Java程序辅导
C C++ Java Python Processing编程在线培训 程序编写 软件开发 视频讲解
C C++ Java Python Processing编程在线培训 程序编写 软件开发 视频讲解
CSC1051 Data Structures and Algorithms I Dr. Papalaskari
Lab
12
Name:________________________
Checked:______
Objectives:
In
this
lab
we
will
learn
how
to
access
text
files
using
Scanner
to
get
their
contents.
We
will
also
use
Scanner
in
other
novel
ways,
including
reading
data
directly
from
a
webpage!
Before
you
begin,
create
a
folder
for
your
Lab
12
files.
It
is
important
to
save
all
the
data
and
program
files
for
this
lab
in
the
same
folder.
Preparation:
Lab
11
Review
In
part
(e)
of
Lab
11
we
explored
how
to
input
values
from
the
user
and
store
them
in
an
array.
Review
your
program
Lab11e.java
–
recall
that
it
uses
Scanner
to
input
integer
values
from
the
user.
Save
a
copy
of
this
program
in
your
Lab
12
folder,
renaming
it
as
Lab12a.java
A.
Reading
values
from
a
file
and
storing
them
in
an
array
Suppose
you
have
a
long
list
of
values
stored
in
a
file,
and
you
want
to
store
them
in
an
array
of
100
elements.
For
example,
the
file:
www.csc.villanova.edu/~map/1051/s15/examples/oneHundredInts.txt
You
would
not
want
to
re-‐type
these
numbers
in
order
to
enter
them
in
your
program
interactively!
In
Java,
you
can
direct
the
Scanner
to
input
from
a
file,
instead
of
from
the
keyboard.
You
do
this
by:
1)
Replacing
System.in
by
the
file
from
which
you
are
“reading”
when
you
set
up
the
Scanner
object,
i.e.:
Scanner scan = new Scanner(new File("oneHundredInts.txt")); 2)
In
order
to
use
the
File class,
above,
you
need
to
import
some
classes
from
java.io:
import java.io.*;
3)
You
also
need
to
add
“throws IOException”
to
the
heading
of
your
main
method,
so
it
should
read:
public static void main(String[] args) throws IOException
4)
Since
you
want
to
input
100
numbers,
you
need
to
enlarge
the
array
to
hold
100
integers.
5)
Finally,
you
need
to
make
sure
that
the
file
oneHundredInts.txt
is
saved
in
the
same
folder
as
your
program
and
contains
the
correct
data
(a
list
of
100
numbers).
• Some
browsers
add
extra
junk
at
the
beginning
and
end
of
the
file
when
saving
files
from
the
web—open
the
file
in
jGrasp
and
verify
that
your
file
consists
of
exactly
those
100
numbers
and
nothing
else
CSC1051 Data Structures and Algorithms I Dr. Papalaskari
B.
Processing
a
text
file,
line
by
line
www.csc.villanova.edu/~map/1051/s15/examples/FileInput.java
//***************************************************************
// FileInput.java Author: MAP
// Demonstrates the use of Scanner to read text file input.
//***************************************************************
import java.util.Scanner;
import java.io.*;
public class FileInput
{
//------------------------------------------------------------
// Reads text from a file and prints it in uppercase.
//------------------------------------------------------------
public static void main (String[] args) throws IOException
{
String line;
Scanner fileScan;
File myFile = new File("sample.txt");
fileScan = new Scanner (myFile);
// Read and process each line of the file
while (fileScan.hasNext())
{
line = fileScan.nextLine();
System.out.println (line.toUpperCase());
}
}
}
1) Download and compile this program; create a small text file named sample.txt and
save it in the same folder as the program (you can create the file directly in JGrasp – under
File-> New, choose “Plain text” instead of “Java” for the file type). Run FileInput – what
does it do?
____________________________________________________________________
____________________________________________________________________
2) Modify it to use the parameter args[0] of main() as the file name. Do this as follows:
• Replace the use of
File myFile = new File("sample.txt");
with
File myFile = new File(args[0]);
• In jGrasp, select “Run Arguments” from the Build menu, and provide the file name as
an argument (parameter) to main by typing sample.txt in the box that appears
above your program. In this way, you can run your program with different files,
without modifying the code. Try it with the dataset file for next project:
• www.csc.villanova.edu/~map/1051/s15/examples/titanic.txt
CSC1051 Data Structures and Algorithms I Dr. Papalaskari
C.
Scanning
from
a
String
Just
as
we
can
use
a
Scanner
to
input
from
a
file
or
from
System.in,
we
can
also
use
a
Scanner
to
“input”
from
a
String!
1)
Try
this
code:
Lab12c.java
//********************************************************************
// Lab12c.java MA Papalaskari
// Simple example: scanning from a String
//********************************************************************
import java.util.Scanner;
public class Lab12c
{
public static void main(String[] args)
{
Scanner scan = new Scanner(System.in);
System.out.print("Please type 3 words: ");
String line = scan.nextLine();
Scanner scanLine = new Scanner(line);
String word1 = scanLine.next();
String word2 = scanLine.next();
String word3 = scanLine.next();
System.out.println("Word 1: " + word1);
System.out.println("Word 2: " + word2);
System.out.println("Word 3: " + word3);
}
}
Run :
Lab12c.java
– what does it do?
____________________________________________________________________
____________________________________________________________________
D.
