//Ex.1 The sum of a range
numbers=[];
function range(start,end,step){
numbers.length=0;
if (step==undefined){step =1;};
if (step<0){
for(i=start;i>=end;i=i+step){
numbers.push(i);
}
}else{
for(i=start;i<=end;i=i+step){
numbers.push(i);
}
}return numbers;
}
function sum(array){
arraySum=0;
for (i=0;i<array.length;i++){
arraySum=arraySum+array[i];
} return arraySum;
}
//Ex.2 Reversing an array
numbers1=[1,2,3];
console.log(numbers1);
function reverseArray(array){
reversedArray=[];
for (i=0;i<array.length;i++){
reversedArray[i]=array[array.length-1-i];
}return reversedArray;
}
console.log(reverseArray(numbers1));
numbers2=[4,5,6];
console.log(numbers2);
function reverseArrayInPlace(array){
for (i=0;i<Math.ceil(array.length/2);i++){
temp=array[i];
array[i]=array[array.length-1-i];
array[array.length-1-i]=temp;
}return array;
}
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Reversing an Array
function reverseArray(arrayToReverse){
var newArray = [""];
var lengthOfArray = (arrayToReverse.length)-1;
var counter = lengthOfArray;
var newArrayCounter = 0;
while (counter >= 0){
console.log("counter "+ counter);
newArray[newArrayCounter] = arrayToReverse[counter];
counter --;
newArrayCounter++;
}
return newArray
}
var myArray = [âLionsâ, âTigersâ, âBearsâ, âWalrusâ, âElephantâ];
var animals = reverseArray(myArray);
console.log(animals);
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Hi @CryptoVic!
Sorry for the delay in giving you some feedback.
In order for me to check your code for this first exercise, I need to also see your function which produces the array in the first place i.e.the function which generates an array based on start, end and step parameters, so we have the functionality to be able to sum different array variations.
Without seeing this initial function, I canât guage how much of the task youâve actually understood. So if what Iâm asking for doesnât make sense, just let me know and we can discuss further! 
In terms of your sum function, itâs looking good, but have another think about the position ofâ return accumulator; within the function. At the moment itâs going to cause your for loop to be exited on the first iteration, only returning the number at index 0, rather than the sum.
Iâm going to have a look at your second exercise now, but I just wanted to get back to you on the first exercise first.
Hi @JustinP!
Sorry for the delay in replying.
Did you forget to add the code for Reversing an Array? 
If you send it again Iâll review it for you.
I can see youâve sent the code for the 2nd exercise - Iâll get to that in a bit.
Hi again, @CryptoVic!
Great effort with Reversing an Array !
It nearly works.
You just have 2 problems with the for loop:
- Your condition causes the loop to be exited before index 0 has been âpushedâ to the end of the array, and not immediately after. See if you can work out what the problem is and how to solve it, then repost your code here.
- Iâm not sure if itâs just a formatting error when posting your code, but your decrement operator is a dash rather than 2 minus signs. This throws an error.
Concerning formatting, Iâm not sure if you know, but before you enter your code to post it here in the forum, click on the </> icon in the menu at the top of the text editor. That will give you 2 sets of 3 back ticksâŚ
```
```
If you now add your code between these, you will end up with it nicely formatted, which is then also easier for you to organise with the correct spacing and indentation etc. Itâs also easier to spot any copy and paste issues such as the dash/minus sign one Iâve mentioned above.
For your Reversing an Array you should then end up with something like the following (Iâve also added a couple of additional points for you to consider concerning best practice):
var reverse = function(arr) {
var result = [], // Change the comma to a semi-colon;
ii = arr.length; // Better practice to declare variable with a keyword
for (var i = ii - 1; i !== 0; i--) { // Condition needs correcting
result.push(arr[i]);
}
return result;
}
console.log(reverse(['A', 'B', 'C', ]));
Have you had a go at the second part yet (reverseArrayInPlace)? Or did you reach saturation point?
If you didnât really understand what was expected, have a look at the hints (which you can display in the online course book by clicking below the exercise instructions. If you still donât get it, have a look at other studentsâ solutions posted here. Then if youâre still stuck let us know and weâll help you out!
Remember, these exercises are very challenging and require you to take your time and really wrestle with the concepts.
Keep up the great work! 
Hey @ClapTrap!
Sorry for the delay in reviewing your answers to the Chapter 4 exercises.
Iâve just reviewed your code for The Sum of a Range so I wanted to get back to you without any further delay with my comments, then Iâll send you another post when Iâve reviewed the second exercise.
Youâve done a great job on the first exercise!
Youâve dealt effectively with the issue of reverse stepping 
The only situation it fails to handle correctly is if step is undefined AND we have a reverse range e.g.
console.log(sum(range(5, 2)));
It would be great to see if you can adapt your code to handle that too! I hope youâre up for the challenge!
