What does %.21f mean in C language?

%.21f is the format string of floating point numbers in C language, which means: 21 digits are retained after the decimal point, for example: float num = 123.456789; printf("%.21f", num); // Output: 123.456789012345678901

The meaning of %.21f in C language

%.21f It is a format string in C language, used to control the output of floating point numbers. It means:

• %: This is the starting identifier of the format specifier.
• .: This is a decimal point symbol, indicating that a decimal point should be displayed in the output.
• 21: This is the number of digits to be displayed after the decimal point.
• f: This is a floating point numeric character, indicating that a floating point number is to be output.

Thus, %.21f formats a floating point number into a string retaining 21 digits after the decimal point. For example:

float num = 123.456789;
printf("%.21f", num); // 输出：123.456789012345678901

In the above example, the printf() function formats the floating point number num into a string with 21 digits after the decimal point, and which prints to the console.

It’s worth noting:

• If a floating-point number has fewer digits after the decimal point than the specified number of digits after the decimal point, the output will be padded with zeros.
• If the floating-point number has more digits after the decimal point than the specified number of digits after the decimal point, it will be rounded or truncated to meet the specified number of digits after the decimal point.
• If the specified number of decimal places is 0, no decimal point will be displayed in the output.

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