How Does fmt.Printf Handle Leading Zeros When Using Width and Precision for Floating-Point Values?

fmt.Printf Width and Precision Syntax Discrepancy

The usage of fmt.Printf with width and precision fields for floating-point values can be confusing due to a discrepancy in the documentation. The statement:

"For floating-point values, width sets the minimum width of the field and precision sets the number of places after the decimal, if appropriate, except that for %g/%G it sets the total number of digits."

While grammatically correct, it leads to ambiguity as "it" refers to precision rather than width. This can result in unexpected formatting behavior.

Precision for Floating-Point Notations

Consider the following floating-point values:

123.45
12312.2
1.6069
0.6069
0.0006069

When using fmt.Printf("%.4g"), the output is:

123.5
1.231e+04
1.607
0.6069
0.0006069

Observe that the precision field (.4) specifies the total number of digits, excluding decimal points and exponents. Hence, even though the last two values have leading zeros, they still count as digits and are not truncated.

Width Specifications

On the other hand, the width field (e.g., g) specifies the minimum width of the field, including decimal points and exponents. If the formatted value exceeds this width, it will not be truncated. Leading zeros, however, are not counted towards the width until there are four or more leading zeros.

Example

In your case, the format string .9g specifies a minimum width of 10 and a total digit count of 9, excluding leading zeros. The resulting formatting is:

0.0606060606060606: 9 digits, 12 width
0.3333333333333333: 9 digits, 11 width
0.05: 3 digits, 10 width (padded with spaces)
0.4: 2 digits, 10 width (padded with spaces)
0.1818181818181818: 9 digits (rounded), 11 width

This demonstrates the interplay between precision and width fields and how leading zeros can affect the final formatting.

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