Using if Statements

if statement

Make the first 10 lights (red) except for the 5th light, which should be green.

While there are a number of ways to do this, we'll use an if statement to make the change. We code the logical statement:

• LOOP through the first 10 pixels (Line 6).
  • IF it's the 5th pixel make it green (Lines 7,8).
  • ELSE make the pixel red (Line 9,10).

              import board
              import neopixel

              pixels = neopixel.NeoPixel(board.GP0, 20)

              for i in range(10):
                if (i == 4):
                  pixels[i] = (0, 100, 0) # green
                else:
                  pixels[i] = (100, 0, 0) # red
            

Every turn through the loop, the if statement checks to see if i is equal to 4. If it is, it makes that LED green, else, it make the LED red.

You will notice that the if statement uses a double equal sign ("==").

              if (i == 4):
            

== is a comparison operator that tests to see if the things on either side are the same: "equal to" each other.

The other comparison operators are:

Since the operators have opposites of each other, and they all test to see if the statement is true or false, you can be flexible and creative.

For example, the same program, to have 10 red lights and the fifth one green could be written as:

• LOOP through the first 10 pixels.
  • IF it's NOT the 5th pixel make it red.
  • ELSE make it green.

              import board
              import neopixel

              pixels = neopixel.NeoPixel(board.GP0, 20)

              for i in range(10):
                if (i != 4):
                  pixels[i] = (100, 0, 0) # green
                else:
                  pixels[i] = (0, 100, 0) # red
            

Using the Modulus Operator: %

The modulus is the remainder if you divide two numbers. So:

You can use this to check if a number is exactly divisible by another number, because if it is the remainder is zero.

So, if I want to make every third light green I could do:

            import board
            import neopixel

            pixels = neopixel.NeoPixel(board.GP0, 20)

            for i in range(20):
              if (i % 3) == 0:
                pixels[i] = (0,40,0)
            

Logical Operators: and, or, nor, and not

If you have two conditions to be met you can nest if statements or you can use the logical operators to chain the conditions together.

For example, write a program that makes the pixels that are divisible by both 2 and 3 green:

            import board
            import neopixel

            pixels = neopixel.NeoPixel(board.GP0, 20)

            for i in range(20):
              if (((i % 2) == 0) and ((i % 3) == 0)):
                pixels[i] = (0,10,0)
            

not negates the condition it's applied to. So to do the opposite of the program above: Write a program that makes the pixels that are not divisible by both 2 and 3 green:

            import board
            import neopixel

            pixels = neopixel.NeoPixel(board.GP0, 20)

            for i in range(20):
              if not(((i % 2) == 0) and ((i % 3) == 0)):
                pixels[i] = (0,10,0)