Use Higher-Order Functions map, filter, or reduce to Solve a Complex Problem

JavaScript
1 
const squareList = arr => {
  // Only change code below this line

  // Use filter to find the positive integers (No negative numbers or decimals) 

  // Use reduce to mutiply the positive numbers by their own value (example: answer should be [25, 9])

  // return the new array
  return arr;
  // Only change code above this line
};

const squaredIntegers = squareList([-3, 4.8, 5, 3, -3.2]);
console.log(squaredIntegers);

I wrote notes about what I think should be done, but I am struggling with how I should go about it. Can someone please help me understand without giving me the answer?

3

In my own words, I would say if a value in the array is less than 0…then it is not positive. And if a value doesn’t have a remainder of 0…then it is not an integer but instead a decimal. Saying it in my own words seems simple but writing it as code is a different story.

Ahhhh so I should use the map method to mutiply the remaining numbers in the array by their own value. Something like:

[ ].map(squareList(i) i * i);
4 
const squareList = arr => {
  // Only change code below this line

  // Step 1: Use filter to find the positive integers (No negative numbers or decimals) 

  // Step 2: Use map to mutiply the positive numbers by their own value (example: answer should be [25, 9])
  
  return arr;
  // Only change code above this line
};

const squaredIntegers = squareList([-3, 4.8, 5, 3, -3.2]);
console.log(squaredIntegers);

When I am confused I write down the steps I think need to happen as if I were explaining it to a kid. I have done that but I am stuck when it comes to actually applying methods I have learned in previous lessons. Can someone help me out without giving me the answer?

5

You have something for this part, but it has a small issue

This syntax doesn’t look like what you use inside of the map method.

Can you write a function that takes a number and returns it’s square? That function should be the argument for your map

6

Would it be something like this?

.map(num => num * num);
7 
const squareList = arr => {
  // Only change code below this line
  arr.filter(num => num > 0 && num //(num must have a remainder of 0 which indicates it is not a decimal))
  arr.map(num => num * num);
  return arr;
  // Only change code above this line
};

const squaredIntegers = squareList([-3, 4.8, 5, 3, -3.2]);
console.log(squaredIntegers);

This is what I have now but I know I am doing something wrong

8

Calling map or filter doesn’t change arr. That makes a brand new array.

9

Ohhhh okay, so instead I should keep it how I had it the first time and indicate that these changes are going into a brand new array. Like this? :

const squareList = arr => {
  // Only change code below this line
  [].filter(num => num > 0 && num //(num must have a remainder of 0 which indicates it is not a decimal))
  [].map(num => num * num);
  return arr;
  // Only change code above this line
};

const squaredIntegers = squareList([-3, 4.8, 5, 3, -3.2]);
console.log(squaredIntegers);

I am having trouble remembering how I can indicate that the numbers must not be decimals. I believe the % sign is for remainders but I don’t know how to go about using it for the second half of the filter syntax

10

No.

This line right here is fine, but its making a brand new array instead of changing arr. You need to use the return value instead of throwing it away.

See the example here:

11 
const squareList = arr => {
  // Only change code below this line
  arr = [].filter(num => num > 0 && num //(num must have a remainder of 0 which indicates it is not a decimal))
  arr = [].map(num => num * num);
  return arr;
  // Only change code above this line
};

const squaredIntegers = squareList([-3, 4.8, 5, 3, -3.2]);
console.log(squaredIntegers);

Like this?

12

Does that look like the example I linked to?

const words = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];

const result = words.filter(word => word.length > 6);

console.log(result);

It doesn’t look like that example to my eye

13 
const squareList = arr => {
  // Only change code below this line
  squaredIntegers = arr.filter(num => num > 0 && num //(num must have a remainder of 0 which indicates it is not a decimal))
  squaredIntegers = arr.map(num => num * num);
  return arr;
  // Only change code above this line
};

const squaredIntegers = squareList([-3, 4.8, 5, 3, -3.2]);
console.log(squaredIntegers);

I tried doing this first to make it look just like the example, but this gives me an error and says TypeError: “squaredIntegers” is read-only

14

You declared squaredIntegers outside of the function as a const. That’s confusing. I would use a different function name inside of your function

15

Ahh I see, so I should use result as my function name?

16

Huh? I don’t know what “result as your function name” would mean. The function name is given to you:

17 
const squareList = arr => {
  // Only change code below this line
  squareList = arr.filter(num => num > 0 && num) //(num must have a remainder of 0 which indicates it is not a decimal))
  squareList = arr.map(num => num * num);
  return arr;
  // Only change code above this line
};

const squaredIntegers = squareList([-3, 4.8, 5, 3, -3.2]);
console.log(squaredIntegers);

So it should be this?

18

No.

You seem to be confused about how variables work?

Do you know how to declare variables?

arr will not be changed by calling map or filter, so you should not return arr.

You need to declare a new variable that doesn’t share any name with something else to hold the result here.

19

Yes you can declare variables using var, let, and const. People don’t really use var a lot because it causes a lot of problems because it can be easily changed. Using const keeps whatever you’re declaring constant. So in this case I can use let like:

let positiveIntegersSquared = arr.filter(num => num > 0 && num) //(num must have a remainder of 0 which indicates it is not a decimal))

Sorry for being so slow, I know it’s probably frustrating

20

This is closer. You haven’t squared anything yet, but this does get you positive numbers

21

Okay I think this is looking a lot better, I am just having trouble indicating that the number must be an integer and not a decimal. I’m sure there’s multiple ways to do this but I was trying to find a way to say if the number has a remainder of 0 because then the computer should know that number is an integer

const squareList = arr => {
  // Only change code below this line
  let positiveIntegersSquared = arr.filter(num => {
    if (num > 0 && num) return num; //(num must have a remainder of 0 which indicates it is not a decimal))
  }).map(num => { return num * num });
  return positiveIntegersSquared;
  // Only change code above this line
};

const squaredIntegers = squareList([-3, 4.8, 5, 3, -3.2]);
console.log(squaredIntegers);

But right now I just have num for the second half of that if statement