Multiply char with float

Question

I have a value for led brightness that is stored as an unsigned char (0-255)

unsigned char* colors[3];
colors[0] = 255;
colors[1] = 0;
colors[2] = 0;

I want to multiply a brightness percentage to that value.

float percent = 0.5;
auto red_brightness = colors[0] * percent

Since colors[0] is of type char and percent is of type float. It gives error Invalid operands of types 'unsigned char*' and 'float' to binary 'operator*'

unsigned char brightness;
float percent;
unsigned char* colors[3];
void setup() {
 Serial.begin(9600);
 colors[0] = 255;
 colors[1] = 0;
 colors[2] = 0;
}
void loop() {
 brightness = 127;
 float percent = (float)brightness / 255;
 float newRed = (float)colors[0] * percent;
 
 Serial.println(percent);
 Serial.print("red ");
 Serial.print(newRed);
 Serial.println("");
 delay(1000);
}

How can I multiply colors[0] (255) by 0.5 to get 127?

---

What I've tried

1. Cast colors[0] to a float or int

 # invalid cast from type 'unsigned char' to type 'float'
 float newRed = (float)colors[0] * percent;

2. strtod

Using this conversion website, I've tried converting like so:

 float tempColor = (float)strtod(colors[0],NULL);
 float newRed = tempColor * percent;

While this compiles, it always returns 0 (not 127 like I expect)

unsigned char brightness;
float percent;
unsigned char* colors[3];
void setup() {
 Serial.begin(9600);
 colors[0] = 255;
}
void loop() {
 brightness = 127;
 float percent = (float)brightness / 255;
 float tempColor = (float)strtod(colors[0],NULL); // <- returns 0.00
 Serial.print("tempColor");
 Serial.println(tempColor);
 delay(1000);
}

How can I multiply 255 by 0.5 to get ~127 ?

---

Solution

Thanks to the pointer by st2000 and this sparkfun article on datatypes

I avoided floating point and char arrays and converted everything to byte and now my code works as expected. Thank you

byte brightness;
byte percent;
byte colors[3];
void setup() {
 Serial.begin(9600);
 colors[0] = 255;
}
void loop() {
 percent = 50;
 auto foobar = colors[0] * percent / 100;
 Serial.print("foobar: ");
 Serial.println(foobar);

11:12:19.833 -> foobar: 127

Answer 1

Using "floats" is expensive both in required space and execution time. Consider avoiding the problem entirely and use uint8_t or byte for everything.

Then just assume

percent = 50; 

... is 50%. Then try:

tempColor = colors[0] * percent / 100;

... where all variables are of type uint8_t or byte.

• st2000: @spuder, please follow up to let us know if the above code suggestion works. I think C uses type int for intermediate values in which case the worst expected case (255*100) should not exceed the limits of a type int variable. Otherwise casting one of the variables to a type uint16_t would likely make the code more portable / less likely to break.

Answer 2

You wrote:

I have an led brightness that is stored as an unsigned char (0-255)

unsigned char* colors[3];

This is not an array of unsigned char, this is an array of pointers to unsigned char. If you instead write

unsigned char colors[3];

Everything should work as expected.

But then, st2000 has very good advice on how to avoid expensive floating point operations.

• st2000: Oooo, good catch @edgar!