How to Create Loops in Python

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Learn how to create a loop using Python
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In Python, and many other programming languages, you will need to loop commands several times, or until a condition is fulfilled. It is easy, and the loop itself only needs a few lines of code.

Things You Should Know

  • If you are using IDLE, make sure all subprograms are off.
  • For a loop that goes for a definite amount of times, you will need a for loop, and for a loop that goes on forever you'll need a while loop.
  • You can use the break command to forcibly stop a loop.
  • Remember the "if", "elif" and "else" words for your loops. They are useful keywords that you can use on your on your loop.
1

Open your shell or program.

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    Make sure all subprograms are off if using IDLE.
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2

Write a for loop.

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    This is the structure for a for loop. The example prints hello, world! 10 times:
 >>> 
 for 
 i 
 in 
 range 
 ( 
 0 
 , 
 10 
 ): 
 # The first number specifies the start and the last one specifies the end 
 >>> 
 print 
 ( 
 "hello, world!" 
 )
3

Write a while loop.

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    A method for both is shown. That will throw a SyntaxError unless you use Python version 2. It is recommended to use the above example of the print statement.
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 >>> 
 while 
 True 
 : 
 # This loop goes on forever 
 >>> 
 print 
 "Hello World" 
 # Prints the message which will not work on recent versions
4

Make the while loop last until a condition is met.

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    In this example, True will always be True as long as the variable's answer and grade are Yes and 6. If they never changed, it will run forever so the while loop just makes the code loop while something is happening.
 >>> 
 while 
 answer 
 == 
 "Yes" 
 and 
 grade 
 == 
 "6" 
 : 
 # While the answer is equal to the string Yes and the grade to 6
5

Stop a loop.

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    You can always kill the loop in Task Manager as well. The other way is using the break command or using while loops.
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  • Question
    How do you do a nested loop?
    Adam Blalock
    Community Answer
    Here is a loop within a loop in Python: x = [0,1,2,3,4], y= [10,11,12,13]. For i in x: For ii in y: print i + ii. (Make sure to do it as a single tab before the second "For" and two tabs for the "print" statement.)
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  • Question
    What about web development?
    Community Answer
    You need to learn about Django, the Python framework for web development.
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  • Question
    How do you use a while loop to repeat a calculator program?
    Community Answer
    First create a function. For example, if you wanted to square numbers 1 through 100 and print the results to the screen, I would write: def square(x): return x*x for i in range(1, 101): # stop = 101 because the stop value is not included. This will go from 1-100. print(square(i)).
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      • You can exit a loop prematurely with the break command. This might be needed, for example, if you're iterating over a list to find something and can stop looking once you've found it.
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      • You can use the continue command to skip to the top of the loop. This is useful if you wanted to skip certain code if a condition is met.
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      • Avoid using the while True loop without any "break" commands or keywords that end it.
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