C if the User Does Not Enter a Valid Choice Then the Program Should Ask the Same Question Again

Making Choices

Overview

Teaching: thirty min
Exercises: 0 min

Questions

• How can my programs do different things based on data values?

Objectives

• Write conditional statements including if, elif, and else branches.

• Correctly evaluate expressions containing and and or.

In our last lesson, we discovered something suspicious was going on in our inflammation information by cartoon some plots. How can we use Python to automatically recognize the different features nosotros saw, and have a different activeness for each? In this lesson, nosotros'll learn how to write code that runs only when certain atmospheric condition are true.

Conditionals

We can ask Python to have different actions, depending on a condition, with an if statement:

                          num              =              37              if              num              >              100              :              print              (              'greater'              )              else              :              print              (              'not greater'              )              print              (              'done'              )                      

The second line of this code uses the keyword if to tell Python that we want to make a choice. If the test that follows the if statement is truthful, the body of the if (i.due east., the set of lines indented underneath it) is executed, and "greater" is printed. If the test is false, the body of the else is executed instead, and "non greater" is printed. Only one or the other is ever executed before continuing on with plan execution to impress "done":

Conditional statements don't have to include an else. If there isn't ane, Python simply does null if the test is faux:

                          num              =              53              impress              (              'before conditional...'              )              if              num              >              100              :              print              (              num              ,              'is greater than 100'              )              print              (              '...after conditional'              )                      

            before conditional... ...after conditional                      

We can also chain several tests together using elif, which is brusk for "else if". The following Python code uses elif to print the sign of a number.

                          num              =              -              3              if              num              >              0              :              impress              (              num              ,              'is positive'              )              elif              num              ==              0              :              impress              (              num              ,              'is nix'              )              else              :              print              (              num              ,              'is negative'              )                      

Annotation that to test for equality we employ a double equals sign == rather than a single equals sign = which is used to assign values.

Comparing in Python

Along with the > and == operators we take already used for comparison values in our conditionals, in that location are a few more options to know nigh:

• >: greater than
• <: less than
• ==: equal to
• !=: does not equal
• >=: greater than or equal to
• <=: less than or equal to

We can likewise combine tests using and and or. and is only truthful if both parts are truthful:

                          if              (              1              >              0              )              and              (              -              1              >=              0              ):              impress              (              'both parts are truthful'              )              else              :              print              (              'at least 1 office is false'              )                      

            at to the lowest degree one part is false                      

while or is truthful if at least one part is true:

                          if              (              1              <              0              )              or              (              1              >=              0              ):              print              (              'at to the lowest degree one test is true'              )                      

            at least ane examination is true                      

Truthful and False

True and False are special words in Python called booleans, which represent truth values. A statement such as 1 < 0 returns the value False, while -1 < 0 returns the value True.

Checking our Information

Now that we've seen how conditionals piece of work, we can use them to check for the suspicious features we saw in our inflammation data. We are virtually to use functions provided by the numpy module over again. Therefore, if yous're working in a new Python session, make sure to load the module with:

From the start couple of plots, we saw that maximum daily inflammation exhibits a strange behavior and raises one unit a mean solar day. Wouldn't it exist a good idea to detect such behavior and report it equally suspicious? Let's do that! Withal, instead of checking every single day of the study, permit'southward just bank check if maximum inflammation in the beginning (twenty-four hours 0) and in the middle (mean solar day 20) of the study are equal to the corresponding day numbers.

                          max_inflammation_0              =              numpy              .              max              (              data              ,              axis              =              0              )[              0              ]              max_inflammation_20              =              numpy              .              max              (              data              ,              centrality              =              0              )[              20              ]              if              max_inflammation_0              ==              0              and              max_inflammation_20              ==              20              :              impress              (              'Suspicious looking maxima!'              )                      

We also saw a different problem in the tertiary dataset; the minima per day were all zero (looks like a healthy person snuck into our study). Nosotros can also bank check for this with an elif status:

                          elif              numpy              .              sum              (              numpy              .              min              (              data              ,              axis              =              0              ))              ==              0              :              print              (              'Minima add upwardly to zero!'              )                      

