1. Computer Programming Methodology Files and Colors
COS 130
Computer Programming Methodology
Files and Colors

2. Files May be a device, like a printer
Or a section of the disk (document)
By default the input is the keyboard
By default the output is the screen
We can redirect this with > <
To get things into the program we use read instructions
To get things out of the program we use write or print instructions

3. Letters vs. Numbers “11” is not the same as 11
“11” is two letters that happen to be ‘1’ and ‘1’
11 is a number that we can do math with
In the computer
“11” is
11 is 1011

4. Text Files Text files have only characters (letters)
Many programs know how to look at text files (Notepad, etc.)
It is possible to use only text files
However,
For some things text files become very large and inefficient.

5. Binary Files May be much smaller
Allows numbers to be saved in the format we need for computation, without translating back and forth to characters
However,
Programs must be written specifically for the format used.

6. Binary Numbers “Natural” for the computer Base 2
Roll to the next column every time we reach 2

7. Decimal
123
Ones 100
Tens 101
Hundreds 102
Binary
1010
Ones 20
Twos 21
Fours 22
Eights 23

8. Let’s Count
Decimal Binary

9. Red, Green, Blue (RGB) Model
How to Represent a Color with 1’s and 0’s

10. RGB Model For each pixel (spot) use 3 numbers
Each number is between 0 – 255 (inclusive)
First number is the Red value
Second number is the Green value
Third number is Blue value
The higher the number the greater the color intensity

11. Examples
Red
Green
Blue

12. PPM Format A file header A sequence of pixels
Tells what the rest of the file is like
Metadata
A sequence of pixels
As many as we said in the header
Each pixel is an RGB triple

13. PPM Header P6 White spaces Width using ASCII (letters) in decimal
Height using ASCII (letters) in decimal
Max color value using ASCII (letters) in decimal
A SINGLE White space
# to end of line are comments, they may occur anywhere in the header

14. PPM Header Example P6 800 600 255 P6 #Image by John Hunt
#This is a rainbow
800 #Width
600 #Height
255 #This many shades

15. Writing to Files System.out.print(…); Prints characters
Always takes a single argument
Converts argument to a String
Example
System.out.print(“The “);
System.out.print(“quick “);
System.out.print(“brown fox”);
Outputs
The quick brown fox

16. Writing to Files System.out.println(…); Like System.out.print
But, goes to the next line
This varies depending on system
Example
System.out.println(“The “);
System.out.println(“quick “);
System.out.println(“brown fox”);
Outputs
The
quick
brown fox

17. Writing to Files Both print and println can use
special control characters
\n puts a new line character in.
In most situations this causes the text to go to
the next line.
\t puts a tab character in.
In most situations this causes multiple spaces to
be displayed.

18. Writing to Files Example Outputs System.out.print(“The \n“);
System.out.println(“quick \n“);
System.out.println(“brown fox\n”);
Outputs
The
quick
brown fox

19. Writing to Files A Caution
System.out.println(); uses two characters to move to a new line on MS Windows
PPM allows only a single white space character at the end of a header
To avoid this use System.out.print(“\n”); which will always use just a single new line character

20. Writing to Files System.out.write(…); Writes binary
Will use the least number of bytes
Example
System.out.write(255);
System.out.write(0);
Example
int red = 255;
int green = 0;
int blue = 0;
System.out.write(red);
System.out.write(green);
System.out.write(blue);

21. Write the whole file
When writing to disk, using write, your program puts data in memory, then moves it in big blocks to disk
When your program ends the data in memory is thrown away without being written
You must tell your program when to wrap it up
Use System.out.flush();

22. Example public class Flush { public static void main(String args[]) {
for(int i = 0; i < 800; i=i+1){
System.out.write(i); }
System.out.flush();

23. What does this print out?
A Last Trick
int numTimes = 100;
for(int i=0; i < numTimes; i=i+1) {
System.out.println(i%4); }
What does this print out?