Just a couple of further comments:
I donât really understand this line of code in your solution.
numbers.length = 0;
It works, and at first I thought it was redundant, but if itâs removed, it doesnât execute successfully. Iâd be interested to know what your logic is behind it.
Personally, I think itâs clearer to remove this line, and then move the declaration of the array variable into the scope of the function like this:
function range(start, end, step) {
let numbers = []; /* Get into the habit of always declaring your
varaiables with keywords let, const or var */
// .... etc.
}
A more concise and clearer way to handle undefined arguments, is directly in the function header, like this:
function range (start, end, step = 1) { ... }
This achieves the same thing as your if statement, and so avoids having to add it.
if (step == undefined) {
step = 1;
}
I hope that helps!
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Hi again, @ClapTrap!
Reversing an array (returning a new array) Nice!
Yours is a good solution. Now thinking about how we can improve things even furtherâŚ
Can you also see how using .unshift() in the for loop bodyâŚ
reversedArray.unshift(array[i]);
⌠is more concise (and clearer?) than yourâŚ
reversedArray[i] = array[array.length-1-i];
Reversing an array (in place) Great!
Youâre progressing really well! Keep on learning!
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let numbers=[];
function range(start,end,step){
numbers.length = 0;
if (step==undefined){if (start<=end) step =1; else step=-1;};
if (step<0){
for(i=start;i>=end;i=i+step){
numbers.push(i);
}
}else{
for(i=start;i<=end;i=i+step){
numbers.push(i);
}
}return numbers;
}
Thanks for getting in touch, I just updated a correct version of the code which can handle BOTH undefined steps and reverse range.
numbers.length=0; is used to reset the Array so that it does not add a new array to an old one. You can easily see what happens removing and adding this command if you add these 2 lines to the bottom of the code:
console.log(range(1,10,2));
console.log(sum(numbers));
console.log(range(5,2));
console.log(sum(numbers));
Without numbers.length=0; the second array returns [1,3,5,7,9,5,4,3,2] instead of [5,4,3,2].
Defining numbers=[]; inside the function returns me an error of âUncaught ReferenceErrorâ, so I just added it outside.
With regard to the second exercise, I was somewhat insecure about the actual behavior of that function so I just coded it my self. I will be sure to check out that one as well as all the other jQuery functions.
Thanks again for the review.
1 Like
Thatâs great coding, @JustinP!
Here are a few comments to help you build even more upon what already looks like excellent progress:
-
[""]no need for the quotes when you define an empty array i.e. itâs just[]. In fact what youâve actually defined is an array with one element which is an empty string atnewArray[0]. It doesnât affect your code executing correctly, as you replacenewArray[0]in yourwhileloop anyway. However, itâs always good practice to tidy things up, in case it causes a bug if the code is developed futher. -
Have a think about how you can merge these two lines into just one, to make it more concise:
var lengthOfArray = (arrayToReverse.length)-1; // parentheses not needed
var counter = lengthOfArray;
- Your solution is good, but did you try using a
forloop as well? Aforloop would be a more concise solution than using awhileloop. Can you see why?
Iâm not sure if you know, but before you enter your code to post it here in the forum, if you click on the </> icon in the menu at the top of the text editor. That will give you 2 sets of 3 back ticksâŚ
```
```
If you now add your code between these, you will end up with it nicely formatted, which is then also easier for you to organise with the correct spacing and indentation etc. Itâs also easier to spot any copy and paste issues that might have crept in.
For your Reversing an Array you should then end up with something like the following:
function reverseArray(arrayToReverse) {
var newArray = [""]; // quotes should be removed
var lengthOfArray = arrayToReverse.length -1; // combine this line...
var counter = lengthOfArray; // ...with this line
var newArrayCounter = 0;
while (counter >= 0) {
console.log("counter" + counter);
newArray[newArrayCounter] = arrayToReverse[counter];
counter--;
newArrayCounter++;
}
return newArray
}
var myArray = ["Lions", "Tigers", "Bears", "Walrus", "Elephant"];
var animals = reverseArray(myArray);
console.log(animals);
Have you had a go at the second part yet ( reverseArrayInPlace )? Or did you reach saturation point?
If you didnât really understand what was expected, have a look at the hints (which you can display in the online course book by clicking below the exercise instructions. If you still donât get it, have a look at other studentsâ solutions posted here. Then if youâre still stuck let us know and weâll help you out!
Remember, these exercises are very challenging and require you to take your time and really wrestle with the concepts.
Keep up the great work!
1 Like
Hi @Ondrej.S!
Sorry for the delay in getting back to you with some feedback.
