Truncating numbers

truncation.md

The problem

So you are interested in rounding a number to its lowest multiple. Let's say you want numbers to be rounded to 10:

>>> for number in range(30):
...     print(f"{number}: {10 * (number // 10)}")
...
0: 0
1: 0
2: 0
3: 0
4: 0
5: 0
6: 0
7: 0
8: 0
9: 0
10: 10
11: 10
12: 10
13: 10
14: 10
15: 10
16: 10
17: 10
18: 10
19: 10
20: 20
21: 20
22: 20
23: 20
24: 20
25: 20
26: 20
27: 20
28: 20
29: 20

An alternative

An aternative is to use a power of two instead of 10 -- say, 8. Then we subtract the number bit-and-ed with a mask (the power of two minus 1 -- 7, in our case):

>>> mask = 8 - 1
... for number in range(30):
...     print(f"{number}: {number - (number & mask)}")
...
0: 0
1: 0
2: 0
3: 0
4: 0
5: 0
6: 0
7: 0
8: 8
9: 8
10: 8
11: 8
12: 8
13: 8
14: 8
15: 8
16: 16
17: 16
18: 16
19: 16
20: 16
21: 16
22: 16
23: 16
24: 24
25: 24
26: 24
27: 24
28: 24
29: 24

The benefit

See how the numbers go up by 8 every 8 numbers? If you choose a power of two, then you get this result at very low computation cost.

Unfortunately, Scratch does not have bit-operators:

No, Scratch, a visual programming language, does not have built-in bitwise operators (like AND, OR, XOR) for directly manipulating individual bits within numbers. However, you can simulate bitwise operations using arithmetic and conditional logic.

Instead, you can use the mod operator in Scratch:

chunk = 8
truncated = number - (number mod chunk)

This also works for the original value of 50 that you were using:

chunk = 50
truncated = number - (number mod chunk)