Time estimate: 90 min.

2.8. For Loops

Another type of loop (or iteration) in Java is a for loop. It is usually used when you know how many times you want the loop to execute, as a simple counter-controlled loop to do the loop body a set number of times.

If you took AP CSP with a block programming language like App Inventor, you probably used a for loop block like below that looks very similar to Java for loops. If you have used a language like Scratch or Snap!, you may remember the repeat(n) block where you type in a number of times you want the code to be repeated, just like the AP pseudocode REPEAT block. In fact, almost every programming language has a for or repeat loop.

../_images/loopAppInv.png

Figure 1: Comparing App Inventor and Java for loops

2.8.1. Three Parts of a For Loop

A for-loop combines all 3 parts of writing a loop in one line to initialize, test, and update the loop control variable. The 3 parts are separated by semicolons (;). Each of the three parts of a for loop declaration is optional (initialization; Boolean expression; update), but the semicolons are not optional.

for (initialize; test; update)
{
   loop body
}

The for-loop is almost a shortcut way to write a while loop with all three steps that you need in one line. A for loop can be rewritten into an equivalent while loop (and vice versa).

../_images/compareForAndWhile.png

Figure 2: Showing how a for loop maps to a while loop

Watch the following video which compares a while loop and for loop line by line.

Here is a control flow diagram for a for loop. In a for loop, the initialization statement is only executed once before the first Boolean expression evaluation. The variable being initialized is referred to as a loop control variable. The Boolean expression is evaluated immediately after the loop control variable is initialized and then followed by each execution of the increment statement until it is false. In each iteration, the update is executed after the entire loop body is executed and before the Boolean expression is evaluated again. When the loop condition is finally false, we jump out of the loop and continue at the next statement after the body of the loop.

../_images/ForLoopFlow.png

Figure 2: Control flow in a for loop

coding exercise Coding Exercise

Here is a for loop that counts from 1 to 5. Can you change it to count from 2 to 10?

Here is a while loop that counts from 5 to 10. Run it and see what it does. Can you change it to a for-loop? Run your for-loop. Does it do the same thing?

Note

Two common patterns in for-loops are to count from 0 up to an number (using <) or count from 1 to the number including the number (using <=). Remember that if you start at 0 use <, and if you start at 1, use <=. The two loops below using these two patterns both run 10 times. The variable i (for index) is often used as a counter in for-loops.

// These loops both run 10 times
// If you start at 0, use <
for(int i = 0; i < 10; i++)
{
   System.out.println(i);
}
// If you start at 1, use <=
for(int i = 1; i <= 10; i++)
{
   System.out.println(i);
}

exercise Check your understanding

2.8.2. Decrementing Loops

You can also count backwards in a loop starting from the last number and decrementing down to 0 or 1. All 3 parts of the loop must change to count backwards including the test of when to stop. For example, for (int i=5; i > 0; i--) counts from 5 down to 1.

coding exercise Coding Exercise

What do you think will happen when you run the code below? How would it change if you changed line 11 to initialize i’s value to 3? Try the Code Lens button to visualize and trace through this code.

The method printPopSong prints the words to a song. It initializes the value of the variable i equal to 5 and then checks if i is greater than 0. Since 5 is greater than 0, the body of the loop executes. Before the condition is checked again, i is decreased by 1. When the value in i is equal to 0 the loop stops executing.

Can you make the loop count by 2s backwards? It should print out 5 3 1? Remember to change all 3 parts of the for loop.

2.8.3. groupwork Coding Challenge : Turtles Drawing Shapes

Let’s use a for loop to have a Turtle draw different shapes. We encourage you to work in pairs.

The following code has 4 methods to draw shapes using a Turtle t passed in as an argument.

  1. Complete the drawSquare method to use a for loop to have the Turtle t draw a square.

  2. Complete the drawTriangle method to use a for loop to draw an equilateral triangle. How many times should the loop run? It ran 4 times for a square, so how many for a triangle? What angle should you use for the turns? One way to figure this out is to notice that to complete a shape, all the exterior angles should add up to 360 degrees. So, for a square 4x90 = 360. What angle times 3 will give you 360?

  3. Complete the drawPentagon method to use a for loop to draw a pentagon (which has 5 sides and looks like a stop sign). What external angle should you use for the turns? Remember they have to add up to 360 degrees after 5 turns.

  4. Complete the drawPolygon method to use a for loop to draw any polygong with n sides of length pixels given as arguments. Use n in your loop for the number of sides (or the number of iterations). Use pixels for the amount to move forward. Calculate the angle to turn by using a formula that uses n and 360, so that n turns add up to 360 degrees.

  5. In the main method, call the drawPolygon method to draw a hexagon (6 sides). This method can draw a variety of shapes by just changing the value of the argument n. The power of abstraction! Try drawing other shapes with it. Note that if the turtle runs into walls, it stays there and will mess up the shape, so you may have to move the turtle or go forward smaller amounts.

Complete the methods below with for-loops to draw a square, triangle, pentagon, and then any polygon using a variable n that holds the number of sides. Call 1 more call to the method drawShapes in main. Note that the angles in the turns have to add up to 360.

2.8.4. Summary

  • (AP 2.8.A.1) A for loop is a type of iterative statement. There are three parts in a for loop header: the initialization (of the loop control variable or counter), the Boolean expression (testing the loop variable), and the update (to change the loop variable).

  • (AP 2.8.A.2) In a for loop, the initialization statement is only executed once before the first Boolean expression evaluation. The variable being initialized is referred to as a loop control variable.

  • (AP 2.8.A.2) The for loop Boolean expression is evaluated immediately after the loop control variable is initialized and then followed by each execution of the increment (or update) statement until it is false.

  • (AP 2.8.A.2) In each iteration of the for loop, the update is executed after the entire loop body is executed and before the Boolean expression is evaluated again.

  • (AP 2.8.A.3) A for loop can be rewritten into an equivalent while loop (and vice versa).

2.8.5. AP Practice

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