JS Evening

CoreJS: Loops, Arrays, and Functions

20 Jul 2015

While-loops

Consider the following fragments of code: execute them by hand, as shown in class, and then compare your answers to what you get from running the examples in scratchpad. The second is almost the same as the first. Do you understand why they behave differently?

Be sure you understand the examples before you run them!

1.

var x;
var i = 0;
while (!x) {
    i = i + 1;
    if (i == 5) {
        x = true;
    }
    console.log("running");
}

2.

var y;
var j;
while (!y) {
    j = j + 1;
    if (j == 5) {
        y = true;
    }
    console.log("running");
}

For-loops

Consider the following fragment of code: execute it by hand, as shown in class, and then compare your answer to what you get from running the examples in scratchpad.

var i;

for (i = 0; i < 8; i=i+1){
    if(i == 5){
        console.log("It's five!");
        i = i+1;
    }
    console.log(i);
}

What numbers are printed out by this code?

Arrays

var x;
var nums = [3, 1, 4, 1, 5, 9, 2, 7]

Find the outcome of each of the these expressions:

nums[3];
nums[nums.length-1];
nums[nums[4]];

x = nums.concat(nums);
x[x[x[nums[2]]]];
nums[x[x[x[2]]]];

nums.push(x.pop());
x.push(nums.length);
x.push(x.push(0));

x.length = x[6];

Arrays with Loops 1

Examine the following code and step through it by hand. Explain, in words, what this program does. Compare your answer with other members of your group.

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

for (var i=0; i < x.length; i = i + 1){
    if (i % 2 == 0){
        console.log(i + " : " + x[i]);
    }
}

Now, write a small program that

  1. declares a variable holding an empty array;
  2. iterates from 1 through 10 and puts each odd number in the array;
  3. prints out the array.

Evaluate it by hand when you're done (at least for a couple of iterations) and confirm that it works correctly.

Arrays with Loops 2

Given the following array definition: var nums = [3, 1, 4, 1, 5, 9, 2, 7];

  1. Write a loop to console.log all the elements of nums.

  2. Write a loop to change nums so that each of its elements is doubled.

  3. Write a loop to duplicate nums, copying each element of nums into a different array.

Fun with Functions

  1. Write a function which receives a number parameter and returns the square of that number.

  2. Write a function which receives one parameter, an array of numbers, and returns the mean of those numbers. (Hint: you'll need a loop.)

  3. Write a function which receives an integer up to 10 and returns the corresponding word (i.e. 'one', 'two', 'three', etc). (Hint: you'll need an array.)

  4. Use your three functions together to print the word for the square of the mean of these numbers: [2,4,7,-1]

Still More Fun

Examine the following code and execute it by hand.

var thing = function (fun, arg, num) {
    var result = arg;
    for (var i = 0; i < num; i = i+1) {
        arg = fun(arg);
    }
    return arg;
}

var mult = function (num){
    return num*num;
}

console.log(thing(mult,2,3));

Explain in words what this program does.

Summary

Explain, in your own words, the following programming constructs and concepts:

After you're done, share your answers within your group and discuss them.

Samples

scratchpad console

Diagrams

misc