3.6 if…else and if…elif…else Statements

  • Performs different suites, based on whether a condition is True or False.
  • Pseudocode:

If student’s grade is greater than or equal to 60
      Display 'Passed'
Else
      Display 'Failed'

  • Correspondong Python code with variable grade initalized to 85
In [1]:
grade = 85
In [2]:
if grade >= 60: 
    print('Passed')
else:
    print('Failed')

Passed
  • Assign 57 to grade, then shows the if…else statement again to demonstrate that only the else suite executes
In [3]:
grade = 57
In [4]:
if grade >= 60: 
    print('Passed')
else:
    print('Failed')

Failed

In IPython:

  • The up and down arrow keys navigate backwards and forwards through the current interactive session’s snippets.
  • Pressing Enter re-executes the snippet that’s displayed.
  • In JupyterLab, you can select a cell in its left margin, press C to copy it and V to paste it below the currently selected cell.
In [5]:
grade = 99
In [6]:
if grade >= 60: 
    print('Passed')
else:
    print('Failed')

Passed

if…else Statement Flowchart

Flowchart of the if…else statement’s flow of control.

Conditional Expressions

  • Sometimes the suites in an if…else statement assign different values to a variable, based on a condition
In [7]:
grade = 87
In [8]:
if grade >= 60:
    result = 'Passed'
else:
    result = 'Failed'
In [9]:
result
Out[9]:
'Passed'
  • Can write statements like this using a concise conditional expression.
  • The parentheses are not required, but they make it clear that the statement assigns the conditional expression’s value to result.
In [10]:
result = ('Passed' if grade >= 60 else 'Failed')
In [11]:
result
Out[11]:
'Passed'
  • In interactive mode, you also can evaluate the conditional expression directly.
In [12]:
'Passed' if grade >= 60 else 'Failed'
Out[12]:
'Passed'

Multiple Statements in a Suite

In [13]:
grade = 49
In [14]:
if grade >= 60:
    print('Passed')
else:
    print('Failed')
    print('You must take this course again')

Failed
You must take this course again
  • If you do not indent the second print, then it’s not in the else’s suite.
  • In that case the statement always executes, creating strange incorrect output.
In [15]:
grade = 100
In [16]:
if grade >= 60:
    print('Passed')
else:
    print('Failed')
print('You must take this course again')

Passed
You must take this course again

if…elif…else Statement

  • Can test for many cases.
  • Only the action for the first True condition executes.
In [17]:
grade = 77
In [18]:
if grade >= 90:
    print('A')
elif grade >= 80:
    print('B')
elif grade >= 70:
    print('C')
elif grade >= 60:
    print('D')
else:
    print('F')

C

if…elif…else Statement Flowchart

Flowchart of the if…`elif`…else statement’s flow of control.

else Is Optional

  • Handle values that do not satisfy any of the conditions.
  • Without an else, if no conditions are True, the program does not execute any of the statement’s suites.

Logic Errors

  • For a nonfatal logic error, code executes, but produces incorrect results.
  • For a fatal logic error in a script, an exception occurs, Python displays a traceback, then the script terminates.
  • A fatal error in interactive mode terminates the current snippet, then IPython waits for your next input.

©1992–2020 by Pearson Education, Inc. All Rights Reserved. This content is based on Chapter 3 of the book Intro to Python for Computer Science and Data Science: Learning to Program with AI, Big Data and the Cloud.

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