how to convert binary data to float value?

Question

For my data logger project I'm logging data to a SD card. Here I'm storing the data in binary format. I'm logging accelerometer and temp&humidity sensors. I could read data byte by byte. Hence to print ADXL data (two bytes), I combine each two byte data. Hence four bytes has to be combined to get temparature data. Unfortunately for temparature I failed to print actual data. There is some conversion problem. I'm looking some help regarding how to convert data to floats.

Code to read and print binary to uint16_t ( accelerometer data): working

while ((nr = fgetc(logFile)) != EOF){
         pc.printf(" \r\n %d ",nr);
          acc_con[i] = nr;
          if (i == 1){ 
          acc = (acc_con[1]<<8) | acc_con[0];
          pc.printf(" \r\n %i ",acc);
          i = 0;} 
          else i++; 
         }

Code to read and print binary to float ( temparature data): Not working

while ((nr = fgetc(logFile)) != EOF){
         pc.printf(" \r\n %d ",nr);
           humicon[i] = nr;
           if (i == 3){
            hum = (humicon[0] << 24) | (humicon[1] << 16) | (humicon[2] << 8) | humicon[3];
            pc.printf(" \r\n %f ",hum);
            i = 0;}
            else i++;
            }

The byte by byte raw data from variable nr is: 24, 58, 46, 66, and the output from variable hum is: 406466112.000000.

I could read humidity or temparature (floats) with fread. Unfortunatly to use fread I must know th no of bytes stored in the file. With below code I could print floats also. int main() {

int nr;
FILE *fp = fopen("/sd/PCE000.bin", "rb");
float read[1];
fread(read,sizeof(float),1,fp);
pc.printf("\r\n %f",read[0]);
  fclose(fp);

}

I would like to know the formula for the conversion like (acc_con[1]<<8) | acc_con[0]. Thank you.

See struct

Open AI - Opting Out
– Open AI - Opting Out

2016-12-14 09:41:24 +00:00
Commented Dec 14, 2016 at 9:41 — Open AI - Opting Out
– Open AI - Opting Out, Commented Dec 14, 2016 at 9:41
Is this a python or C question?

cdarke
– cdarke

2016-12-14 09:46:42 +00:00
Commented Dec 14, 2016 at 9:46 — cdarke
– cdarke, Commented Dec 14, 2016 at 9:46
It is C. Done as below answer.

Ghani
– Ghani

2016-12-14 10:06:14 +00:00
Commented Dec 14, 2016 at 10:06 — Ghani
– Ghani, Commented Dec 14, 2016 at 10:06

unwind · Accepted Answer · 2016-12-14 09:50:52Z

2

If you want to do this in Python, it sounds like a job for the struct module:

>>> struct.unpack("f", "".join([chr(x) for x in [24, 58, 46, 66]]))
(43.556732177734375,)

This assumes little-endian byte-order.

In C, you can use memcpy():

float x;
const unsigned char bytes[] = { 24, 58, 46, 66 };
memcpy(&x, bytes, sizeof x);
printf("%f\n", x);

edited Dec 14, 2016 at 9:50

answered Dec 14, 2016 at 9:45

unwind

402k64 gold badges492 silver badges620 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Rishikesh Raje · Accepted Answer · 2016-12-14 09:59:50Z

0

With

unsigned int hum;
....
hum = (humicon[0] << 24) | (humicon[1] << 16) | (humicon[2] << 8) | humicon[3];

You are converting 8 bit data to 32 bit integer data. You can store this data directly. Or if you absolutely want to store in floats, you can use.

float hum_f;
....
hum_f = hum;

answered Dec 14, 2016 at 9:59

Rishikesh Raje

8,6222 gold badges18 silver badges33 bronze badges

Comments

Julian Declercq · Accepted Answer · 2016-12-14 12:50:22Z

0

Yap, is not simple function, transform an array of bytes to a floating point function. I am lookin also to implement it to avoid portable reasons. There are differences also between little endian and big endian machines. But there are several hacks to this problem. First the file size, you have to include "sys/stat.h", with small differences is also portable. But is not required. Read it as buffer of char :

char f[sizeof(float)];
if (fread(f,1,sizeof(float), yourfile) ==sizeof(float))
{
    float *flt_ptr=f;
    printf("%f\n",*flt_ptr);
}

C is wonderful for those things, there is also no need to cast everithing.

edited Dec 14, 2016 at 12:50

Julian Declercq

1,6253 gold badges18 silver badges33 bronze badges

answered Dec 14, 2016 at 12:30

jurhas

6631 gold badge4 silver badges12 bronze badges

Collectives™ on Stack Overflow

how to convert binary data to float value?

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related