Science:Math Exam Resources/Courses/MATH152/April 2011/Question A 06

MATH152 April 2011

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[hide] Question A 06
What is the output after the following lines of MATLAB code? A = [1 2 3; 4 5 6; 7 8 9]; A(2,:)

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If you are stuck, check the hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it!

[show] Hint
What is the difference between a space and a semicolon when defining a matrix. What does the colon do?

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[show] Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. For the first line of code A=[1 2 3;4 5 6;7 8 9]; we recall that when declaring a matrix in Matlab, square brackets [ and ] start and end the declaration. While inside a space indicates to move to the next column while a semicolon instructs to start a new row. Therefore we place the entries 1, 2, and 3 in the three columns of the first row; 4, 5, and 6 in the three columns of the second row and 7, 8, and 9 in the three columns of the last row. Therefore the matrix generated is $A={\begin{bmatrix}1&2&3\\4&5&6\\7&8&9\end{bmatrix}}.$ The semicolon at the end of the line indicates that output is suppressed and so the matrix will not be displayed. For the second line of code A(2,:) we recall that Matlab recognizes entries in a matrix first by row and column so a call A(i,j) would return an element in row i and column j. The colon instructs Matlab to take all the entries of a given field. Therefore A(:,j) instructs Matlab to output all entries in column j while A(i,:) instructs Matlab to output all of the entries in row i. Therefore A(2,:) instructs Matlab to output all of the entries from row 2. If we look at out matrix then the output would be [4 5 6] Notice the lack of semicolon on this output indicates that the output is not suppressed.