Fix for minimal-printf when printing sign of double between 0.0 and -1.0

[helpfulchicken]

Summary of changes

Fixes "minimal-printf doesn't display decimals between -1 to 0 correctly" listed in #13783

This is pull request is a suggested alternative to #13865, which also succeeds in fixing the above issue.

In mbed_minimal_formatted_string_double(), a given value of type double is printed in two parts, first the integer part and then the decimal part. The integer part is printed using mbed_minimal_formatted_string_signed(), by first casting the double datatype value into an integer datatype. For any values between 0.0 and -1.0, this casting produces the integer 0. Print the integer 0 does not result in a '-' sign, so the overall printing function for the double datatype prints values between 0.0 and -1.0 as positive values. E.g. printing the double value "-0.0567" produces a positive value "0.0567".

Impact of changes

Migration actions required

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR


double signed_small_decimal = -0.0567;
printf("value = %f\r\n", signed_small_decimal);

// Should print "value = -0.0567" rather than "value = 0.0567".
// I might have the default number of decimal places shown wrong.
// I think that this might also require enabling floating point values for minimal-printf.

--->

value = -0.0567

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Reviewers

@0xc0170

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[helpfulchicken]

I thought it would be worth submitting my pull request for consideration. It's only my second pull request ever, so I'm not sure if I've done it right, feel free to edit it.

[ciarmcom]

@helpfulchicken, thank you for your changes.
@0xc0170 @ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[helpfulchicken]

I saw that it failed a style check, so I've fixed that and squashed the commits back down to a single commit.

[0xc0170]

Thank you! We will review

[0xc0170]

@Linguanghsou Please review this alternative

[0xc0170]

@helpfulchicken can you add a test case to catch this bug: https://github.com/ARMmbed/mbed-os/tree/105c39e693e48f6bfe6a0ef8f1a608f4652b59d5/platform/tests/TESTS/mbed_platform/minimal-printf/compliance ? There are tests for minimal printf. We must have missed this use case if the test passed previously.

[Linguanghsou]

@Linguanghsou Please review this alternative

@0xc0170 I think helpfulchicken's fix is also good.

[helpfulchicken]

@helpfulchicken can you add a test case to catch this bug: https://github.com/ARMmbed/mbed-os/tree/105c39e693e48f6bfe6a0ef8f1a608f4652b59d5/platform/tests/TESTS/mbed_platform/minimal-printf/compliance ? There are tests for minimal printf. We must have missed this use case if the test passed previously.

I will do this. It's been a busy time for me lately, but I haven't forgotten about this.

[evedon]

Hi @helpfulchicken thank you for your fix. I reviewed #13548 and your fix will need to be adapted in that PR which adds support for width modifier, see my comment #13548 (comment)
So I don't think it makes sense to add the missing test case here.

Thank you very much for finding the issue and providing a solution; this is much appreciated.

[evedon]

This PR should be closed in favour of #13548