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		<h1>Introductory Programming in Python: Lesson 3<br />
		Basic Output</h1>

		<div class="centered">
			[<a href="invocation.html">Prev: Running Python and Python Code</a>]&nbsp;[<a href="index.html">Course Outline</a>]&nbsp;[<a href="basic_input.html">Next: Basic Input</a>]
		</div>

		<h2>The print Statement</h2>

		<p>The most basic statement in Python is "print". The <a
		class="doclink" href="http://docs.python.org/ref/print.html">print
		statement</a> causes whatever follows it to be outputted to the screen.
		We've already encountered it previously, now it's time to understand
		how it works. Start up the python interactive shell, and let's explore.
		Type the following:</p>

		<pre class='listing'>
Python 2.6.4 (r264:75706, Dec 7 2009, 18:43:55) [MSC v.1310 32 bit (Intel)] on win32
[GCC 3.4.6 (Gentoo 3.4.6-r1, ssp-3.4.5-1.0, pie-8.7.9)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
&gt;&gt;&gt; print 1
1
&gt;&gt;&gt; print 173+92
265
&gt;&gt;&gt; print 173+92.0
265.0
&gt;&gt;&gt; print "hello"
hello
&gt;&gt;&gt;
</pre>

		<p>As one can see, the print statement outputs the
		<strong>value</strong> of the expression immediately following it to
		the screen, and moves to the next line. Note that the third print
		statement produces slightly different output, namely the extra '.0'.
		This is because 92.0 and 92 are different to a computer. 92 is an
		integer, whilst 92.0 is a real number, or in computing terms a
		<strong>floating point number</strong> or float for short. The
		differences will be covered later.</p>

		<p>Also of importance is the expression "hello" (note the double
		quotes). The value of "hello" is <em>hello</em>, and this is what is
		outputted to the screen by the print statement. <em>hello</em> is
		designated as a <strong>string</strong> by enclosing it in quotes. Try
		to print hello without the quotes and see what happens.</p>

		<pre class='listing'>
&gt;&gt;&gt; print hello
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in ?
NameError: name 'hello' is not defined
&gt;&gt;&gt;
</pre>

		<p>What's going on? Welcome to your first bug! We will soon learn to dissect and
		understand what all that means, but for the moment it is sufficient to
		understand that something has gone wrong. But what? Recall from basic
		concepts we were able to store values in 'labels' or 'variables'.
		Python consists of a limited set of key words that have special
		meaning. These key words form the list of atomic statements and
		expressions that python knows how to handle. Whenever python encounters
		a word is doesn't recognise, it treats this as a label name. So it
		obviously doesn't recognise hello as a statement, it thus treats it as
		a <strong>variable</strong>. Variables must have a value, because
		variables are expressions in and of themselves. But we haven't told
		python what the value of hello is, hence it complains "'hello' is not
		defined".</p>

		<p>The print statement is not so plain and boring as it seems. It can
		do a few more things that bear mention. Try entering <code>print "Jane
		has", 7, "apples."</code></p>

		<pre class='listing'>
&gt;&gt;&gt; print "Jane has", 7, "apples"
Jane has 7 apples
&gt;&gt;&gt;
</pre>

		<p>Of course we could just as easily use <code>print "Jane has 7
		apples"</code> and get the same result. However, separating the number
		7 out illustrates two important things about the print statement.
		Firstly, we can in fact output the values of any number of expressions
		in a comma separated list, and secondly the outputs of each of the
		expressions in the comma separated list are separated by a single space
		each.</p>

		<p>Finally, you will notice that the print statement always ends off
		the line, and starts a new one. Simply leaving a comma on the end
		prevents this, e.g. <code>print "Enter your name:",</code>. In summary:</p>

		<ul>

			<li>The print statement prints out the <em>values</em> that follow
			it and then prints a new line</li>

			<li>An empty print statement ends the current line and starts a new
			one.</li>

			<li>The print statement can print multiple expressions if they are
			given in a <em>comma separated</em> list. In this case, a space is
			included between each expression's outputted value.</li>

			<li>The automatic newline outputted by print can be avoided by
			putting a trailing comma in the expression list. This will print a
			trailing space instead.</li>

			<li>If a print statement with a trailing comma is followed by an
			<em>empty</em> print statement, the trailing space is
			suppressed.</li>

		</ul>

		<h2>Some String Basics</h2>

		<p>Python treats all text in units called strings. A string is formed
		by enclosing some text in quotes. Double quotes, or single quotes may
		be used. There is a small limitation to this however, being that a
		string cannot be broken across multiple lines.</p>

		<pre class='listing'>
'this is a string of text'
"this is also a string of text"
'this string will
    cause an error, because it spans multiple lines'
</pre>

		<p>Trying to enter the third string into the interactive shell yields</p>

		<pre class='listing'>
&gt;&gt;&gt; 'this string will
    File "&lt;stdin&gt;", line 1
            'this string will
                            ^
SyntaxError: EOL while scanning single-quoted string
&gt;&gt;&gt;
</pre>

