Iii. Subscript:
Column J of row I of matrix A is represented as a (I, j ). That is, a (2, 4) indicates the elements in the 2nd columns of 4th rows.
Therefore, the sum of the elements in column 4 can be calculated as follows:
A () + a (2, 4) + a (3, 4) + a (4, 4)
This will generate:
Ans =
34
However, this is not the best way to solve the sum of elements in a column.
Of course, you can use a subscript to describe data members, such as a (k). This is mainly used to indicate the k elements of a row vector or column vector. But it can also be applied to two-dimensional arrays. On the one hand, a matrix can be described as a long column vector composed of the columns of the original matrix, so a (8) can represent
A (4, 2 ).
If you try to read the elements of a (), an error will occur because the subscript cannot cross-border.
In other words, if you access an element outside the original matrix, the size is automatically adjusted to a new order.
For example:
X =;
X (4, 5) = 17
Will generate:
X =
16 3 2 13 0
5 10 11 8 0
9 6 7 12 0
4 15 14 1 17
4. colon (:) Operator
The colon operator is a particularly important operator in MATLAB. It appears in MATLAB in different forms.
Expression:
1: 10
Represents a group of row vectors composed of integers from 1 to 10.
1 2 3 4 5 6 7 8 9 10
To get a non-unit space and create a growth volume, you can:
100:-7:50
Indicates:
100 93 86 79 72 65 58 51
At the same time
0: PI/4: pi
Indicates:
0 0.7854 1.5708 2.3562 3.1416
The subscript expression introduces the colon operator to determine the position of the matrix:
A (1: K, j): 1 to k elements in column J of matrix
Sum (A (, 4): calculates the sum of the elements in the fourth column.
But this is not the best. The colon itself represents all elements of a certain row vector or a certain column vector in the matrix. At the same time, the keyword end indicates the last column or the last row.
Therefore:
Sum (A (:, end): calculates the last column of matrix.
Ans =
34
Why is the magic and value of the 4*4 cube 34?
Because the integers from 1 to 16 are evenly divided into four groups with the following values:
Sum (1:16)/4
Result:
Ans =
34
5. Magic function:
MATLAB has a built-in function to build a cube of any size. This function is magic.
B = magic (4)
Will generate:
B =
16 2 3 13
5 11 10 8
9 7 6 12
4 14 15 1
This matrix is roughly the same as matrix A. The only difference is the sequence of the second and third columns.
To change matrix B to A, that is, to swap the two columns in the middle
A = B (:, [1, 3, 2, 4])
That is to say, each row of B is rearranged in the order of 1, 3, and 2, and the result is:
A =
16 3 2 13
5 10 11 8
9 6 7 12
4 15 14 1
This section describes the following topics:
1. Underlying applications
2. Add new elements to the existing matrix
3. Colon Operator
4. End keyword
5. How to rearrange the matrix order