The Sum of a Range Great, your solution correctly sums a fixed array from 1-10 using âstepsâ of 1. However, the idea was to write 2 functions. Youâve posted the second one, sum , but what about the first one, range , which should be able to generate an array of any range of numbers based on start and end parameters? The sum function calls the range function within console.log âe.g.
console.log(sum(range(6, 9));
// an array [6, 7, 8, 9] will be passed to the sum function
// => 30
So, we want two functions that will be able to handle any combination of start and end arguments.
Have you tried the bonus assignment which introduces the step (interval) argument? If you havenât, then I encourage you to do so. Itâs a good challenge and will teach you a lot.
Post your additional code for what Iâve mentioned above and weâll review it for you. If you didnât really understand what was expected, have a look at the hints (which you can display in the online course book by clicking below the exercise instructions. If you still donât get it, have a look at other studentsâ solutions posted here. Then if youâre still stuck let us know and weâll help you out!
Remember, these exercises are very challenging and require you to take your time and really wrestle with the concepts.
Iâm going to have a look at your second exercise now, but I just wanted to get back to you on the first exercise first.
1 Like
Hi again, @Ondrej.S!
Reversing an array (returning a new array) Nice!
Your use of the decrement operator in the for loop body is an interesting one. I havenât come across that solution before! Very creative and innovative! 
However, can you see how in the code you posted, the two minus signs (--) have changed to a dashâ( â ) ? Iâm assuming your original code did contain the correct decrement operator. If so, this is an excellent example of why itâs important to format your code when posting it.
Yours is a good solution. Now thinking about how we can improve things even furtherâŚ
Can you also see how using .unshift() in the for loop bodyâŚ
newArray.unshift(myArray[i]);
⌠is more concise (and clearer?) than yourâŚ
newArray[--newArrayIndex] = myArray[i];
Please feel free to disagree! There is nearly always more than one way to get an optimal result.
Have you had a go at the second part yet ( reverseArrayInPlace )? Or did you reach saturation point?
Keep up the great work!
1 Like
Thanks for all of your help. I tried reverseArrayInPlace and got a partial solution (Iâve changed a few things based on your input). I got stuck because I couldnât figure out how to get the new array returned properly. Then I looked at other solutions and realised it wasnât really what was intended! Iâll have another go using the other strategy I sawâŚ
Hereâs my failed attempt:
function reverseArrayInPlace(arrayToReverse){
var newArray = [];
var counter = (arrayToReverse.length)-1;
var newArrayCounter = 0;
while (counter >= 0){
newArray[newArrayCounter] = arrayToReverse[counter];
counter --;
newArrayCounter++;
}
console.log("Array to reverse"+arrayToReverse);
console.log("new array"+ newArray);
arrayToReverse = newArray;
return arrayToReverse;
}
var myArray = ["Lions", "Tigers", "Bears", "Walrus", "Elephant"];
reverseArrayInPlace(myArray);
console.log(myArray);
</script>
type or paste code here
Got it! I think⌠I donât fully understand though, why does this permanently change the contents of myArray? The function is working on itâs own âversionâ of myArray called arrayToReverse, and thatâs what it returned so why has myArray become arrayToReverse permanently?
<script>
function reverseArrayInPlace(arrayToReverse){
var lengthOfArray = arrayToReverse.length;
var halfLengthOfArray = Math.floor(arrayToReverse.length/2);
for (var counter = 0; counter <= halfLengthOfArray; counter++){
var holdingPlace = arrayToReverse[counter];
arrayToReverse[counter]=arrayToReverse[lengthOfArray-1];
arrayToReverse[lengthOfArray-1]=holdingPlace;
lengthOfArray--;
}
return arrayToReverse;
}
var myArray = ["Lions", "Tigers", "Bears", "Walrus", "Elephant"];
console.log(reverseArrayInPlace(myArray));
console.log("MyArrayis now: " + myArray);
</script>
1 Like
Thank you so much for this thorough feedback. Iâm currently working on a trading bot in a different course but I was still planning to come back to this before starting C++. This will help me a lot. Iâll post another possible solution here later.
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Hi, Iâm stuck on the reversing an array exercise Iâve looked at the solution and kinda understand it, but I still donât know how I should solve the exercise, because I know that if I would try to do this exercise without looking at the solution I wouldnât be able to do it. If I could get some help on âthe reversing arrayâ, I would be very grateful.
