I try to reverse an array but without the reverse function.
Like you see I try to decrement the length of the array num=(tab.length-i)-1, so it works. But when I try to add tab[i]=tab[num], it works but the value push inside tab[i] is random [ 10, 9, 8, 7, 6, 6, 7, 8, 9, 10 ].
Do you have any idea why ?
const tab=[1,2,3,4,5,6,7,8,9,10]
let num=0
for(let i=0;i<tab.length;i++){
num=(tab.length-i)-1
console.log(num)
tab[i]=tab[num]
}
the problem is you're modifying the array at the same time you look into it, so when you get to the middle element it was already modified.
here's a manual execution of your script :
i | tab
0 | [1,2,3,4,5,6,7,8,9,10]
1 | [10,2,3,4,5,6,7,8,9,10]
2 | [10,9,3,4,5,6,7,8,9,10]
3 | [10,9,8,4,5,6,7,8,9,10]
4 | [10,9,8,7,5,6,7,8,9,10]
5 | [10,9,8,7,6,6,7,8,9,10]
6 | [10,9,8,7,6,6,7,8,9,10]
7 | ...
8 | ...
9 | ...
I think you can see the problem there
there are 2 ways to solve this problem:
// either create a temp array to keep previous values
const reverseWithTmpArray = arr => {
let tmp = [...arr]
for(let i = 0; i < arr.length; i++){
let num = (arr.length - i) - 1
arr[i] = tmp[num]
}
return arr
}
// or reverse the element by pairs
const reverseByPairs = arr => {
for(let i = 0; i < arr.length / 2; i++){
let num = (arr.length - i) - 1
let tmp = arr[i]
arr[i] = arr[num]
arr[num] = tmp
// can also be written
// [arr[i], arr[num]] = [arr[num], arr[i]]
// using destructuring assignement
}
return arr
}
console.log(reverseWithTmpArray([1,2,3,4,5,6,7,8,9,10]))
console.log(reverseByPairs([1,2,3,4,5,6,7,8,9,10]))As I wrote them these 2 function mutate the input array, it can be rewritten to prevent this side effect
You have to use a temporary variable. Without that, you are just overwriting values like so.
Step 01: 1,2,3,4,5,6,7,8,9,10
Step 02: 10,2,3,4,5,6,7,8,9,10
Step 03: 10,9,3,4,5,6,7,8,9,10
Step 04: 10,9,8,4,5,6,7,8,9,10
Step 05: 10,9,8,7,5,6,7,8,9,10
Step 06: 10,9,8,7,6,6,7,8,9,10
// Now it will just take the values, but they are the same, so you are left with a mirrored array.
Step 07: 10,9,8,7,6,6,7,8,9,10
Step 08: 10,9,8,7,6,6,7,8,9,10
Step 09: 10,9,8,7,6,6,7,8,9,10
Step 10: 10,9,8,7,6,6,7,8,9,10
Do it like this instead:
let tab = [1,2,3,4,5,6,7,8,9,10],
temp = []; // Create temp
let num=0
for(let i=0;i<tab.length;i++){
num=(tab.length-i)-1
temp[i]=tab[num] // Add to temp
}
tab = temp; // Assign temp to tab
console.log(tab)
var tab = [1,2,3,4,5,6,7,8,9,10, 11];
console.log("Before reverse:", tab);
for (var k = 0; k < parseInt(tab.length/2); k++) {
var reverseIndex = (tab.length - 1) - k;
var tmp = tab[reverseIndex];
tab[reverseIndex] = tab[k];
tab[k] = tmp;
}
console.log("After reverse:", tab);
reverse? You could clone the array first if you want to avoid changing the original array. E.g.tab.slice(0).reverse().