And if neither of these conditions are true, we can utilise else to give the all-clear:

Let's examination that out:

                          data              =              numpy              .              loadtxt              (              fname              =              'inflammation-01.csv'              ,              delimiter              =              ','              )              max_inflammation_0              =              numpy              .              max              (              data              ,              axis              =              0              )[              0              ]              max_inflammation_20              =              numpy              .              max              (              data              ,              axis              =              0              )[              twenty              ]              if              max_inflammation_0              ==              0              and              max_inflammation_20              ==              20              :              print              (              'Suspicious looking maxima!'              )              elif              numpy              .              sum              (              numpy              .              min              (              data              ,              axis              =              0              ))              ==              0              :              print              (              'Minima add upwards to zero!'              )              else              :              impress              (              'Seems OK!'              )                      

            Suspicious looking maxima!                      

                          data              =              numpy              .              loadtxt              (              fname              =              'inflammation-03.csv'              ,              delimiter              =              ','              )              max_inflammation_0              =              numpy              .              max              (              information              ,              axis              =              0              )[              0              ]              max_inflammation_20              =              numpy              .              max              (              data              ,              axis              =              0              )[              20              ]              if              max_inflammation_0              ==              0              and              max_inflammation_20              ==              xx              :              print              (              'Suspicious looking maxima!'              )              elif              numpy              .              sum              (              numpy              .              min              (              information              ,              axis              =              0              ))              ==              0              :              print              (              'Minima add up to zero!'              )              else              :              print              (              'Seems OK!'              )                      

In this way, we have asked Python to do something unlike depending on the status of our data. Here nosotros printed letters in all cases, simply we could also imagine non using the else grab-all so that letters are merely printed when something is incorrect, freeing us from having to manually examine every plot for features we've seen before.

How Many Paths?

Consider this lawmaking:

                              if                4                >                five                :                print                (                'A'                )                elif                4                ==                five                :                print                (                'B'                )                elif                4                <                five                :                print                (                'C'                )                          

Which of the following would be printed if you were to run this code? Why did y'all pick this respond?

1. A
2. B
3. C
4. B and C

Solution

C gets printed considering the first 2 conditions, iv > v and four == 5, are not truthful, but 4 < five is truthful.

What Is Truth?

True and False booleans are not the only values in Python that are true and false. In fact, whatsoever value tin can be used in an if or elif. After reading and running the code below, explain what the rule is for which values are considered true and which are considered faux.

                              if                ''                :                print                (                'empty string is truthful'                )                if                'word'                :                print                (                'discussion is true'                )                if                []:                impress                (                'empty list is true'                )                if                [                1                ,                2                ,                3                ]:                print                (                'non-empty list is truthful'                )                if                0                :                print                (                'zero is truthful'                )                if                one                :                print                (                'one is truthful'                )                          

That's Not Not What I Meant

Sometimes it is useful to check whether some condition is not true. The Boolean operator non can exercise this explicitly. After reading and running the code below, write some if statements that use non to examination the rule that you formulated in the previous challenge.

                              if                not                ''                :                print                (                'empty string is non truthful'                )                if                non                'discussion'                :                impress                (                'word is not truthful'                )                if                not                not                True                :                impress                (                'not not True is true'                )                          

Close Plenty

Write some atmospheric condition that print True if the variable a is within ten% of the variable b and Faux otherwise. Compare your implementation with your partner's: do yous get the same answer for all possible pairs of numbers?

Hint

There is a congenital-in office abs that returns the absolute value of a number:

Solution 1

                                  a                  =                  5                  b                  =                  v.i                  if                  abs                  (                  a                  -                  b                  )                  <=                  0.i                  *                  abs                  (                  b                  ):                  print                  (                  'Truthful'                  )                  else                  :                  impress                  (                  'False'                  )                              

Solution 2

                                  print                  (                  abs                  (                  a                  -                  b                  )                  <=                  0.1                  *                  abs                  (                  b                  ))                              

This works because the Booleans True and False have string representations which tin can be printed.

