Arrays and Iteration

Arbitrary Sized Data

Most data has an arbitrary length.

How do we tell our programs how to process this data?

We use the data collection type array. Then we look at each item using a loop.

What is iterating over an array?

If an array is data that is of different possible lengths, we need syntax that will look at every value in the array.

We can combine a loop with an array to get this syntax.

Looking at one element at a time

This is the simplest way of iterating over an array. We write a for loop that starts at 0 and goes while the value of i is less than the length of the array.

Notice that this for loop correctly won't print anything out for an empty array.

var a = [1,4,2,3,6];

var i = 0;
while( i < a.length ){

  console.log( a[i] )

  i = i + 1;
}

For Loop

For loop is a syntax that takes a while loop condition and combines it with iteration into one structure.

for( var i=0; i<10; i++ ){

  console.log( i + " times" );
}

Exercise 1:

Create an index.html file and script.js file. Run each example one at a time, . Create a console.log for each interation of the array.

Make sure to format the output well so it is clear what is happening.

e.g. console.log('iteration: '+i) instead of console.log(i).

Further:

Investigate different ways go get values out of an array:

• what if you start i at different values?
• what if you change the condition to <=?
• what if you change the iteration? i.e., i = i + 2;

Further:

Use a loop to iterate over this array:

var numbers = [4,2,3,1,2,3,4,5];

Write code that sums all the numbers and console.logs them.

Write code that sums all the even numbers in the array and console.logs them.

Further:

var singaporeTemperaturesThisMonth = [30,31,38,37,35,28];

Write code that averages the numbers.

Further:

Look at the above array in the reverse order.

Further:

Use the interactive template. When the user enters a number, put it into an array. Average the numbers in the array and show the number to the user.

Further:

Using the code above, make some changes:

Show all the values in the array to the user.

The user will enter in a number corresponding to the index of the array. They have selected this value from in the array.

Then the user will enter in a number. Add together the number and the value they selected and show it to the user.