Encoding JSON array of an array of floats

[Jorcis]

Hi,

I'm developing Software for the nRF52840. I'm using NCS version 2.2.0 and Zephyr OS build v3.2.99-ncs1.

I try to encode a JSON message that will be send via MQTT.

I managed to encode, objects, strings and other primitives,

But now I'm blocked trying to encode a double array of floats.

The JSON message I'm trying to encode would be like this:

{"observations":[[3.0, 0.20],[5.0, 0.30],[4.5, 0.25]]}

I have tried to follow the JSON_OBJ_DESCR_ARRAY_ARRAY example, like in line 390 in "zephyr/data/json.h" but I think the example shown is for an array of an array of objects (not floats), and I could not manage to get the desired result, although I have tried with several options.

Could you please give me some advice? An example of JSON_OBJ_DESCR_ARRAY_ARRAY, for floats would be appreciated.

Thanks and regards,

Jordi.

[Sigurd Hellesvik]

Hi Jordi,

The JSON library is likely using a zephyr library.
In that case I think you will get better help on the Zephyr Discord.

But if you upload some test code here, I can have a look at it and see if I can help.
(Insert->File) or (Insert->Code)

Regards,
Sigurd Hellesvik

[Jorcis]

Hi Sigurd,

It's OK, I finally make it.

As JSON_TOK_FLOAT, can accept any formatted string, I first encode the double array of floats and then I lie the json_obj_token.start to point to that "fake" string.

I insert a code example here:

#include <stdlib.h>
#include <kernel.h>
#include <data/json.h>

static float vector[3][2] = {
	{2.34, 0.64},
	{3.41, 0.67},
	{2.56, 0.59}
};

struct json_data{
	struct json_obj_token vector;
	char arr_arr_floats[100];
};

static struct json_obj_descr parent_descr[] = {
	JSON_OBJ_DESCR_PRIM(struct json_data, vector, JSON_TOK_FLOAT)
};

size_t encode_aray_array_float(float** data, size_t M, size_t N, char* buf_ini)
{
	char* buf = buf_ini;

	*buf++ = '[';
	for(int i = 0; i < M; i++){
		*buf++ = '[';
		for(int j = 0; j < N; j++){
			gcvt(data[i][j], 6, buf);
			while(*buf != '\0') buf++;
			if(j < N - 1){
				*buf++ = ',';
			}
		}
		*buf++ = ']';
		if(i < M - 1){
			*buf++ = ',';
		}
	}
	*buf++ = ']';
	*buf = '\0';
	return (buf - buf_ini);
}

int main()
{
	struct json_data data;

	/** Prepare JSON data */
	data.vector.length = encode_aray_array_float((float**)vector, 3, 2, data.arr_arr_floats);
	data.vector.start = data.arr_arr_floats;

	/** Encode the json obj */
	char json[200];
	int err = json_obj_encode_buf(parent_descr, ARRAY_SIZE(parent_descr), &data, json, sizeof(json));
	printk("err = %d\n", err);
	printk("%s\n", json);

	while(1){
		k_msleep(1000);
	}
}

I wonder if there is a more elegant way to do it.

I close the case, many thanks for your attention.

Jordi.

[Sigurd Hellesvik]

Hi Jordi,

Good to hear that you found JSON_TOK_FLOAT!

I do not know if it runs faster, but it ups readability to use "sprintf":

#include <zephyr/kernel.h>
#include <stdio.h>

static float vector[3][2] = {
    {2.34, 0.64},
    {3.41, 0.67},
    {2.56, 0.59}
};


void encode_aray_array_float(int M, int N, float data[M][N], char* buf_ini){

    sprintf(buf_ini, ""); // Initialize buf_init to be empty
    for(int m=0; m<M; m++){
        for(int n=0; n<N; n++){
            if(n==N-1){
                sprintf(buf_ini, "%s [%.2lf];", buf_ini, data[m][n]);
            }
            else {
                sprintf(buf_ini, "%s [%.2lf],", buf_ini, data[m][n]);
            }
        }
    }
}


void main(void)
{
    char test_string[200];
    encode_aray_array_float(3, 2, vector, test_string);
    printk("%s\n", test_string);
}

and prj.conf:

# To make sprintf work with floats
CONFIG_NEWLIB_LIBC=y
CONFIG_NEWLIB_LIBC_FLOAT_PRINTF=y

# To make printk work with floats
CONFIG_CBPRINTF_FP_SUPPORT=y

Regards,
Sigurd Hellesvik

[Jorcis]

Great! Thanks Sigurd!