Conditional Statements
Some decisions are inevitable
Under certain conditions some decisions are sometimes in normal life inevitable,
as we can can see in our photo.
It's the same for every program, which has to solve some useful problem.
There is hardly a way to program without having branches in the flow of code.
In programming and scripting languages, conditional statements or conditional
constructs are used to perform different computations or actions depending on
whether a condition evaluates to true or false. (Please note that true and false are always
written as True and False in Python.)
The condition usually uses comparisons and arithmetic expressions with variables.
These expressions are evaluated to the Boolean
values True or False. The statements for the decision taking are called conditional
statements, alternatively they are also known as conditional expressions or
conditional constructs.
The if-then construct (sometimes called if-then-else) is common across many programming
languages, but the syntax varies from language to language.
The if Statement
The general form of the if statement in Python looks like this:
if condition_1:
statement_block_1
elif condition_2:
statement_block_2
else:
statement_block_3
If the condition "condition_1" is True, the statements in the block statement_block_1 will
be executed. If not, condition_2 will be executed. If condition_2 evaluates to True, statement_block_2
will be executed, if condition_2 is False, the statements in statement_block_3 will be executed.
Example Dog Years
It's a generally accepted belief, to assume that one year in the life a dog corresponds to
seven years in the life of a human being. But apparently there are other more subtle methods to
calculate this haunting problem, haunting at least for some dog owners.
Another subtler method works like this:
- A one year old dog roughly corresponds to a fourteen year old child
- A dog who is two years old corresponds to a 22 year old human
- Every further dog year corresponds to five human years
age = input("Age of the dog: ")
print
if age < 0:
print "This can hardly be true!"
elif age == 1:
print "about 14 human years"
elif age == 2:
print "about 22 human years"
elif age > 2:
human = 22 + (age -2)*5
print "Human years: ", human
###
raw_input('press Return>')
There is one drawback to the script. I works only for integers, i.e. full years.
True or False
Unfortunately it is not as easy in real life as it is in Python to differentiate between
true and false:
The following objects are evaluated by Python as False:
- numerical zero values (0, 0L, 0.0, 0.0+0.0j),
- the Boolean value False,
- empty strings,
- empty lists and empty tuples,
- empty dictionaries.
- plus the special value None.
All other values are considered to be True.
Abbreviated IF statement
C programmers usually know the following abbreviated notation for the if construct:
max = (a > b) ? a : b;This is an abbreviation for the following C code:
if (a > b) max=a; else max=b;C programmers have to get used to a different notation in Python:
max = a if (a > b) else b;