How to modify a created byte?

Question

I have created a byte for 16x2 LCD. It is basically a custom character which I will have to change later.

byte char[8]{
 B10000,
 B01000,
 B00100,
 B00010,
 B00001,
 B11000,
 B11100
};

I want to edit the third number from fourth row. So, the fourth row will become B00110 from B00010. Is this possible? Are there any alternatives for this?

Answer 1

Well first of all you are trying to name the variable char? which is already a keyword for the variable type of char. But you can access it by the index of the array. Also not sure if you intended to make the array 8 bytes in length, and then only use 7, but thats what you had done in your example

//define it all at once
// byte byteArray[8] = {B10000,B01000,B00100,B00010,B00001,B11000,B11100};
//or by index
byte byteArray[8];
byteArray[0]=B10000;
byteArray[1]=B01000;
byteArray[2]=B00100;
byteArray[3]=B00010;
byteArray[4]=B00001;
byteArray[5]=B11000;
byteArray[6]=B11100;
//if you need to change one in the code elsewhere
byteArray[3]=B00110;
//or
byteArray[3]=6;
}

• Nouman: In the second last line, why did you set the byteArray[3] equal to 6? Why not 3 because we want to change the third element?
• Michel Keijzers: B00110 = B110 = 1 (first 1) * 2^2 + 1 (second 1) * 2^1 + 0 (last 0) * 2^0 = 1 * 4 + 1 * 2 + 0 * 1 = 6
• Nouman: @MichelKeijzers Well, i didn't get it. What is the purpose of it? Is there simpler way? I just don't want to involve other elements.
• Michel Keijzers: E.g. in the normal (10 or decimal system) number 76 = 7 * 10^1 + 6 * 10^0 = 7 * 10 + 6 * 1 = 70 + 6 = 76. In the binary (2 system) number 110 = 1 * 2^2 + 1 * 2^1 + 0 * 2^0 = 1 * 4 + 1 * 2 + 1 * 0 = 4 + 2 + 0 = 6.
• Chad G: Was just showing that it could be set using decimal rather then binary x=B00110(Binary), x=6(Dec),x=0x06(Hex) Are all equal. It doesn't really pertain to your question, so I should have just left it out to not be confusing.

Answer 2

You can use the following array initialization:

In the setup I shows how to set a bit. For this, the bit operator or (|) is used. To reset a bit, you can use &. You can set/reset multiple bits this way.

Also you can use 1 << 2 which means 1 (most right bit) shifted left two places (thus B100).

Btw, it is common practice to initialize all values.

byte lcd[8] = 
{
 B10000,
 B01000,
 B00100,
 B00010,
 B00001,
 B11000,
 B11100,
 B00000 // Also initialize last element
};
void setup() 
{
 lcd[3] |= B100; // Set 4th row (element 3), 3th bit (from the right)
 lcd[3] |= 1 << 2; // Alternative
}
void loop() 
{
}

Explanation about the or arithmetics: OR means: if at least one bit is 1, the result is 1, otherwise 0.

Truth table:

A | B | A or B
--+---+-------
0 | 0 | 0
0 | 1 | 1
1 | 0 | 1
1 | 1 | 1

For your example it means:

Original value of lcd[3]: B00010
Or mask (in setup): B00100
 ------ OR
Result B00110

• Nouman: Why have we done this? lcd[3] |= B100; .Where did the other elements go?
• Michel Keijzers: lcd[3] will only change the 3th element, the other elements will remain equal. With the or (|) you can set a bit, all 0-values in an or 'mask' will remain unchanged.
• Nouman: Can I do directly like lcd[3][3] = B1; where first 3 is the 4th row and second is the 3rd element?
• Michel Keijzers: No you cannot use lcd[3][3] because lcd[3] is one element (a byte) and not an array.