Converting float value to string

[garrettb]

Hi

I am using nRF Connect SDK, wioth nRF5340_DK.
I am trying to convert a float value to a string, with two decimal places.
Have a look at the code below, and then see the outputs.
I don't know why the 2nd and 3rd efforts don't work.

    float dbg_float_val;
    char dbg_buf1[8];
    int dbg_len1;
    char dbg_buf2[8];
    int dbg_len2;
    char dbg_buf3[8];
    int dbg_len3;

    dbg_float_val = 1.234F;

    int wholePart = (int)dbg_float_val;
    int decimalPart = (int)((dbg_float_val - wholePart) * 100); // Multiply by 100 to get two decimal places    
    dbg_len1 = snprintf(dbg_buf1, sizeof(dbg_buf1), "%d.%02d", wholePart, decimalPart);
    printf("Result 1: dbg_buf1 = %s, dbg_len1: %d\n", dbg_buf1, dbg_len1);

    dbg_len2 = sprintf(dbg_buf2,"%.2f", dbg_float_val);
    printf("Result 2: dbg_buf2 = %s, dbg_len2: %d\n", dbg_buf2, dbg_len2);

    dbg_len3 = snprintf(dbg_buf3, sizeof(dbg_buf3), "%.2f", dbg_float_val);
    printf("Result 3: dbg_buf3 = %s, dbg_len3: %d\n", dbg_buf3, dbg_len3);

The output I get:

Result 1: dbg_buf1 = 1.23, dbg_len1: 4
Result 2: dbg_buf2 = %.2f, dbg_len2: 4
Result 3: dbg_buf3 = %.2f, dbg_len3: 4

[Hussam Aldean]

How about sprintf(dbg_buf1,"%0.2f",dbg_float_val); ?

[garrettb]

Hi Hussam, thanks for the reply.

No, that didn't work for me either.

I wonder is it to do with includes, or project settings?

I tried the following code:

    char dbg_buf1[16];
    char dbg_buf2[16];
    char dbg_buf3[16];
    char dbg_buf4[16];
    char dbg_buf5[16];
    float dbg_float = 12.345;
    double dbg_double = 54.321;
    int dbg_int = 1357;
    sprintf(dbg_buf1,"%0.2f",dbg_float);
    printk("Result 1: dbg_buf1 = %s\n", dbg_buf1);
    printk("Result 2: dbg_float = %0.2f\n", dbg_float);
    printk("Result 3: dbg_float = %f\n", dbg_float);
    sprintf(dbg_buf2,"%0.2f",dbg_double);
    printk("Result 4: dbg_buf2 = %s\n", dbg_buf2);
    printk("Result 5: dbg_double = %0.2f\n", dbg_double);
    printk("Result 6: dbg_double = %f\n", dbg_double);
    sprintf(dbg_buf3,"%0.2f",dbg_int);
    printk("Result 4: dbg_buf3 = %s\n", dbg_buf3);
    printk("Result 5: dbg_int = %0.2f\n", dbg_int);
    printk("Result 6: dbg_int = %f\n", dbg_int);
    sprintf(dbg_buf4,"%d",dbg_int);
    printk("Result 7: dbg_buf4 = %s\n", dbg_buf4);
    printk("Result 8: dbg_int = %d\n", dbg_int);
    printk("Result 9: dbg_int = %d\n", dbg_int);
    

With this result:

Result 1: dbg_buf1 = %0.2f
Result 2: dbg_float = %0.2f
Result 3: dbg_float = %f
Result 4: dbg_buf2 = %0.2f
Result 5: dbg_double = %0.2f
Result 6: dbg_double = %f
Result 4: dbg_buf3 = %0.2f
Result 5: dbg_int = %0.2f
Result 6: dbg_int = %f
Result 7: dbg_buf4 = 1357
Result 8: dbg_int = 1357
Result 9: dbg_int = 1357

I get the same result if I use LOG_INF() instead of printk().

Here are the file's includes:

#include <bluetooth/services/nus.h>
#include <dk_buttons_and_leds.h>
#include <errno.h>
#include <math.h>
#include <soc.h>
#include <stdio.h>
#include <zephyr/bluetooth/bluetooth.h>
#include <zephyr/bluetooth/gatt.h>
#include <zephyr/bluetooth/hci.h>
#include <zephyr/bluetooth/uuid.h>
#include <zephyr/device.h>
#include <zephyr/devicetree.h>
#include <zephyr/logging/log.h>
#include <zephyr/settings/settings.h>
#include <zephyr/sys/printk.h>
#include <zephyr/zephyr.h>

And the prj.conf file:

# Enable bonding
CONFIG_BT_SETTINGS=y
CONFIG_FLASH=y
CONFIG_FLASH_PAGE_LAYOUT=y
CONFIG_FLASH_MAP=y
CONFIG_NVS=y
CONFIG_SETTINGS=y

# Enable DK LED and Buttons library
CONFIG_DK_LIBRARY=y

# This example requires more workqueue stack
CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=2048

# Config logger
CONFIG_LOG=y
CONFIG_USE_SEGGER_RTT=y
CONFIG_LOG_BACKEND_RTT=y
CONFIG_LOG_BACKEND_UART=n

CONFIG_ASSERT=y

CONFIG_NVS_LOG_LEVEL_DBG=y
CONFIG_REBOOT=y
CONFIG_MPU_ALLOW_FLASH_WRITE=y

[Hussam Aldean]

add this to prj.conf

CONFIG_CBPRINTF_FP_SUPPORT=y

[garrettb]

Hi Hussam,

thank you, that worked.

Now I am trying to reverse the process, and convert a string to a float.

However if I try use atof() or strtof(), I get an "undefined reference" error.

Is there another library I need to include, or and extra switch to set in prj.conf?

Thanks

Garrett

[Hussam Aldean]

Take the start of the float number and the end of it, create new string and pass it ti strtof.

[garrettb]

Well, the problem is right now that I can't use the strtof() function for anything. I get the following error:

undefined reference to `strtof'

So, similar to the issue of not being able to use the sprintf() with floats, until I set CONFIG_CBPRINTF_FP_SUPPORT=y in prj.conf, I wonder is there another setting I need to use the strtof() function?