In Programming a Continuous or Repeated Action That You Want to Perform Over and Over Again Is

Repeating Actions with Loops

Overview

Teaching: 30 min
Exercises: 0 min

Questions

• How can I do the same operations on many different values?

Objectives

• Explain what a for loop does.

• Correctly write for loops to repeat simple calculations.

• Trace changes to a loop variable as the loop runs.

• Trace changes to other variables as they are updated by a for loop.

In the episode virtually visualizing information, we wrote Python code that plots values of interest from our first inflammation dataset (inflammation-01.csv), which revealed some suspicious features in it.

Nosotros have a dozen data sets correct at present and potentially more on the fashion if Dr. Bohemian can continue upwards their surprisingly fast clinical trial rate. We desire to create plots for all of our data sets with a single statement. To do that, we'll have to teach the computer how to echo things.

An example job that we might desire to repeat is accessing numbers in a list, which nosotros will do past press each number on a line of its own.

In Python, a listing is basically an ordered collection of elements, and every element has a unique number associated with information technology — its index. This ways that we can access elements in a list using their indices. For example, we can get the commencement number in the list odds, by using odds[0]. Ane fashion to print each number is to utilise four print statements:

                          print              (              odds              [              0              ])              impress              (              odds              [              i              ])              print              (              odds              [              2              ])              print              (              odds              [              3              ])                      

This is a bad approach for iii reasons:

1. Non scalable. Imagine you need to print a listing that has hundreds of elements. It might be easier to type them in manually.

2. Difficult to maintain. If we want to decorate each printed element with an asterisk or any other character, we would take to modify 4 lines of code. While this might not be a problem for small lists, it would definitely be a problem for longer ones.

3. Fragile. If we utilise it with a list that has more than elements than what nosotros initially envisioned, it will just display function of the listing'due south elements. A shorter listing, on the other manus, volition cause an error because it will be trying to brandish elements of the listing that do not exist.

                          odds              =              [              1              ,              3              ,              5              ]              print              (              odds              [              0              ])              print              (              odds              [              1              ])              impress              (              odds              [              2              ])              print              (              odds              [              3              ])                      

            --------------------------------------------------------------------------- IndexError                                Traceback (most contempo call last) <ipython-input-three-7974b6cdaf14> in <module>()       iii print(odds[i])       4 print(odds[2]) ----> five print(odds[three])  IndexError: list index out of range                      

Hither's a better approach: a for loop

                          odds              =              [              ane              ,              3              ,              5              ,              seven              ]              for              num              in              odds              :              print              (              num              )                      

This is shorter — certainly shorter than something that prints every number in a hundred-number list — and more robust likewise:

                          odds              =              [              1              ,              3              ,              5              ,              7              ,              9              ,              11              ]              for              num              in              odds              :              print              (              num              )                      

The improved version uses a for loop to repeat an operation — in this case, printing — in one case for each affair in a sequence. The full general course of a loop is:

                          for              variable              in              collection              :              # do things using variable, such every bit print                                    

Using the odds instance above, the loop might look like this:

where each number (num) in the variable odds is looped through and printed one number after another. The other numbers in the diagram announce which loop cycle the number was printed in (ane being the offset loop cycle, and 6 being the last loop bicycle).

We can call the loop variable anything nosotros like, simply at that place must be a colon at the terminate of the line starting the loop, and we must indent anything we want to run inside the loop. Different many other languages, there is no command to signify the end of the loop body (eastward.g. end for); what is indented after the for statement belongs to the loop.

What's in a proper noun?

In the case in a higher place, the loop variable was given the name num as a mnemonic; it is short for 'number'. We can cull any proper noun we want for variables. We might just equally easily have chosen the name banana for the loop variable, equally long every bit we employ the same name when we invoke the variable inside the loop:

                              odds                =                [                1                ,                3                ,                five                ,                7                ,                ix                ,                eleven                ]                for                banana                in                odds                :                print                (                banana                )                          

It is a good idea to cull variable names that are meaningful, otherwise it would be more hard to sympathize what the loop is doing.

Hither'south another loop that repeatedly updates a variable:

                          length              =              0              names              =              [              'Curie'              ,              'Darwin'              ,              'Turing'              ]              for              value              in              names              :              length              =              length              +              1              impress              (              'There are'              ,              length              ,              'names in the list.'              )                      

            There are 3 names in the list.                      

