The if statement

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• The most common form of the if statement has syntax

if ( logical-expression ) :

    action₁

else :

    action₂

where logical-expression is a test expression that evalutes to either True or False. Both action₁ and action₂ must be non-empty statement lists. The actions are indented one-level further than the start of the if statement. The else is indented at the same level as the if. Both if and else are keywords

• The semantics of the if statement is that logical-expression is first evaluated. If the expression is True, action₁ is executed; otherwise, action₂ is executed. No matter what only one of the two actions are executed.

• Consider the following code segment from if_else.py.

reply = input( 'Enter one or two words: ' )

list = reply.split()

n = len( list )

if ( n == 1 ) :

    # list size is 1

    print( 'one word' )

else :

    # otherwise, list size is 2

    print( 'two words' )

By getting the input and turning it into a list, we can easily determine the number of words entered (i.e., the length of the list). By determining whether the list size is one. We can produce the appropriate response.

• Suppose our only concern was whether the user could follow instructions. The following code segment from if.py does so without using the optional else part of the if statement.

reply = input( 'Enter one or two words: ' )

list = reply.split()

n = len( list )

# assume they followed directions

response = 'You followed directions'

if ( ( n != 0 ) and ( n != 1 ) ) : # what does it take to be wrong? n is neither 0 nor 1

    # they did not, so response needs updating

    response = 'You did not follow directions'

print( response)

To correctly determine whether the input is too short or too long, the if test expression has multiple terms that are anded-ed together. My advice always parenthesize each term in a logical expression. Although the problem is artificial and can be done other ways. It shows what was intended.

• Some programmers hate not having an else for every if. They want one there to explicitly show they considered the possibility. Because Python requires that a statement list be non-empty, some statement must be there. Python provides thea pass statement for such a purpose (pass is a keyword). When Python encounters a pass statement, it just continues on to the next statement. Some programmers will use a pass statement as a temporary action as they develop code. The pass statement is a placeholder for code to be written.

• The below code segment from if_else_pass.py uses a pass statement statement in support of empty else for our previous problem.

reply = input( 'Enter one or two words: ' )

list = reply.split()

n = len( list )

# assume they followed directions

response = 'You followed directions'

if ( ( n != 0 ) and ( n != 1 ) ) : # what does it take to be wrong? n is neither 0 nor 1

    # they did not, so response needs updating

    response = 'You did not follow directions'

else :

    # no action necessary

    pass

print( response)

• Let's revisit the word entry problem again. Now are concern is to alert people who did not follow directions on what they did wrong and to acknowledge people who did follow directions.

• Python recognizes that often our decision process is not necessarily choosing one thing or another. Sometimes we need to test which of several conditions is in effect, and then take the appropriate action. We can use elif's to test for the various possibilites (when you read the elif think else if ).

• Consider the following code segment from if_elif_else.py. The code segment uses the optional elif part of an if statement.

reply = input( 'Enter one or two words: ' )

list = reply.split()

n = len( list )    

# what are the possibilities for the user response

# enters no words

# enters one word

# enters two words

# enters more than two words (i.e., it is not of the prior cases)    

response = '' # none so far    

if ( n == 0 ) :

    # they did not follow directions -- no words were supplied

    response = 'You did not enter any words'

elif ( n == 1 ) :

    # they did follow directions by entering one word

    response = 'Thanks for following directions by entering one word'

elif ( n == 2 ) :

    # they did follow directions by entering two words

    response = 'Thanks for following directions by entering two words'

else :

    # they did not follow directions -- too many words supplied

    response = 'You entered too many words'    

print( response)

The above if statement recognizes there are four possibilities of concern when users react to the prompt: they did not enter any words, they entered one word, they entered two words, or they did something else (i.e., entered more than two words). For each of these possibilities, the if statement takes the possibility into account and correctly responds.

 

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© 2019 Jim Cohoon

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