The "print" statement (and related material)
--------------------------------------------

In class 2, we spent a lot of time using the "print" statement.
I wanted to fill in the details on "print" in case people are
confused or wondering.

The basic form of print is

print something

where something is an item or expression of _any_ data type (most
often a string, but potentially anything).  Some data types don't make
sense to print directly (we'll see examples when we get to
object-oriented programming, where a single variable can represent a
_huge_ collection of data and state information), but most that we've
encountered can be printed out by the print statement in a
straightforward way.

So for example

print "hello"              # a string
print 7.5                  # a number
print ["E", "F", "F"]      # a list of strings
print [2, 4, 6, 8]         # a list of numbers
print [[], "", [], "", ""] # a list of empty strings and empty lists
print "That's funny."      # a string again
print len("Shikaripura")   # the result of a function, in this case
                           # the len function, whose results ("return
                           # type") are numbers
print raw_input()          # the result of a function, in this case
                           # the raw_input function, whose results
                           # are strings
print 3.14159*(r*r)        # the result of a calculation, which will
                           # be performed by Python and then printed
                           # out (assuming that some number has been
                           # assigned to the variable r)

Many other examples are possible, and we'll see more examples as we
learn more features of Python.

The print statement also has a feature where it can print out
_more than one item at once_ on a single line; to use this, you
separate the items to be printed with commas:

print "I", "am", "an", "electronic", "computer"
print "I'm", "a", "Python", "interpreter!"
print 3,1,4,1,5,9,2,6,5,3,5
print "I'm",22,"years","old."
print "I'm", age, "years", "old."  # if the variable age has a value

You can mix types arbitrarily here (so you can print strings and
numbers and lists and anything else you can think of on the same
line).

Note that the print statement will insert spaces between the items
which are printed out this way!  There is no way to prevent the
spaces from being added; one space is added for each comma.

Note that

print "Hi","there,","how are you?"
print "Hi", "there,", "how are you?"
print "Hi"   , "there," , "how are you?"

all do exactly the same thing.  (How come?)

On the other hand,

print "Hi","there,","how are you?"
print "Hi","there, ","how are you?"

are different.

Each print statement outputs an entire complete line and then goes on
to the next line for subsequent outputs.  (For programmers: this means
that print outputs a newline after whatever it prints.)

If you want to print a blank line by itself, you can just say

print

and a line will be skipped.

print

is equivalent to

print ""

because it outputs nothing (the empty string) and then goes to the
next line.

This can be inconvenient if it's not what you intended; for
example,

print "I am"
print age
print "years old."

results in output like

I am
54
years old.

instead of

I am 54 years old.

One of the ways to deal with this is to use string concatenation,
which is accomplished in Python with the "+" operator:

string1 + string2

equals a new string whose contents are the contents of string1
followed by the contents of string2.

You can use this inside a print statement

print "I'm " + "glad to see you."

print "This" + " " + "is" + "fun."

print "Me" + "ow!"

or anywhere else in Python (because it's not specific to print
statements, but is a general feature of the language).  So for
example, you could write something like

statement = "I am"
statement = statement + " "
statement = statement + str(age)
statement = statement + " "
statement = statement + "years old."
print statement

to build up a string out of smaller pieces and then print it out.
(Please be sure that you understand how this works; we'll be using
code like this frequently!)

Or

weird_string = "ergro"
prefix = "und"
place = prefix + weird_string + prefix
print "It's buried " + place

We could also have said

print "It's buried", place

to get the same effect.

We spent a fair amount of time in class observing that

print "I have", 2, "eyes"

works but

print "I have " + 2 + "eyes"

doesn't.  Numbers can't be concatenated with strings.  On the other
hand, there is a function called str which turns a number into its
(base 10) string representation.  So str(2) is the same as "2".
(More usefully, str(x) is the same as whatever number x equals,
but represented as a string instead of a number.)

What if you want to go the other way?  We did a brief example along
these lines.  Remember that raw_input(), which reads a line of input
from the user and returns that line as a string, _always returns a
string_.  (That's actually why it's called "raw".)

So sometimes this is good:

print "What is your name?"
name = raw_input()
name_length = len(name)
print "Your name is " + name + " and has " + str(name_length) + " letters."

But sometimes it's annoying:

this_year = 2002
print "How old are you?"
age = raw_input()
print "You were probably born around " + str(this_year-age)

Oops!  That doesn't work!  How come?

That's right, we have to do

this_year = 2002
print "How old are you?"
age = int(raw_input())
print "You were probably born around " + str(this_year-age)

or

this_year = 2002
print "How old are you?"
age = raw_input()
print "You were probably born around " + str(this_year-int(age))

in order to be able to do arithmetic on the age (because it has to be
a number type in order to do arithmetic on it).

Does all this make sense?  If not, please experiment and then ask me
questions about it.

One last thing about print: if you wanted to have several
successive print statements give output all on one line, you
can put a comma at the _end_ of the print statement to indicate
that the next print statement should output on the same line.

print 2,
print 4,
print 6,
print 8,
print 10

Note that you still get a space in between items (but now the items
can be printed with subsequent statements).  Of course, you can do
other things in between:

x = 2
print x,
x = x * 2
print x,
x = x * 2
print x,
x = x * 2
print x,
x = x * 2
print x,
x = x * 2
print x,
x = x * 2
print x,
x = x * 2
print x,
x = x * 2
print x

This sure seems repetitive; when we're all done with sequence
types, we'll talk about more concise ways to write programs which
repeat the same action over and over (looping, or iteration).