[pythonvis] Re: Yet another indenting explanation
- From: "Richard Dinger" <rrdinger@xxxxxxxxxx>
- To: <pythonvis@xxxxxxxxxxxxx>
- Date: Thu, 15 May 2014 06:19:49 -0700
Joseph is correct a function or class definition is one level deeper than the
declaration, although I was trying to limit the discussion to execution
control-statements for now.
I don’t want to beat this to death, but it seems to be a source of confusion.
Richard
From: Jaffar.sidek10@xxxxxxxxx
Sent: Wednesday, May 14, 2014 10:01 PM
To: pythonvis@xxxxxxxxxxxxx
Subject: [pythonvis] Re: Yet another indenting explanation
Hi Richard. Thanks for this. Just a point of clarification.
following the same satement as in your email, ;do the second and third lines of
the statement occupy the same indent level?
what about if the statement incapsulates a class like this:
class window:
dif __Init__(self, ...):
display.window()
then what should be the levels of indent?
Cheers and thanks.
From: Richard Dinger
Sent: Thursday, May 15, 2014 12:21 PM
To: pythonvis@xxxxxxxxxxxxx
Subject: [pythonvis] Yet another indenting explanation
Many years ago the computer scientist Edsger Dijkstra demonstrated that any
program can be structured out of combinations of the following four basic
constructs:
1 A sequence of statements: execution of the statements is sequential.
2 Branch or conditional control-statement: execution branches to a different
subordinate statement sequence depending on some condition.
3 loop control-statement: execution of a subordinate sequence of statements is
repeated until some condition is met.
4 function call statement: execution jumps to a subordinate statement sequence
and then continues immediately after the point of the function call.
So how is a subordinate statement sequence specified in the source code?
Different programming languages use different approaches. Note the term
statement block may also be used. Algol languages use begin and end to delimit
the sequence as in the following if-statement:
if x < 0 then
begin
x = abs(x)
count = count + 1
end
The C family of languages use braces for a more mathematical looking approach.
In C languages the same if-statement is:
if (x < 0)
{
x = abs(x)
count += 1
}
And Python simply uses the indenting alone like this:
if x < 0:
s = abs(x)
count += 1
You must tell the computer somehow what statements are to be executed when a
control-statement is executed. Python uses the indentation and nothing else.
So in our example, the two statements are indented one more level from the
level of the controlling if statement's level and the subordinate statement
sequence continues until the indenting returns to a lower level. Note that
this is laid out like an outline or a set of help files where each subsequent
level is indented one more level to the right.
The rules for indenting are simple. Start at the left margin in a file or
level0. Control-statements introduce subordinate statement sequences of one or
more statements, which are indented to the next level. You cannot increase
indentation except after a control statement such as a loop or conditional.
Note all such statements end in a colon. The block of indented statements ends
when the level of indentation decreases to a previous level.
If you are still uncertain, try writing some short nonsense programs where you
can test indenting. Try a sequence of two print statements and then a third
print one level deeper. What happens?
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