convert multiple chats to a float

[wraith nimbus]

I have a char array that contains both chars but also other stuff.

Let’s call the array char stuff[69]

stuff[5] = 3F / 00111111 stuff[6] = 80 / 1000000 stuff[7] = 00 / 00000000 stuff[8] = 00 / 00000000

The iee float value for 1 is
3F800000 / 00111111100000000000000000000000

I need to be able to convert those four chats into a float.

This will be done on an esp device using arduino

[harsh bronze]

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[pseudo raven]

is it hex value divided by binary value?

[wraith nimbus]

It’s on Arduinos coming from ‘WiFIUdp’s udp.read() function

That’s the data portion of the udp packet. The final 4 bytes are an iee float

[pseudo raven]

iee?

[green hound]

Compiler Output

<source>: In function 'int main()':
<source>:3:15: error: unable to find numeric literal operator 'operator""f'
    3 | char fl[4] = {3f, 80, 00, 00};
      |               ^~
Build failed

dux7#6470 | c++ | x86-64 gcc 12.2 | godbolt.org

[pseudo raven]

;compile cpp

unsigned char fl[4] = {0x3f, 0x80, 0x00, 0x00};
float f = (float)*fl;
std::cout << f;

[green hound]

Program Output

63

dux7#6470 | 42ms | c++ | x86-64 gcc 12.2 | godbolt.org

[stone rune]

;compile c

char fl[4] = {0x3f, 0x80, 0x00, 0x00};
float* f = (float*)fl;
printf("%f", *f);

[green hound]

Program Output

0.000000

Lord Dimwit Flathead#1478 | 45ms | c | x86-64 gcc 12.2 | godbolt.org

[stone rune]

If you do *fl then you are dereferencing the first element in the char array, which gives you 63 (0x3f).

Mine's no better because 3f 80 00 00 in big endian should be 1.0

[pseudo raven]

ye

;compile cpp

unsigned char fl[4] = {0x3f, 0x80, 0x00, 0x00};
float f = (float)*fl;
memcpy(&f, (float*)fl, 4);
std::cout << f;

[green hound]

Program Output

4.6006e-41

dux7#6470 | 67ms | c++ | x86-64 gcc 12.2 | godbolt.org

[pseudo raven]

well does it use little endian or big endian

[stone rune]

You're right ofc.

;compile c

char fl[4] = { 0x00, 0x00, 0x80, 0x3f};
float* f = (float*)fl;
printf("%f\n", *f);

[green hound]

Program Output

1.000000

Lord Dimwit Flathead#1478 | 35ms | c | x86-64 gcc 12.2 | godbolt.org

[stone rune]

Alas, there is no such thing as ntohf() in C.

[terse fox]

float f;
memcpy(&f, &stuff[5], sizeof(f));

[harsh bronze]

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