Comparisons and logic

A comparison is a number too, and it can be worked with like one

easyYou need: op: arithmetic

A comparison in mlog is not a special construct but an op operation like addition. Its answer is a number too: 1 if true and 0 if not. It can be put into a variable, added to another, passed on — it is no different from an ordinary number.

steptick 0

Set0

=

Set1

=

Operation2

=

<

Operation3

=

>

Operation4

=

and

Operation5

=

or

@counter0number

coppernullnull

limitnullnull

lownullnull

plentynullnull

bothnullnull

atLeastOnenullnull

There is 40 copper and the limit is 100. The first comparison answers with a one, the second with a zero.

Operation	In the block	True when
equal	==	the numbers are equal (with a tolerance of 0.000001)
notEqual	not	they are not equal
lessThan	<	the first is less than the second
lessThanEq	<=	less than or equal
greaterThan	>	greater
greaterThanEq	>=	greater than or equal

The second column is what is written on the button in the block. It does not always match: “not equal” is labelled not, with no equals sign at all, and a confusion of its own comes with that — see below.

About equal’s tolerance and how strictEqual differs from it, see the lesson «Whole numbers and precision».

Two conditions at once

land is “and”: a one only if both numbers are non-zero. In the example both came out zero, because the second condition did not hold.

There is, however, no separate “or” operation in mlog. or is used instead — it is bitwise, but on ones and zeroes it behaves exactly like “or”, because 1 or 0 in bits is just 1 or 0. In the example atLeastOne came out a one.

Note

or only lets you down when the variables hold arbitrary numbers rather than ones and zeroes: or of 2 and 4 gives 6. For a check of “is at least one of these true” that still does — 6 is not zero — but if you expected exactly a one, you will not get it. It is safer to bring the numbers to 0 and 1 with a comparison first and join them after.

not is not a negation

The group’s most common trap, and it starts with the labels. The operation list has not — a bitwise inversion — but on the button it is labelled flip. And the label not goes to a completely different operation: that is how “not equal”, notEqual, is written.

In the reference	On the button	What it is
notEqual	not	not equal
not	flip	bitwise inversion

Now for the second row: op not is a bitwise inversion, not a logical “not”:

steptick 0

Operation0

= flip

Operation1

=

==

@counter0number

bitwisenullnull

logicalnullnull

not 1 gives −2, not 0. What happens to the bits is in the lesson «Bitwise operations»; what matters here is something else: the operation is no good for negation.

A logical “not” is written as a comparison with zero: op equal answer condition 0. If the condition was a one it becomes zero; if it was zero it becomes one.

Silent failure

op not in a condition is a mistake you cannot see: the program does not crash, nothing is underlined, it is just that instead of “not” you get −2, and −2 is not zero, that is, “yes”. The check works backwards, and silently.

Why store a comparison at all

Branching in mlog is done not by op but by jump, whose condition is written right into the jump block. A comparison through op is for when the answer has to be kept rather than checked once: to add up several conditions and see how many held, to hand a number to print, to put it into memory, to give it back to another processor.

Remember

• A comparison is an ordinary op operation; its answer is 1 or 0.
• land is “and”; there is no separate “or”, or is used instead.
• not is a bitwise inversion, not a negation: not 1 is −2.
• A logical “not” is equal with zero.

Task

Change copper in the example from 40 to 150 and watch all four answers change: low becomes zero, plenty a one, both stays zero, atLeastOne a one.

Then add the line op equal notLow low 0 and see that it gives a one when low is zero. That is a logical “not”, written correctly.