How does precision work with width in fmt.Printf\'s %g format specifier?

How Does fmt.Printf Work with Width and Precision Fields in %g?

fmt.Printf's behavior with width and precision fields in %g can be confusing. To understand it, we need to clarify the documentation.

The following statement in the documentation is particularly important:

precision sets the number of places after the decimal, if appropriate, except that for %g/%G it sets the total number of digits.

In this statement, "it" refers to precision, not width. This means that while width specifies the minimum field width, precision specifies the total number of digits displayed, excluding leading zeros.

For example, when we format 123.45 using fmt.Printf("%.4g"), the result is 123.5. This is because precision is set to 4, and 123.5 has 4 digits (excluding the decimal point).

However, when there are leading zeros, the behavior becomes more complex. Leading zeros are not counted as digits, and they are not removed when there are less than 4 of them. For example, when we format 0.00012345 using fmt.Printf("%.4g"), the result is 0.0001234. This is because the precision is set to 4, and 0.0001234 has 4 digits (excluding the decimal point and leading zeros).

Now that we understand how precision behaves, we can examine the width field. Width specifies the minimum width of the field, including the decimal place and any exponents. If the formatted value has more digits than the width specified, it will extend beyond the width. However, precision takes priority, so the value must first satisfy the precision requirement.

In your example, you specify .9g, which means a total of 9 digits (excluding leading zeros) and a minimum width of 10, including the decimal point and any exponents. Let's examine each value individually:

• 0.0606060606060606: 9 digits without leading zeros is 0.0606060606, which is already 12 characters wide, exceeding the minimum width of 10.
• 0.3333333333333333: Similar to the previous value, 9 digits without leading zeros is 0.333333333, which is 11 characters wide, exceeding the minimum width of 10.
• 0.05: 9 digits without leading zeros is 0.05, which is less than the minimum width of 10. It will be padded with 6 spaces to meet the minimum width.
• 0.4: Similar to the previous value, 9 digits without leading zeros is 0.4, which is less than the minimum width of 10. It will be padded with 6 spaces to meet the minimum width.
• 0.1818181818181818: 9 digits without leading zeros is 0.181818182, which is due to rounding. It exceeds the minimum width of 10 with 11 characters.

This explains the unexpected widths you observed.

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