Print with round towards zero instead of round to nearest

Question

When printing with one decimal place, Arduino's Serial.println() rounds numbers like 123.89 to “123.9”. Is there any way to change the rounding mode? For some reasons I want the value to be rounded towards zero (i.e., truncated) to one decimal place, like 123.89 → “123.8”.

here is the code

void setup() {
Serial.begin(9600);

}

void loop() 
{
float A = analogRead(A0);
A = (A/1023)*5.0;
Serial.print("A = ");
Serial.println(A,3);
float B = A*10;
Serial.print("A * 10 = ");  
Serial.println(B,1);
delay(1000);
}

the output is

A = 2.473
A * 10 = 24.7
A = 2.488
A * 10 = 24.9
A = 2.517
A * 10 = 25.2
A = 2.527
A * 10 = 25.3
A = 2.507
A * 10 = 25.1

instead I want the output to be like this

A = 2.473
A * 10 = 24.7
A = 2.488
A * 10 = 24.8
A = 2.517
A * 10 = 25.1
A = 2.527
A * 10 = 25.2
A = 2.507
A * 10 = 25.0

is it possible ?

Answer 1

A cheap trick to round a number to one decimal place (in "chop-off mode") is to multiply it by 10, convert it to an integer, and divide by 10.0f again:

float myVal = 123.89f;
myVal = (long)(myVal * 10) / 10.0f;
// = ((long)1238.9) / 10.0f
// = 1238 / 10.0f
// = 123.8

You can then print this modified number to the serial monitor.

This method can also be generalized to other precisions. For 2 decimal places, we just have to multiply by 10^2 (100) and divide by 10^2. A macro would be:

#define TRUNCATE(val, numDecimalPlaces) \
    (float)(((long)((double)(val) * pow(10, (numDecimalPlaces) * 1.0f))) / (pow(10, (numDecimalPlaces)* 1.0f)))

Note: You don't have to actually dynamically compute pow(10, numDecimalPlaces), you can also create macros for each decimal precision from 1 to x and simply precompute that value (100, 1000, 10000, ..).

The same can also be achieved by using the standard library function trunc from math.h:

#define TRUNCATE_TO_ONE(val) \
    (trunc((val) * 10.0f) / 10.0f)

See http://www.cplusplus.com/reference/cmath/trunc/.

Here's a small sketch in which it is demonstrated:

#include <Arduino.h>

#define TRUNCATE(val, numDecimalPlaces) \
    (float)(((long)((double)(val) * pow(10, (numDecimalPlaces) * 1.0f))) / (pow(10, (numDecimalPlaces)* 1.0f)))

#define TRUNCATE_TO_ONE(val) \
    (trunc((val) * 10.0f) / 10.0f)


void printValueFormatted(float value, int numDecimalPlaces) {
    //format correctly
    char buf[20];
    char* res = dtostrf(value, sizeof(buf)-1, numDecimalPlaces, buf);

    //skip over empty chars
    while(*res == ' ')
        res++;

    //print buffer
    Serial.println(res);
}


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

void loop(void)
{
    float myVal = 123.8978f;
    printValueFormatted(myVal, 4);
    printValueFormatted(TRUNCATE(myVal, 1), 1);
    printValueFormatted(TRUNCATE(myVal, 2), 2);
    printValueFormatted(TRUNCATE(myVal, 3), 3);
    printValueFormatted(TRUNCATE(myVal, 1), 4);
    printValueFormatted(TRUNCATE(myVal, 2), 4);
    printValueFormatted(TRUNCATE(myVal, 3), 4);
    delay(1000);
}

Prints

123.8978
123.8
123.89
123.897
123.8000
123.8900
123.8970

Answer 2

Use the given [function] for your problem. This function works on a pre-defined [algorithm].

int roundTowardZero(float number, int decimal_place) {
    float round_number = round(number);
    if(round_number > number) round_number = round_number - 1;
    float deci = (number - round_number) * pow(10, decimal_place);
    float round_decimal = round(deci);
    if(round_decimal > deci) round_decimal = round_decimal - 1;
    round_decimal = round_decimal / pow(10, decimal_place);
    total_number = round_number + round_decimal;
    return total_number;
}

Place this function above int setup()

like:

int roundTowardZero( ... ) {
    ...
}
void setup() {
    ...
}
void loop() {
    ...
}

Function syntex is like this : roundTowardZero([Enter a Decimal Value], [Enter Round Place Value]) Example :roundTowardZero(123.89, 1) //Output : 123.8

Please comment below is you find any problem or face any error.