# Julia 101

Lesson 3.2: Manipulating Matrices Using the Colon Operator

In Julia, we can use the **colon** operator to extract elements from a matrix. This feature, which is similar to MATLAB, is very useful during data analysis. Consider the matrix. returned by the statement:

`A = [1 2 3 4 5; 6 7 8 9 10; 11 12 13 14 15]`

- When a colon is used in a matrix reference in place of a specific index number, the colon represents the entire row or column. For example, the statement
`A[1,:]`

*(read as “all columns in row 1”)*returns the first row i.e`1 2 3 4 5.`

Similarly`A[:,3]`

*(read as “all rows in column 3”)*returns the third column. - The colon operator can also be used to extract multiple rows or columns. For example, the statement
`A[1:2,:]`

*(read as “all columns from row 1 to row 2”)*returns`1 2 3 4 5; 6 7 8 9 10`

and the statement`A[:,1:2]`

*(read as “all rows from column 1 to column 2”)*returns`1 2; 6 7; 11 12`

- To access the last element, last row or last column of a matrix, the keyword
*“end”*can be used. The statements`A[end]/A[end,end], A[end,:] and A[:,end]`

returns`15`

, the last row and the last column of matrix`A`

, respectively. - Finally, using a single colon with a matrix name, like
`A[:]`

transforms the matrix into a single column. The statement`A[1:end]`

does the same.