Scanning
from
a
String
and
doing
something
useful
Next,
we
will
create
Lab12d.java
by
modifying
Lab12c.java
so
that
it
does
something
more
interesting
with
the
input.
Our
new
program
will
treat
the
input
as
a
command
for
a
simple
numeric
computation.
For
example,
the
input
might
be:
55 * 83
We
want
the
program
to
compute
and
print
the
product
4565.
First,
run
Lab12c.java
with
this
input
and
observe
how
it
picks
out
the
“55”,
“*”,
“83”
as
word1,
word2,
and
word3,
respectively.
Note
that
the
code
uses
scanLine.next()
which
produces
String
tokens
and
that
was
fine
because
word1,
word2,
and
word3
are
Strings.
But
now
you
want
to
use
the
values
55
and
83
as
numbers,
so
the
variables
have
to
be
of
type
double
(we
scanning
from
a
String
CSC1051 Data Structures and Algorithms I Dr. Papalaskari
could
use
int,
but
double
will
allow
you
to
handle
a
wider
range
of
values),
and
you
need
to
obtain
their
values
using
scanLine.nextDouble()
instead
of
scanLine.next().
Can
you
use
these
ideas
to
create
a
simple
calculator?
Change
the
prompt
from
“Please
enter
3
words”
to
“Calculate:
”
Note
that
you
can
test
the
value
of
word2.charAt(0)
to
see
if
it
is
equal
to
‘+’,
‘*’,
etc,
and,
accordingly,
compute
the
result.
(If
you
want
to
be
able
to
handle
more
than
2
operators,
it
is
best
to
use
a
switch
statement.)
Sample
runs:
----jGRASP exec: java Lab12d
Calculate:
9.3 + 44.7
= 54.0
----jGRASP exec: java Lab12d
Calculate:
55 * 83
= 4565.0
E.
Input
data
into
an
array
The
technique
described
in
Parts
C
is
also
useful
for
processing
data
organized
in
columns
and
inputting
that
into
an
array.
Go
back
to
the
code
for
PartC
(NOT
Part
D)
and
modify
the
code
so
that
it
inputs
8
words
into
an
array
of
8
Strings.
(Be
sure
to
replace
the
variables
word1,
word2
etc
by
an
array
word[0],
word[1],
etc.
and
use
a
for-‐loop
to
get
the
input.
The
words
should
then
be
printed
backwards.
Tab
delimited
data:
Sometimes
the
input
tokens
can
contain
spaces.
For
example,
the
“words”
could
be
country
names:
India United States France China Germany Greece South Korea Brazil
These
are
still
just
8
countries!
In
such
situations,
a
tab
can
be
used
as
a
delimiter,
so
the
String
would
be
stored
as:
"India\tUnited States\tFrance\tChina\tGermany\tGreece\tSouth Korea\tBrazil" In
order
for
your
Scanner
to
use
a
delimiter
other
than
whitespace,
you
need
to
specify
this
before
doing
any
input:
scanLine.useDelimiter("\t");
Sample
run:
----jGRASP exec: java Lab12d
Enter 8 country names, all in one line, separated by tabs:
India France Japan India Greece United States South Korea Mali
Mali
South Korea
United States
Greece
India
Japan
France
India
Note:
these
are
tab
characters
CSC1051 Data Structures and Algorithms I Dr. Papalaskari
F.
Processing
data
from
text
files,
organized
in
columns
(Combine
Parts
B
&
E)
The
technique
described
in
Part
E
is
useful
for
processing
text
files
containing
data
organized
in
columns.
We
now
modify
FileInput.java from
Part
B,
above,
so
that
after
it
inputs
each
line,
it
uses
the
technique
of
Lab12e.java,
above
(i.e.,
a
second
Scanner)
to
“scan”
8
words
from
each
line
in
the
file
and
store
these
words
in
an
array,
then
print
the
contents
of
the
array
backwards.
Try
this
with
the
following
file:
http://www.csc.villanova.edu/~map/1051/s15/examples/eightwords.txt
Sample
output:
Line: India France Japan United Arab Emirates Greece United States South Korea Mali
Mali
South Korea
United States
Greece
United Arab Emirates
Japan
France
India
Line:
apple orange asian pear fig persimmon grape raspbery pineapple
pineapple
raspbery
grape
persimmon
fig
asian pear
orange
apple
Line: black white gray light gray dark gray red blue green
green
blue
red
dark gray
light gray
gray
white
black
G.
(Optional)
Input
directly
from
a
website
Would
you
like
your
program
to
access
a
website
directly?
Here
is
how.
You
need
to
1)
Add
another
import
directive
at
the
beginning
or
your
program:
import java.net.URL;
2)
Set
up
your
Scanner
to
read
from
the
url
instead
of
a
file.
Here
is
an
example:
String myurl =
"http://www.csc.villanova.edu/~map/1051/f14/examples/oneHundredInts.txt";
InputStream inStream = new URL(myurl).openStream();
Scanner webScan = new Scanner (inStream);
3)
Now
you
can
use webScan as
any
other
Scanner object,
to
input
from
a
webpage
as
if
it
were
any
other
text
file.
Try
it
with
your
program
lab12a.java.
This
technique
will
work
with
most
webpages,
as
long
as
they
can
be
read
as
text
(including
html
files).