A list is the collection of items, enclosed within [] (square brackets). Herein below, a list of prime numbers are shown as example. You will learn more about different data-types and application of while and for loops in next pages.

for number in [1, 2, 3, 5, 7, 11, 13, 17, 19]:
  print( number )

C:\xampp\htdocs\python2023>py new.py
1
2
3
5
7
11
13
17
19