		<p>EOL meaning End Of Line. If we want to introduce line breaks into
		strings we can use two methods. The first, and simplest, is to use
		'triple quotes' meaning three double or three single quotes to indicate
		both the beginning and the end of the string, as in</p>

		<pre class='listing'>
&gt;&gt;&gt; """this string will not
...  cause an error
...    just because it is split over three lines"""
'this string will not\n  cause an error\n    just because it is split over three lines'
&gt;&gt;&gt;
</pre>

		<p>The immediately obvious disadvantage is that everything between the
		triple quotes is taken as-is, meaning the second and third lines of my
		string which I indented to line up with the beginning of the first line
		are indented in the string itself in the form of three spaces after
		those '\n' thingies. Speaking of which, what the hell are those things?
		Why does our string contain stuff we didn't put there? Well let's try
		to print the string out ...</p>

		<pre class='listing'>
&gt;&gt;&gt; print """this string will not
...    cause an error
...    just because it is split over three lines"""
this string will not
   cause an error
   just because it is split over three lines
&gt;&gt;&gt;
</pre>

		<p>Well the '\n's are gone, but what were they? Strings are sequences
		of characters and are one dimensional. They have no
		<strong>implicit</strong> way to specify a line break, or relative
		position, or which characters are where relative to which other
		characters in the string in two dimensions, as displayed on a screen.
		Hence we get the second method of specifying line breaks within a
		string. There are special characters known as <strong>escape
		characters</strong> which mean special things inside strings. They all
		start with a backslash '\' which escapes the following character from
		the string, or in layman's terms means the following character in the
		string is not a 'normal character' and should be treated specially.
		Some important escape characters are</p>

		<dl>
			<dt>\n</dt>

			<dd>line break or New line (n from the n in new line)</dd>

			<dt>\t</dt>

			<dd>tab</dd>

			<dt>\\</dt>

			<dd>a plain backslash</dd>
		</dl>

		<p>You will see that because a backslash already has a special meaning,
		namely "treat the next character specially", we can't simply put a
		backslash into our string. So we escape the backslash with a second
		backslash, meaning we actually want a backslash and not a special
		character.</p>

		<p>Finally, how do we actually put quotes inside a string since they
		indicate the <strong>end</strong> of a string. The easiest solution is
		to mix your quotes. If you want a single quote in a string, define the
		string with double quotes, e.g.  <code>"I've got this escape thing all
		figured out!"</code>. Alternatively, you can actually escape quotes
		within strings, to give them the special meaning that they don't end
		the string, e.g. <code>'I\'ve got this escaped thing totally figured
		out!'</code></p>

		<h2>Exercises</h2>

		<ol>

			<li>What does the print statement do generally?</li>

			<li>Start the python interactive interpreter:
				<ol>

					<li>Output your first name.</li>

					<li>Output your surname.</li>

					<li>Output your first name followed by a space followed by
					your surname.</li>

					<li>Create a variable called 'firstname' and put your first
					name into it.</li>

					<li>Create a variable called 'surname' and put your surname
					into it.</li>

					<li>Using only the variables you have created, print your
					first name and surname again, making sure there is exactly
					one space between your two names.</li>

				</ol>
				Quit the python interactive interpreter.</li>
			</li>

			<li>What special rules does the print statement adhere to regarding
			trailing spaces and newlines?</li>

		</ol>

		<p>Consider the following code...</p>

		<pre class='listing'>print "MUCH madness is divinest sense,"
print "To a discerning eye;"
print "Much sense the starkest madness."
print "'T is the majority", "In this, as all, prevails."
print "Assent, and you are sane;"
print "Demur,-you're straightway dangerous",
print
print "And handled with a chain."</pre>

		<ol start='4'>

			<li>What output, exactly, does the above code produce? Indicate
			space with underscores.</li>

			<li>Write a program that prints "Hello".</li>

			<li>Write a program that outputs favourite piece of poetry or other
			prose, over multiple lines.</li>

			<li>How can you print a value stored in the variable 'x'?</li>

			<li>How can you print the values of multiple expressions on one line?</li>

			<li>Write a program that outputs your name, age, and height in
			metres in the following format. Make sure age is an integer, and
			height is a float, and not simply part of your string.<br />
			<code class='value'>My name is James, I am 30 years old and 1.78 metres
			tall.</code></li>

			<li>Explain three possible ways to print a string containing an
			apostrophe, for example the string<br /> <code class='value'>The cat's
			mat</code>.</li>

		</ol>

		<div class="centered">
			[<a href="invocation.html">Prev: Running Python
and Python Code</a>]&nbsp;[<a href="index.html">Course
Outline</a>]&nbsp;[<a href="basic_input.html">Next: Basic Input</a>]
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