It's worth tracing the execution of this little plan footstep by footstep. Since at that place are three names in names, the statement on line four will exist executed three times. The commencement time around, length is zero (the value assigned to it on line i) and value is Curie. The statement adds ane to the sometime value of length, producing 1, and updates length to refer to that new value. The next time effectually, value is Darwin and length is 1, so length is updated to be two. After one more than update, length is three; since there is nothing left in names for Python to procedure, the loop finishes and the print function on line v tells us our final answer.

Annotation that a loop variable is a variable that is being used to tape progress in a loop. It still exists after the loop is over, and we can re-utilise variables previously divers as loop variables also:

                          proper name              =              'Rosalind'              for              name              in              [              'Curie'              ,              'Darwin'              ,              'Turing'              ]:              impress              (              proper name              )              impress              (              'after the loop, name is'              ,              proper noun              )                      

            Curie Darwin Turing afterwards the loop, name is Turing                      

Notation besides that finding the length of an object is such a common operation that Python really has a born function to do it called len:

len is much faster than whatever function we could write ourselves, and much easier to read than a two-line loop; it volition also give us the length of many other things that we haven't met yet, so we should always use it when we tin can.

From 1 to Due north

Python has a built-in function called range that generates a sequence of numbers. range tin can accept 1, ii, or 3 parameters.

• If 1 parameter is given, range generates a sequence of that length, starting at zero and incrementing by 1. For example, range(3) produces the numbers 0, 1, 2.
• If ii parameters are given, range starts at the first and ends just before the second, incrementing by 1. For example, range(ii, 5) produces 2, 3, 4.
• If range is given 3 parameters, it starts at the first ane, ends merely earlier the second one, and increments past the third one. For example, range(three, 10, ii) produces 3, 5, 7, 9.

Using range, write a loop that uses range to print the commencement three natural numbers:

Solution

                                  for                  number                  in                  range                  (                  1                  ,                  4                  ):                  impress                  (                  number                  )                              

Understanding the loops

Given the following loop:

                              give-and-take                =                'oxygen'                for                char                in                word                :                impress                (                char                )                          

How many times is the trunk of the loop executed?

• 3 times
• iv times
• v times
• 6 times

Solution

The body of the loop is executed 6 times.

Calculating Powers With Loops

Exponentiation is built into Python:

Write a loop that calculates the aforementioned upshot as 5 ** iii using multiplication (and without exponentiation).

Solution

                                  result                  =                  one                  for                  number                  in                  range                  (                  0                  ,                  3                  ):                  upshot                  =                  result                  *                  5                  print                  (                  event                  )                              

Summing a list

Write a loop that calculates the sum of elements in a list by adding each element and printing the final value, then [124, 402, 36] prints 562

Solution

                                  numbers                  =                  [                  124                  ,                  402                  ,                  36                  ]                  summed                  =                  0                  for                  num                  in                  numbers                  :                  summed                  =                  summed                  +                  num                  print                  (                  summed                  )                              

Computing the Value of a Polynomial

The built-in function enumerate takes a sequence (e.g. a listing) and generates a new sequence of the aforementioned length. Each element of the new sequence is a pair composed of the index (0, 1, 2,…) and the value from the original sequence:

                              for                idx                ,                val                in                enumerate                (                a_list                ):                # Practice something using idx and val                                          

The code above loops through a_list, assigning the alphabetize to idx and the value to val.

Suppose y'all have encoded a polynomial as a list of coefficients in the following way: the kickoff chemical element is the constant term, the 2d chemical element is the coefficient of the linear term, the third is the coefficient of the quadratic term, etc.

                              10                =                5                coefs                =                [                2                ,                four                ,                3                ]                y                =                coefs                [                0                ]                *                ten                **                0                +                coefs                [                i                ]                *                x                **                1                +                coefs                [                2                ]                *                x                **                2                impress                (                y                )                          

Write a loop using enumerate(coefs) which computes the value y of whatsoever polynomial, given x and coefs.

Solution

                                  y                  =                  0                  for                  idx                  ,                  coef                  in                  enumerate                  (                  coefs                  ):                  y                  =                  y                  +                  coef                  *                  x                  **                  idx                              

Key Points

• Use for variable in sequence to process the elements of a sequence one at a time.

• The torso of a for loop must be indented.

• Use len(thing) to determine the length of something that contains other values.

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Source: https://swcarpentry.github.io/python-novice-inflammation/05-loop/

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