In-Identify Operators

Python (and most other languages in the C family unit) provides in-place operators that work like this:

                              x                =                ane                # original value                                x                +=                ane                # add one to x, assigning result back to ten                                x                *=                3                # multiply x by iii                                print                (                x                )                          

Write some lawmaking that sums the positive and negative numbers in a list separately, using in-place operators. Practice y'all think the result is more or less readable than writing the same without in-place operators?

Solution

                                  positive_sum                  =                  0                  negative_sum                  =                  0                  test_list                  =                  [                  3                  ,                  4                  ,                  6                  ,                  1                  ,                  -                  1                  ,                  -                  5                  ,                  0                  ,                  7                  ,                  -                  8                  ]                  for                  num                  in                  test_list                  :                  if                  num                  >                  0                  :                  positive_sum                  +=                  num                  elif                  num                  ==                  0                  :                  pass                  else                  :                  negative_sum                  +=                  num                  print                  (                  positive_sum                  ,                  negative_sum                  )                              

Hither pass means "don't exercise annihilation". In this particular example, it's not actually needed, since if num == 0 neither sum needs to alter, but it illustrates the use of elif and pass.

Sorting a Listing Into Buckets

In our information folder, big information sets are stored in files whose names get-go with "inflammation-" and small data sets – in files whose names beginning with "small-scale-". We also have some other files that nosotros do not care about at this signal. We'd like to break all these files into three lists chosen large_files, small_files, and other_files, respectively.

Add code to the template below to do this. Note that the string method startswith returns True if and only if the string it is chosen on starts with the string passed as an argument, that is:

                              'Cord'                .                startswith                (                'Str'                )                          

But

                              'Cord'                .                startswith                (                'str'                )                          

Utilise the post-obit Python code as your starting point:

                              filenames                =                [                'inflammation-01.csv'                ,                'myscript.py'                ,                'inflammation-02.csv'                ,                'small-01.csv'                ,                'small-02.csv'                ]                large_files                =                []                small_files                =                []                other_files                =                []                          

Your solution should:

1. loop over the names of the files
2. figure out which grouping each filename belongs in
3. suspend the filename to that listing

In the stop the three lists should be:

                              large_files                =                [                'inflammation-01.csv'                ,                'inflammation-02.csv'                ]                small_files                =                [                'small-scale-01.csv'                ,                'small-02.csv'                ]                other_files                =                [                'myscript.py'                ]                          

Solution

                                  for                  filename                  in                  filenames                  :                  if                  filename                  .                  startswith                  (                  'inflammation-'                  ):                  large_files                  .                  suspend                  (                  filename                  )                  elif                  filename                  .                  startswith                  (                  'small-scale-'                  ):                  small_files                  .                  suspend                  (                  filename                  )                  else                  :                  other_files                  .                  suspend                  (                  filename                  )                  print                  (                  'large_files:'                  ,                  large_files                  )                  impress                  (                  'small_files:'                  ,                  small_files                  )                  print                  (                  'other_files:'                  ,                  other_files                  )                              

Counting Vowels

1. Write a loop that counts the number of vowels in a character string.
2. Test it on a few private words and full sentences.
3. Once you are done, compare your solution to your neighbour'southward. Did yous brand the same decisions about how to handle the letter 'y' (which some people think is a vowel, and some do non)?

Solution

                                  vowels                  =                  'aeiouAEIOU'                  judgement                  =                  'Mary had a little lamb.'                  count                  =                  0                  for                  char                  in                  judgement                  :                  if                  char                  in                  vowels                  :                  count                  +=                  1                  print                  (                  'The number of vowels in this string is '                  +                  str                  (                  count                  ))                              

Key Points

• Use if condition to start a provisional statement, elif condition to provide additional tests, and else to provide a default.

• The bodies of the branches of conditional statements must be indented.

• Use == to exam for equality.

• X and Y is simply true if both X and Y are true.

• X or Y is truthful if either 10 or Y, or both, are true.

• Zero, the empty cord, and the empty list are considered false; all other numbers, strings, and lists are considered true.

• True and False represent truth values.

schultzromuffel.blogspot.com

Source: https://swcarpentry.github.io/python-novice-inflammation/07-cond/index.html

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