What are the rules on declaring integers?

Bluenick2100 · April 26, 2018, 7:01pm

Are there any rules on when integers should be declared or defined? Sometimes I see this done before setup and sometimes I see them declared in setup. What are the rules that tell me when I should declare stuff?

system · April 26, 2018, 7:02pm

You should define any variable (not just the integer ones) with the minimum scope possible.

Some of the provided examples are not very good in this respect.

groundFungus · April 26, 2018, 7:16pm

Some information on Arduino variables and scope.

olf2012 · April 26, 2018, 8:27pm

groundFungus:
Some information on Arduino variables and scope.

Like most of the tutorial pages on Arduino.cc you should not rely on the information given here. Some are outright wrong e.g. the part about "variable rollover" using a (signed) int as an example.
There are many good C and C++ tutorials on the web. Google is your friend here

Bluenick2100 · April 26, 2018, 9:05pm

Thanks!

Coding_Badly · April 26, 2018, 11:06pm

olf2012:
Some are outright wrong e.g. the part about "variable rollover" using a (signed) int as an example.

No doubt. That missing semicolon and the two commas are definitely a problem.

You are referring the syntax mistakes, right?

pert · April 27, 2018, 5:46am

olf2012:
Like most of the tutorial pages on Arduino.cc you should not rely on the information given here. Some are outright wrong e.g. the part about "variable rollover" using a (signed) int as an example.

I have submitted recommendations for improvements to that page:

github.com/arduino/Arduino

Issues with "Variables" reference page

opened 05:42AM - 27 Apr 18 UTC

closed 11:40AM - 27 Apr 18 UTC

per1234

Component: Documentation

https://www.arduino.cc/en/Reference/VariableDeclaration Change: ``` int inp…utVariable1; int inputVariable2 = 0; // both are correct ``` To: ``` int inputVariable1; int inputVariable2 = 0; // both are correct ``` (Remove blank line from end of code block) Change: ``` int calibrationVal = 17; // declare calibrationVal and set initial value ``` To: ``` int calibrationVal = 17; // declare calibrationVal and set initial value ``` Notes: - Remove one space indent. - Remove blank line from end of code block --- Change: ``` int x; x = -32,768; x = x - 1; // x now contains 32,767 - rolls over in neg. direction x = 32,767; x = x + 1; // x now contains -32,768 - rolls over ``` To: ``` byte x; x = 0; x = x - 1; // x now contains 255 - rolls over in neg. direction x = 255; x = x + 1; // x now contains 0 - rolls over ``` Notes: - Use a type that will be a consistent size when using any official Arduino core (thanks Coding Badly). - Signed integer overflow is undefined behavior so it's not a good thing to use in an example. - Add missing semicolon. - Don't use decimal separators. - Standard 2 space indentation rather than weird 3 space indentation. --- Change: >Once variables have been declared, they are used by to: >Once variables have been declared, they **can be defined** by --- Change: ``` int lightSensVal; char currentLetter; unsigned long speedOfLight = 186000UL; char errorMessage = {"choose another option"}; // see string ``` to: ``` int lightSensVal = 1234; char currentLetter = 'a'; unsigned long speedOfLight = 186000UL; char errorMessage[] = "choose another option"; // see string ``` Notes: - Consistent, standard indentation. - These are supposed to be examples of variable definition, not declaration. So define them. - `errorMessage` is an array. - Standard string initialization syntax. - Trim trailing space --- Change: ``` if (inputVariable2 < 100) { inputVariable2 = 100; } delay(inputVariable2); ``` To: ``` if (inputVariable2 < 100) { inputVariable2 = 100; } delay(inputVariable2); ``` (All previous code that would need to be inside a function was indented so either this should be indented for consistency or else other indentation should be removed) --- Change: >and it uses the value of the variable as an input parameter to the delay() function (` delay(inputVariable2)` ) To: >and it uses the value of the variable as an input parameter to the delay() function ( `delay(inputVariable2)` ) (remove leading space from code tag) --- "Some variable types" section: Since a comment in the variable definition examples says "see string", add a link to https://www.arduino.cc/reference/en/language/variables/data-types/string/ --- What's the deal with this page? It appears to be a Language Reference page but it's not in the [Language Reference repository](https://github.com/arduino/reference-en).

I'm happy to add to that issue if anyone has any recommendations.

Any specific suggestions for improvements to the Arduino documentation are welcome. It's unfortunate that the quality isn't better to start with but I've found the Arduino team is very receptive to suggestions, even if it takes them a little while to implement them sometimes.

It's now possible to submit pull requests for change proposals directly to the repository that contains the content the Language Reference pages are generated from:

There is an edit link on every one of those pages.

Other documentation don't have that system so you need to submit an issue report with suggested improvements here:

The specific page in question seems like it might be an orphan which is not linked to from any other page on the website but it will still show up in Google searches so it still needs to either be of high quality or removed.

Coding_Badly · April 27, 2018, 5:59am

  unsigned int x;
  x = 0;
  x = x - 1;       // x now contains 65535 - rolls over in neg. direction

Bear in mind that is only true when sizeof(x) == 2.

pert · April 27, 2018, 6:10am

Good point! I've changed x to a byte. There should be a fairly good guarantee that will be an unsigned 8 bit integer when using any official Arduino core.

olf2012 · April 27, 2018, 11:14am

I am referring to the fact that signed variables are not guaranteed to rollover as described. Any future compiler upgrade could silently break the code

Coding_Badly · April 27, 2018, 6:04pm

olf2012:
Any future compiler upgrade could silently break the code

I believe the point is that a signed integer rolling over is a serious problem regardless of the final outcome. But, the description is terse so I could be mistaken.

pert · April 27, 2018, 6:12pm

Signed integer overflow is undefined behavior. It may happen to roll over but this should not be counted on. There actually is a way to make it not undefined behavior:

github.com/arduino/Arduino

Make signed overflow behave "correctly"

arduino:master ← cousteaulecommandant:patch-1

opened 06:52PM - 28 Feb 16 UTC

cousteaulecommandant

+4 -4

Fix for issue #4511 In short, the documentation says that signed integer overflo…w causes the value to "roll over". Actually, this results in undefined behavior, with potentially unpredictable results (see #4511 for an example). This patch adds a GCC option to treat signed integers the "right" way so that they roll over as the documentation claims.

Unsigned integer rollover is not undefined behavior so I think the change to an unsigned type in the example is an improvement. Maybe it would be worth adding an explanation about signed overflow being UB to that page.

My suggestions have already been accepted. Pretty impressive response time from Arduino! Thank you olf2012 for pointing out that page needed some work and Coding Badly for the improvement. The world is now a very slightly better place.

Coding_Badly · April 27, 2018, 7:51pm

Thank you @pert!

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What are the rules on declaring integers?

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