Why Does Converting Array with Length of One to JSON Remove Array?

Question

MathWorks Support Team 2023-11-10

回答： MathWorks Support Team 2023-12-19

When using 'jsonencode' to convert a struct to JSON, arrays of size one are not converted as arrays.

The following example is an array of size two being converted to JSON correctly:

>> jsonencode(struct('test', [string('1'), string('2')]))

The output is: {"test":["1","2"]}

The following example shows what happens when the array is of size one:

>> jsonencode(struct('test', [string('1')]))

The output is: {"test": "1"}

The desired output is: {"test": ["1"]}

How do I achieve this result?

Answer 1

In MATLAB, there is not a distinction between a single element string array and a string scalar. The following code snippet shows this case:

>> a = [string("s")]
a = 
    "s"
>> b = string("s")
b = 
    "s"
>> isscalar(a)
ans =
  logical
   1
>> isscalar(b)
ans =
  logical
   1

A single element array is considered scalar. Therefore, when converting to JSON, there is not a way to distinguish the two cases.

Using cell arrays instead will result in the desired output. The following code illustrates this workflow:

s.a = {"a"} 
s.b = {"b1", "b2"} 
jsonencode(s) 
ans =  
  '{"a":["a"],"b":["b1","b2"]}' 