I'm quite new to JavaScript and I'm trying to implement my code in pseudocode in JavaScript, however I'm not getting the result that I'm supposed to. I want the function to permute the elements of the array p places to the left. In pseudocode I'm using a queue data structure, but I thought I can as well us an array. As the result of my function, I get an array with [2, 2, 2, 2]. Can you please help me out?
My code in pseudocode:
Function PERMUTEVECTOR(row, p)
If p=0 then
Return row
End if
New Queue q
For 0<= i <4 do
ENQUEUE[row[i], q]
End for
For 1<= i <= p do
ENQUEUE[HEAD[q],q]
DEQUEUE[q]
End for
For 0<=i<4 do
Row[i] <- HEAD[q]
DEQUEUE[q]
End for
Return row
End function
My code in JavaScript:
function permute_vector(row, p)
{
if (p=0)
{return row}
let q = new Array()
for (i=0; i<4; i++)
{
q.push(row[i])
}
for (i=0; i<p; i++)
{
q.push(q[0])
q.pop()
}
for (i=0; i<4; i++)
{
row[i] = q[0]
q.pop()
}
return row
}
px = permute_vector([2,4,1,3], 1)
console.log("px is:", px)
}
I did the same in Python and it works fine:
def permute_vector(row, p):
if p==0:
return row
q = []
for i in range(4):
q.append(row[i])
for i in range(p):
q.append(q[0])
q.pop(0)
for i in range(4):
row[i] = q[0]
q.pop(0)
return row
What am I doing wrong with my JavaScript code?
Many thanks!
In javascript, Array.pop() removes the last element. You need Array.shift() for removing the first one.
function permute_vector(row, p)
{
if (p===0) // in js, checks are with == (loose) or === (strict).
// = is for assignment and you were assigning p to 0 here.
{return row}
let q = new Array()
for (i=0; i<4; i++)
{
q.push(row[i])
}
for (i=0; i<p; i++)
{
q.push(q[0])
q.shift() // shift instead of pop
}
for (i=0; i<4; i++)
{
row[i] = q[0]
q.shift() // shift instead of pop
}
return row
}
px = permute_vector([2,4,1,3], 1)
console.log("px is:", px)
}
Two mistakes:
pop() accepts an index to remove a specific item. You're making use of that to remove the first item. In JavaScript Array.pop() merely is capable of removing the last item. Use Array.shift() to remove the first item.Working code:
function permute_vector(row, p) {
if (p === 0) {
return row;
}
let q = new Array();
for (i = 0; i < 4; i++) {
q.push(row[i]);
}
for (i = 0; i < p; i++) {
q.push(q[0]);
q.shift();
}
for (i = 0; i < 4; i++) {
row[i] = q[0];
q.shift();
}
return row;
}
px = permute_vector([2, 4, 1, 3], 1);
console.log("px is:", px);Now when you've got it, you can just ditch the whole thing and replace it with
vec[n:] + vec[:n] (Python)
[...vec.slice(n), ...vec.slice(0, n)] (JS)
Basic textbook algorithms are rarely useful in scripting languages, because they implement stuff like this out of the box.