Question

In GNU Octave version 3.4.3 I want to round a matrix to 2 units precision on the contents of a matrix like this.

mymatrix=[1.1234567, 2.12345; 3.1234567891, 4.1234];
disp(mymatrix);

This prints:

1.1235   2.1235
3.1235   4.1234

As you can see, the disp forces the precision to ‘5’, I want the units precision to be 2. How do I do this?

Answer 1

How to round off elements in a matrix in Octave:

There are many different ways to round a matrix and round a number in octave.

Option 1, use of sprintf format feature

mymatrix=[100.1234567, 2.12345; 3.1234567891, 4.1234];
rows = rows(mymatrix);
cols = columns(mymatrix);
for i = 1:rows
  for j = 1:cols
    sprintf("%5.2f", mymatrix(j,i))
  endfor
endfor

Output, note the "%5.2f" token. The ‘f’ means expect a float, the 5 means occupy 5 spaces. The 2 means 2 units precision after the decimal point.

ans = 100.12
ans =   3.12
ans =   2.12
ans =   4.12

Option 2, round to significant digits using eval and mat2str

mymatrix2=[100.1234567, 2.12345; 3.1234567891, 4.1234];
j = mat2str(mymatrix2, 3);
mymatrix2=eval(j)

Output, matrix rounded to 3 significant digits, notice 100.123 rounded to 100 while the 2.12345 was rounded to 2.12

mymatrix2 = 100.0000     2.1200
              3.1200     4.1200

Option 3, use the round function

The round function does not have a precision parameter in Octave. However you can hack around it by multiplying each item in the matrix by 100, rounding it to the nearest int, then dividing each item by 100:

mymatrix=[100.1234567, 2.12345; 3.1234567891, 4.1234];
round(mymatrix .* 100) ./ 100

Output, round occurs correctly:

ans = 100.1200     2.1200
        3.1200     4.1200

Option 4, specify a output_precision(num)

You noticed that option 3 above kept the trailing zeros, which may be undesirable, so you can tell them to go away by setting output_precision:

mymatrix=[100.1234567, 2.12345; 3.1234567891, 4.1234];
disp(mymatrix);
output_precision(3)
disp(mymatrix)

Output:

100.1235     2.1235
  3.1235     4.1234

100.123     2.123
  3.123     4.123

Octave has some odd behavior when trying to do rounding because octave tries hard to uniformly apply a uniform rounding to all items in a matrix. So if you have multiple columns with wildly different values, octave sees a tiny value and says: "I should convert that to an exponential like 1.0e-04, and so the same exponential is applied to the entire data structure in the matrix.