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Lesson Two: Everything You Need to Know

Lesson Two: Everything You Need to Know

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Lesson Two: Everything You Need to Know

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  1. Lesson Two: Everything You Need to Know Chapter 6: Loops Learning Processing Daniel Shiffman Presentation by Donald W. Smith Graphics from text

  2. Lesson Two: Everything You Need to Know • 4: Variables • 5: Conditionals • 6: Loops • The concept of Iteration • Two types of loops: • While • For • When do we use them? • Iteration in the context of computer graphics Learning Processing: Slides by Don Smith

  3. What is Iteration? • Something that repeats • Each ‘iteration’ may do something different • Usually has a defined condition to stop repeating • You have already ‘used’ loops • Processing calls draw()over and over until you end your program. Learning Processing: Slides by Don Smith

  4. Why use Iteration? • Without Iteration: // No variables stroke(0); line( 50,60, 50,80); line( 60,60, 60,80); line( 70,60, 70,80); line( 80,60, 80,80); line( 90,60, 90,80); line(100,60,100,80); line(110,60,110,80); line(120,60,120,80); line(130,60,130,80); line(140,60,140,80); line(150,60,150,80); Learning Processing: Slides by Don Smith

  5. Why use Iteration? • Without Iteration: // No variables stroke(0); line( 50,60, 50,80); line( 60,60, 60,80); line( 70,60, 70,80); line( 80,60, 80,80); line( 90,60, 90,80); line(100,60,100,80); line(110,60,110,80); line(120,60,120,80); line(130,60,130,80); line(140,60,140,80); line(150,60,150,80); • Study what changes • x’s increase each time • What is the pattern? • Add 10 each time • When does it stop? • Last line is at x = 150 Learning Processing: Slides by Don Smith

  6. Planning Iteration (Step 1) // With x variable int x = 50; int spacing = 10; int len = 20; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); .. • Without Iteration: // No variables stroke(0); line( 50,60, 50,80); line( 60,60, 60,80); line( 70,60, 70,80); line( 80,60, 80,80); line( 90,60, 90,80); line(100,60,100,80); line(110,60,110,80); line(120,60,120,80); line(130,60,130,80); line(140,60,140,80); line(150,60,150,80); Learning Processing: Slides by Don Smith

  7. Planning Iteration (2) // With x variable int x = 50; int spacing = 10; int len = 20; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); .. • Find the repetitive code Learning Processing: Slides by Don Smith

  8. Planning Iteration (3) // With x variable int x = 50; int spacing = 10; int len = 20; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); .. • Plan the ‘exit’ condition • When to stop drawing lines • x == 150? • x > 150? • x < 150? • x <= 150? • x >= 150? Learning Processing: Slides by Don Smith

  9. The only Loop You Really Need: while • Use a Boolean Expression to test if we are done • Just like in if-else • There are there other types loops available: • [do-]while • for Condition True Action(s) False Learning Processing: Slides by Don Smith

  10. How to plan a loop: • Three Parts in every loop: • Setup variables • Loop condition • Actions (loop body) including the update of loop condition • Make sure the loop will end! • The condition should be false at some point… • Or you have an ‘infinite’ loop! (not good) Setup Test Condition True Action(s) Change test variable False Learning Processing: Slides by Don Smith

  11. Infinite loops • Forgetting to update the loop condition usually results in an infinite loop • Sometimes an infinite loop is needed • Graphical program is polling user input • Operating systems poll user input • many other operations. Learning Processing: Slides by Don Smith

  12. Legs using Iteration // With x variable int x = 50; int spacing = 10; int len = 20; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); x = x + spacing; line(x,60,x,80); .. • Use the same ‘setup’ • Put the repetitive code inside the loop • Put your exit condition in the ‘test’ // Loop Version int x = 50; int spacing = 10; int endLegs = 150; while(x < endLegs) { line(x,60,x,80); x = x + spacing; } Learning Processing: Slides by Don Smith

  13. Three Parts of the Loop • Find the three parts: • Setup • Test • Change // Loop Version int x = 50; int spacing = 10; int endLegs = 150; while(x< endLegs) { line(x,60,x,80); x = x + spacing; } Learning Processing: Slides by Don Smith

  14. Other Examples • Find the sum of the first 100 integers • Setup • Test condition • Update loop condition • Find the sum of the first 50 even (odd) numbers. • Find the average of the first 100 integers int x = 1; int sum = 0; while(x< 100) { sum = sum + x; x = x + 1; } Learning Processing: Slides by Don Smith

  15. Let’s see if we can plan and write a loop… • Look at Figure 6.5 • Use 200 x 200 grid • What changes per bar? • Plan the three parts: • Setup (initial condition) • Initial y (top bar) ___ • Last y: ________ • Test (when to stop) • ___________ • Change • How far apart are bars? • ___________ • Plan the width too • How wide are they? _____ • Where to start? x = _____ • Note: x doesn’t change! Learning Processing: Slides by Don Smith

  16. Graph Paper to the Rescue! 70 50 60 5 10 20 30 40 50 Learning Processing: Slides by Don Smith

  17. And the code for the loop ‘falls out’… void setup() { size(200,200); background(255); fill(125); } void draw() { int y = 5; int last = height; while(y < last) { rect(50,y,100,10); y = y + 20; } } Learning Processing: Slides by Don Smith