Sum of a range:
function range(start, end, step = 1){
let numbers = [];
if(start > end){
if(step < 0){
for(let i = start; i >= end; i += step){
numbers.push(i);
}
}else{
for(let i = start; i >= end; i -= step){
numbers.push(i);
}
}
}else{
for(let i = start; i <= end; i += step){
numbers.push(i);
}
}console.log(numbers)
return numbers;
};
function sum(numbs){
let result = 0;
for(let i = 0; i < numbs.length; i++){
result += numbs[0 + i];
}return result;
};
console.log(sum([2,4,7]))
console.log(sum(range(5,2,-1)));
Reverse Array:
let fruits = ["apple", "kiwi", "orange", "banana", "melon"];
function reverseArray(array){
let reverse = [];
for(let i = array.length-1; i >= 0; i--){
reverse.push(array[i]);
}return reverse;
};
console.log(reverseArray(fruits));
Reverse Array In Place:
I needed a lot of help with this one 
let fruits = ["apple", "kiwi", "orange", "banana", "melon"];
function reverseArrayInPlace(array){
for(let i = 0; i <= Math.floor((array.length-1)/2); i++){
let item = array[i];
array[i] = array[array.length-1-i];
array[array.length-1-i] = item;
}return array;
};
console.log(reverseArrayInPlace(fruits));
In the exercise description there was another thing:
âThinking back to the notes about side effects and pure functions in the previous chapter, which variant do you expect to be useful in more situations? Which one runs faster?â
How can I figure out which one is faster? I feel like the reversing an array by creating a new array will run faster, but I canât back this up with hard proofs.
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// The sum of a range
function createRangeArray(start, end) {
var retval = [];
for(var i = start; i <= end; ++i){
retval.push(i);
}
return retval;
}
console.log(createRangeArray(15, 20));
function sumArrayValues(inputArray) {
var retval=0;
for(var i in inputArray) {
retval += inputArray[i];
}
return retval;
}
console.log(sumArrayValues(createRangeArray(1, 10)));
function createRangeArrayStep(start, end, step = 1) {
var retval = [];
var reverseOrder = (start > end);
for(var i = start; ; i += step) {
// I don't like this if / else if code block
// Using reverse arrays may result in a more "elegant" solution
if(reverseOrder) {
if(i < end)
break;
} else if (i > end){
break;
}
retval.push(i);
}
return retval;
}
console.log(createRangeArrayStep(1, 10, 2));
console.log(createRangeArrayStep(5, 2, -1));
console.log(sumArrayValues(createRangeArrayStep(1, 10, 2)));
console.log(sumArrayValues(createRangeArrayStep(5, 2, -1)));
// Reversing an array
function reverseArray(inputArray) {
var retval = [];
for (var i = inputArray.length - 1; i >= 0; --i){
retval.push(inputArray[i]);
}
return retval;
}
console.log(reverseArray([1, 2, 3, 4, 5]));
function reverseArrayInPlace(inputArray) {
for (var i = 0; i <= inputArray.length/2; ++i){
var tmp = inputArray[i]
inputArray[i] = inputArray[inputArray.length - 1 - i];
inputArray[inputArray.length - 1 - i] = tmp;
}
return inputArray;
}
console.log(reverseArray([1, 2, 3, 4, 5]));
// The reverseArrayInPlace function will be faster than the reverseArray
// because in the second one we iterate over all the input array. While in
// the first one we just need to run the loop inputArray.length/2 times.
// But I think that we could implement the reverseArray function in a simmilar way:
// Reversing an array faster
function reverseArrayV2(inputArray) {
var arrayLength = inputArray.length;
var retval = Array(arrayLength);
for (var i = 0; i <= arrayLength/2; ++i){
retval[i] = inputArray[arrayLength - 1 - i];
retval[arrayLength - 1 - i] = inputArray[i];
}
return retval;
}
console.log(reverseArray([1, 2, 3, 4, 5]));
1 Like
Have you already tried looking at the hints (which you can display in the online course book by clicking below the exercise instructions. If you still donât get it, let us know and weâll give you some easier hints.
Iâll just add that the first part of the exercise needs to return a new array i.e. the input array should remain unmodified. Then in the second part you are reversing an array in place, which means the elements of the input array itself should be reversed within the function body, and so the same array is returned but modified.
1 Like
Thanks for the explanation, I understand whatâs happening now. What confused me was the use of:
numbers.length = 0;
âŚas I would have instinctively gone for:
numbers = [];
Thanks for showing me an alternative 
both seem to do exactly the same thing (they both work anyway).
Placingâlet numbers = [] âinside the function (without ânumbers.length = 0â) doesnât throw an error if you call sum like this:
console.log(sum(range(1, 10, 2)));
console.log(sum(range(5, 2)));
Instead of passingânumbers as the argument.
Anyway, great job!
1 Like
I wouldnât be as harsh as to say âfailedâ, because your code works. Itâs just that you arenât really doing anything different to the first part, and the changes are just âcosmeticâ, in that they donât really provide any different functionality â you are still inputting one array (arrayToReverse), not modifying it, but instead creating a new array (newArray) which is a mirror image. You then make it seem like the input array was modified by reassigning the new array to its name:
1 Like