For Loop Increment Array

I am currently in class and trying to do my assignment. For this assignment we have to create a for loop that creates an array of 0 through 20 with the even numbers. I wrote the code according to my lesson but it keeps inserting a 0 between each number in the array. Is there something obvious I am missing?

% for loop example

for i = 0:2:20

A(i+1) = i^2;

B(i+1) = i;

end

[Stephen23]

"Is there something obvious I am missing?"

Your code assigns numeric values to the odd elements 1, 3, 5, 7, ... etc.

What numeric value do you expect the even elements 2, 4, 6, 8, ... to have ?

Answer 1

[Steven Lord]

I wrote the code according to my lesson but it keeps inserting a 0 between each number in the array.

It doesn't so much insert a 0 between each number in the array so much as not assigning to certain elements in the array. Let me add a little diagnostic code to what you wrote. I'm also only going to work on A as B has the same behavior and I'm changing the vector over which your for loop operates just so we display fewer lines.

A = zeros(1, 0); % Need to create A prior to the loop
for i = 0:2:6
    fprintf("Before assigning to element %d of A, A has length %d.\n", i+1, length(A))
    fprintf("Assigning %d to element %d of A.\n", i^2, i+1)
    A(i+1) = i^2;
    fprintf("After assigning to element %d of A, A has length %d.\n \n", i+1, length(A))
end

Before assigning to element 1 of A, A has length 0.

Assigning 0 to element 1 of A.

After assigning to element 1 of A, A has length 1.
 

Before assigning to element 3 of A, A has length 1.

Assigning 4 to element 3 of A.

After assigning to element 3 of A, A has length 3.
 

Before assigning to element 5 of A, A has length 3.

Assigning 16 to element 5 of A.

After assigning to element 5 of A, A has length 5.
 

Before assigning to element 7 of A, A has length 5.

Assigning 36 to element 7 of A.

After assigning to element 7 of A, A has length 7.
 

A

A = 1x7

0 0 4 0 16 0 36

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All the values in the vector over which you're iterating are even. You only ever assign to the elements at locations that are the values in that vector plus 1, so you only ever assign to the odd element locations. But MATLAB can't leave "holes" in the vector, so when it grows the vector it has to fill in the even element locations (that you're skipping over) with something. That something, for a double array, is 0.

To avoid skipping elements, either perform a computation on the value of i to map between the values 0:2:n and the indices 1:m or keep a variable with the index of the next element to fill in.

A = zeros(1, 0);
ivalues = 0:2:6;
C = zeros(1, 0);
nextCloc = 1;
for i = ivalues
    fprintf("Assigning %d to element %d of A.\n", i^2, (i/2)+1)
    A((i/2)+1) = i^2;
    fprintf("Assigning %d to element %d of C.\n", i^2, nextCloc)
    C(nextCloc) = i^2;
    nextCloc = nextCloc + 1;
end

Assigning 0 to element 1 of A.

Assigning 0 to element 1 of C.

Assigning 4 to element 2 of A.

Assigning 4 to element 2 of C.

Assigning 16 to element 3 of A.

Assigning 16 to element 3 of C.

Assigning 36 to element 4 of A.

Assigning 36 to element 4 of C.

disp(ivalues)

     0     2     4     6

disp(A)

     0     4    16    36

disp(C)

     0     4    16    36