Index Variables: 6.2.1

  • You can use a variable or expression inside the brackets for an array index
double[] testScores = {90.0, 45.5, 83.0};

int index = 1;

testScores[index] = 70.0;

double combinedScores = testScores[index] + testScores[index - 1];

For Loop to Traverse Arrays: 6.2.2

  • You can iterate an array using a for loop
    • called traversing an array
    • start at 0, loop while the index is less than the length of the array
    • variable i is often used in loops
      • short for index
double[] testScores = {90.0, 45.5, 83.0};

for (int i = 0; i < testScores.length; i++) {
    System.out.println("Test score is " + testScores[i]);
}

Arrays Are Objects!

Remember that Arrays in Java are objects! If you pass one in as a parameter, know that you are passing an object reference, and any change to that array will effect the array outside the method too.

Looping back to front: 6.2.3

  • You can loop however you want! Just do the math :)

double[] wheelSpeeds = {1.0, 3.0, 4.7, 2.4};

for (int i = wheelSpeeds.length-1; i >= 0; i--) {
    System.out.println(wheelSpeeds[i]);
}

Common Errors when looping through arrays: 6.2.5

  • The first index is 0!
  • The last index is arrayName.length - 1
  • An off by one error would result in an ArrayIndexOutOfBoundsException!

Summary: 6.2.7

  • Iteration can be used to access all elements in an array
    • traversing an array
    • elements are accessed using an index
  • Arrays start at index 0 and go to length-1!