  18. Exercise 6.1: Learning Processing: Slides by Don Smith

  19. Exit Conditions: When to stop! • How do we know when to stop? • Test is usually based on a comparison with a variable • Until age is 21 • While changeCount is greater than 5 • While not done • Until hitSomething is true • While dayOfMonth is less than daysInMonth • What if we don’t stop? • You probably forgot to change the test variable! • Or you changed it in the wrong direction • You have written an ‘infinite’ loop • Not good. Must ‘kill’ processing to get out! Learning Processing: Slides by Don Smith

  20. Your next loop: for Setup • Use a for loop when: • 1) Your ‘test’ variable is a number • 2) You know when to stop before you start • Put all three parts on one line: for(Setup variables; Test Condition;Change test variable){ // Action(s) go here } Test Condition True Action(s) Change test variable False Learning Processing: Slides by Don Smith

  21. for loop Syntax and Example Use two semicolons (;) as separators: for(Setup variables; Test Condition ;Change test variable){ for( int x = 1; x < 10 ;x = x + 1) { // Action(s) go here } • Who came up with this? • Programmers of course • Why? • It saves typing, and it puts all three parts on one line Learning Processing: Slides by Don Smith

  22. Exercise 6.2: While to for loop Learning Processing: Slides by Don Smith

  23. Exercise 6-3Wuzzit Do? for (int i = 0; i < 10; i++) { rect(i*20,height/2, 5, 5); } int i = 0; while (i < 10) { ellipse(width/2,height/2, i*10, i*20); i++; } for (float i = 1.0; i < width; i *= 1.1) { rect(0,i,i,i*2); } int x = 0; for (int c = 255; c > 0; c –= 15) { fill(c); rect(x,height/2,10,10); x = x + 10; } Learning Processing: Slides by Don Smith

  24. Example 6-4 • Global versus Local variables in loops • Which will these produce? A, B or C? // Global " count " int count = 0; void setup() { size(200,200); } void draw() { count = count + 1; background(count); } ________ // Local " count " void setup() { size(200,200); } void draw() { int count = 0; count = count + 1; background(count); } ________ Learning Processing: Slides by Don Smith

  25. for Loop Variable ‘Block Scope’ for( int x = 1; x < 10;x = x + 1) { // steps to do inside the loop // You can use ‘x’ anywhere in this box } if (x > 100) // but x is gone now! • Scope is the ‘lifetime’ of a variable. • When ‘x’is declared in the for statement: • ‘x’ exists only inside the ‘block’ of the for loop { } • Do you have to declare x inside the for loop? NO int x; // Declare before loop starts (local scope) for( x = 1; x < 10;x = x + 1) { // assign here Learning Processing: Slides by Don Smith

  26. Loop Inside the Main Loop • Remember that draw()is called in a ‘loop’ • This is often called the ‘main’ loop • The last thing that draw()does is.. . • (remember Chapter 1?) ….. Paints the screen • How could you make a set of parallel horizontal lines on the screen? Learning Processing: Slides by Don Smith

  27. Exercise 6.5 • Render one line at a time inside a for loop: • Until the reach the bottom Learning Processing: Slides by Don Smith

  28. Marching Lines Plan • Use a global variable to remember where on the Y axis to start each time (between 1 and 20) • 0, 1, 2, 3…. 19, 0, 1, 2, 3… • Initialize it 0 • Every time through draw() • 1) Clear the screen • 2) Draw a set of parallel lines (20 pixels apart) • Starting at the Y axis where the global variable tells us to • Add 20 to the Y axis each time through • Loop until Y is >= the height of the screen • 3) Increment the global variable by 1 • If it reaches 20, set it back to 0 Learning Processing: Slides by Don Smith

  29. Shutters Plan (what changed?) • Use a global variable to remember where on the Y axis to start each time (between 1 and 20) • 0, 1, 2, 3…. 19, 0, 1, 2, 3… • Initialize it 0 • Every time through draw() • 1) Draw a set of parallel lines (20 pixels apart) • Starting at Y axis where the global variable tells us to • Add 20 to the Y axis each time through • Loop until Y is >= the height of the screen • 2) Increment the global variable by 1 • If it reaches 20, set it back to 0 and… • Clear the screen Learning Processing: Slides by Don Smith

  30. Increment and Decrement Operators • Loops often use a counter and increment or decrement it as they go. • Here is a a “shortcut” way to decrement or increment a variable: increment operator ++x; // increments x by one – BEFORE it is used x++; // increments x by one – AFTER it is used decrement operator --x; // decrements x by one – BEFORE it is used x--; // decrements x by one – AFTER it is used • When these operators are used as part of an expression, then the choice of pre versus post will make a difference. Learning Processing: Slides by Don Smith

  31. How to measure distance between points • Take one axis (X) for example: • float distance = abs(mouseX – i); • abs() function returns a positive number • Absolute value… remember algebra! • Used in example 6.9 • Exercise 6.6: • Rewrite 6.9 using a for loop Learning Processing: Slides by Don Smith

  32. Summary • Iteration (looping) is another way to control the flow • Every loop must have three elements: • 1) Initialization (of something to count or test) • 2) Test if you are done yet • 3) Update the test variable • The ‘while’ loop can do everything you need • Initialize before the test, test up front, don’t forget to update! • A ‘for’ loop puts all three elements on one line • A variable’s ‘scope’ can be: • Global, Local to the method, or inside a block of code { } • Increment (++) or decrement (--) operators save typing! • You can use print() and println() to help debug your programs while you are developing them Learning Processing: Slides by Don Smith