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Format a matrix with entries displayed as exact values

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Aleem Andrew
Aleem Andrew on 18 Sep 2020
Commented: madhan ravi on 19 Sep 2020
I am trying to print a matrix with its entries displayed as exact values not fractional approximations. For example, the following code should print A exactly as it is initialized (not approximate sqrt(2) as 1393/985) but displays fractions. What format specification instead of rat should be used?
format rat
AB = [0 1 1; sqrt(2) 2 0; 0 1 1];
disp("AB: "); disp(AB)

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Accepted Answer

Dana
Dana on 18 Sep 2020
I'm not sure exactly what you're after. Here are some possibilities
>> format short
>> disp(AB)
0 1.0000 1.0000
1.4142 2.0000 0
0 1.0000 1.0000
>> format long
>> disp(AB)
0 1.000000000000000 1.000000000000000
1.414213562373095 2.000000000000000 0
0 1.000000000000000 1.000000000000000
>> format shortg
>> disp(AB)
0 1 1
1.4142 2 0
0 1 1
>> format longg
>> disp(AB)
0 1 1
1.4142135623731 2 0
0 1 1
If you're saying you want it to literally say "sqrt(2)", then you can't do that using numeric arrays. As soon as you set an element of a numeric array equal to sqrt(2), MATLAB forgets how it arrived at that value, and only remembers the floating-point approximation. The only option I can see would be to use the Symbolic Toolbox:
>> AB = sym([0 1 1; sqrt(sym(2)) 2 0; 0 1 1]);
>> disp("AB: "); disp(AB)
AB:
[ 0, 1, 1]
[ 2^(1/2), 2, 0]
[ 0, 1, 1]
You can then convert this to a numeric array any time using eval:
>> eval(AB)
ans =
0 1 1
1.4142 2 0
0 1 1

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Dana
Dana on 18 Sep 2020
Not sure what the relevance of that is here madhan? This use of eval isn't about variable naming, it's about converting a symbolic array to a numeric one.
madhan ravi
madhan ravi on 19 Sep 2020
a simple use of double() would do the trick. Generally eval() is not recommended, if you read that link entirely, it’ll tell you the reasons why not to use that function.

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