Sprintf() a float number

Using Arduino Programming Questions
24

using sprintf()? isn't that the issue?

25

Sprintf() a float number

The OP has deviated from his title and now asking just for a solution?

26

with x10, truncating and / 10 as suggested

void setup() {
  Serial.begin(115200);
  char str_tmp[10];
  sprintf(str_tmp, "%2.1f", trunc(6.38 * 10) / 10.0); // ESP or ARM only, float not supported on Uno and similar 
  Serial.println(str_tmp);
}

void loop() {}
27

dtostrf() - turn your floats into strings - Programming Electronics Academy

dtostrf (float_value, min_width, num_digits_after_decimal, where_to_store_string)

doesn't round

1 Like
28

The sketch of post #26 is uploaded in NodeMCU/ESP8266; there is 6.3 output on the Serial Monitor.
Output:

6.3
6.3
6.3

Anyway, I had to shift the codes in the loop() function to see the output.

void setup() 
{
  Serial.begin(115200);
}

void loop()
{
  char str_tmp[10];
  sprintf(str_tmp, "%2.1f", trunc(6.38 * 10) / 10.0); // ESP or ARM only, float not supported on Uno and similar
  Serial.println(str_tmp);
  delay(1000);
}

@J-M-L
Is there any advantage of using trunc() function instead of (int) casting?

29

no
Arduino sprintf doesn't support "%f"?
isn't that the issue?

30

This is it.

31

divide by 10.0?

32

I suppose it should be enough to make multiplication an int

(int)(6.38 * 10) * 0.1f
33

From an efficiency perspective I don’t know, the compiler is smart - it’s about Using math instead of ugly type manipulation and promotion

1 Like
34

@killzone_kid

Why is the f there? Is 0.1 not enough to bear the notion of float number?

35

0.1 - double
0.1f - float
float is cheaper than double
multiplication * is cheaper than division /

36

Ok! It is understood that float is cheaper than double as float-type variable uses 32-bit space; whereas, double uses 64-bit space.

How is * (multiplication) cheaper than / (division)?

37

Takes fewer cycles, having said that, compiler would probably convert division by 10 to multiplication internally anyway so it won't make a difference to the final code

38

Not true!

void setup() 
{
  Serial.begin(115200);
}

void loop()
{
  char str_tmp[10];
  dtostrf(6.38, 3, 1, str_tmp); //arg1=value, arg2=#symbols. arg3=#digits after point
  Serial.println(str_tmp);
  delay(1000);
}

Output:

 6.4
 6.4
 6.4
39

The ATmega32A/328P takes 2 clock cycles for multiplication; whereas, the said MCU has no div instruction in its list.

40

Remarkable, the makers of ATmega32A/328P must have found a way how to bend Math

41

That you have already said in your post #37.
"[...]compiler would probably convert division by 10 to multiplication internally [...]."

42

The procedure is a decimal shift-left (*10), truncate (remove "100's" decimal place), decimal shift-right (/10). It avoids the rounding up or rounding down.

43

If you use trunc() function, then it does not matter using /10 or /10.0. If using (int) casting, then /10.0 is a must to get correct result; else, you get 0.0.