Controlling the number of significant digits used by jsonencode

Question

I'd like to use jsonencode in Matlab R2022a to encode a structure containing double precision values, like e.g.:

>> s = struct('a', sqrt(2))
s = 
  struct with fields:
    a: 1.4142

>> jsonencode(s)
ans =
    '{"a":1.4142135623730951}'

But I don't need all the digits and I'd like to keep the JSON file short, so I'd like to have an output like:

'{"a":1.414}'

There does not seem to have such an option in jsonencode, so I have tried to remove the less significant digits beforehand. Unfortunately there are random rounding errors:

>> s.a = round(s.a*10^3)*10^-3
s = 
  struct with fields:
    a: 1.4140

>> jsonencode(s)
ans = 
'{"a":1.4140000000000005}'

These errors do not occur all the time, but it seems that the deeper the struct the more often I have these rounding errors.

Then I have tried to use vpa, but it does not seem to be compatible with jsonencode:

>> s.a = vpa(s.a,4)
s = 
  struct with fields:
    a: 1.414

>> jsonencode(s)
ans =
    '{"a":{}}'

Now I'm out of options. Is it possible to get a clean JSON output with control over the significant digits using pure Matlab ?

score 1 · Answer 1 · answered Sep 28 '22 at 11:08

I changed the way I round numbers by using a feature of the round function and it removed the random rounding approximations (probably due to the division by the power of 10):

>> s.a = round(s.a, 4, 'significant')
s = 
  struct with fields:
    a: 1.414

>> jsonencode(s)
ans = 
    '{"a":1.414}'

Controlling the number of significant digits used by jsonencode

1 Answers1