Control Structures

• As in any programming language there are mechanisms to control program flow: if ... else, for ..., while

Comparisons testing

• Control flow requires knowing how to compare values, for instance does one value equal another. In Python there are 6 operators that deal with comparisons:

  • == Tests for equality of two values, e.g. x == 2

  • != Tests for inequality of two values, e.g. x != 2

  • < Tests if lhs is less than rhs, e.g. x < 2

  • > Tests if lhs is greater than rhs, e.g. x > 2

  • <= Tests if lhs is less than or equal to rhs, e.g. x <= 2

  • >= Tests if lhs is greater than or equal rhs, e.g. x >= 2

Control blocks

• In Python bodies within control blocks are defined by indentation: spaces or tabs. Each level of indentation defines a separate control block. Tabs and spaces should never be mixed and each block must have the same indentation level.

If else

• The simplest control structure runs one of two different blocks of code depending on the value of a test,

x = 5
if x == 5:
    print('x has the value 5')
else:
    print('x does not equal 5')
x = 4
if x == 5:
    print('x has the value 5')
else:
    print('x does not equal 5')

Gives the output:

x has the value 5
x does not equal 5

• To test for ranges combine test with the and keyword

x = 2
if x > 0 and x < 5:
    print('x is between 0 and 5 (not inclusive)')
else:
    print('x is outside the range 0->5')
x = 7
if x > 0 and x < 5:
    print('x is between 0 and 5 (not inclusive)')
else:
    print('x is outside the range 0->5')

Gives the output:

x is between 0 and 5 (not inclusive)
x is outside the range 0->5

• Here we show an example of incorrect indentation and the subsequent error,

if x == 5:
    print('In x = 5 routine')
   print ('Doing correct thing')  # Results in error "IndentationError:
                                # unindent does not match any outer
                                # indentation level"
else:
    print('Everything else')

If … elif … else

• For situations with more than 2 possible outcomes there is an enhanced version of if ... else using the keyword elif to add additional blocks, e.g.

x = 3
if x == 1:
    print('Running scenario 1')
elif x == 2:
    print('Running scenario 2')
elif x == 3:
    print('Running scenario 3')
else:
    print('Unrecognized option')

Gives the output:

Running scenario 3

• Tests can also be combined with the not to negate the test or with the or keyword to test one of two values.

x = 2
if x == 1 or x == 2:
    print('Running scenario first range')

Gives the output:

Running scenario first range

Category:Tested Examples