CODEWITHSUNDEEP
javascriptfor-loop
almost a beginner
almost a beginner

Reputation: 1632

Is it possible to reverse a forloop

I am trying to figure out how to reverse a for loop, it is kinda hard to explain so I will provide an example:

var data_base_array = [3, 3, 1, 4, 1];
var points = [];
for (var index = 0; index <= 4; index++){
    points.push(point_xy_cords[data_base_array[index]][index]);
}

data_base_array contains numbers between 1-5. Another array contains coordinates of points called point_xy_cords. I want to know if there are any methods I can use to reverse this forloop. I can't wrap my head around it, so some logical technique would help get me in the right direction.

If the array points change for example, I want to change data_base_array accordingly. The values of point_xy_cords looks something like this:

point_xy_cords[1-5][0-4][0-1]

so if point_xy_cords[1][0] then it would give result of two values, (123.12, 242.11).

If point_xy_cords[1][0][0] then it would give result of 123.12.

I'm looking for a solution something like this:

for (var x = 1; x <= 5; x++){
  for (var y = 0; y <= 4; y++){
    if (points[y][0] == point_xy_cords[x][y][0] && points[y][1] == point_xy_cords[x][y][1]){
      data_base_array[y] = x;
    }
  }
}

The above possible solution didn't work. Any direction would be appreciated. Danke.

Upvotes: 0

Views: 80

Answers (1)

GramThanos
GramThanos

Reputation: 3622

I think you almost got it...

So the relation between your 2 arrays is :

                       i1  i2, ...
data_base_array     [  j1, j2, ... ]

                       i1                    , i2                    , ...
points              [  point_xy_cords[j1][i1], point_xy_cords[j2][i2], ... ]

So iX is the index and jX is the value on the data_base_array array (where X is a number). Also iX is the index and point_xy_cords[jX][iX] is the value on the points array. So the index of the arrays data_base_array and points is the same (iX).

In order to reconstruct he data_base_array array, if we know the points and the point_xy_cords arrays, we need to know the jX, but we don't. So we need to search all the possible jX to find a possible match on the points[iX] == point_xy_cords[jX][iX].

The reconstruction can be done with the code :

// Init array
var data_base_array = [];

// For each `iX` (from 0 to 4)
for (var i = 0; i <= 4; i++){

    // Init to an invalid number
    data_base_array[i] = 0; // Zero is invalid

    // Find correct `jX` (so search from 1 to 5)
    for (var j = 1; j <= 5; j++){
        // Check if `points[iX] == point_xy_cords[jX][iX]`
        if (
            // Javascript can't compare arrays, so compare each array item
            points[i][0] == point_xy_cords[j][i][0] &&
            points[i][1] == point_xy_cords[j][i][1]
        ){
            // We found a possible `jX`
            data_base_array[i] = j;
            // Stop search
            break;
        }
    }

}

But this is 100% correct, only if you are sure that the pairs in the point_xy_cords[i][j] are unique on each column.

Upvotes: 1

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