%i vs. %d in C: When Do These Format Specifiers Differ?

Differences Between Format Specifiers %i and %d in Formatted I/O Functions (printf / scanf)

Formatted I/O functions like printf and scanf utilize conversion specifiers to control how data is represented during input or output. Among these specifiers, %i and %d are commonly used for integers.

Usage for Output (%i vs. %d)

When used as format specifiers for output, there is no distinction between %i and %d. Both produce the same result. They represent an integer value in decimal format.

Input Specifiers

However, the distinction emerges when using %i and %d as input specifiers (in scanf). Here's where the key difference lies:

• %d: Specifies a signed decimal integer. It expects the input to be a base-10 number (decimal).

• %i: Default behavior is similar to %d (signed decimal integer). However, it has an additional feature: it allows input in other bases, such as:

  • Hexadecimal: Preceded by "0x", it interprets the input as a hexadecimal number.
  • Octal: Preceded by "0", it interprets the input as an octal number.

Practical Example

To illustrate the difference, consider the input string "033".

• With %d: It treats it as a signed decimal integer, resulting in the value 33.
• With %i: Since the input starts with "0", it recognizes it as an octal number. The value of "033" in octal is 27.

In summary, while %i and %d behave similarly for output, they differ in their behavior for input. %i provides extended functionality by allowing hexadecimal and octal input, while %d is strictly limited to signed decimal input.

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