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### 3.7 Numeric Prefix Arguments

Many Calculator commands use numeric prefix arguments. Some, such as d s (`calc-sci-notation`), set a parameter to the value of the prefix argument or use a default if you don’t use a prefix. Others (like d f (`calc-fix-notation`)) require an argument and prompt for a number if you don’t give one as a prefix.

As a rule, stack-manipulation commands accept a numeric prefix argument which is interpreted as an index into the stack. A positive argument operates on the top n stack entries; a negative argument operates on the nth stack entry in isolation; and a zero argument operates on the entire stack.

Most commands that perform computations (such as the arithmetic and scientific functions) accept a numeric prefix argument that allows the operation to be applied across many stack elements. For unary operations (that is, functions of one argument like absolute value or complex conjugate), a positive prefix argument applies that function to the top n stack entries simultaneously, and a negative argument applies it to the nth stack entry only. For binary operations (functions of two arguments like addition, GCD, and vector concatenation), a positive prefix argument “reduces” the function across the top n stack elements (for example, C-u 5 + sums the top 5 stack entries; see Reducing and Mapping Vectors), and a negative argument maps the next-to-top n stack elements with the top stack element as a second argument (for example, 7 c-u -5 + adds 7 to the top 5 stack elements). This feature is not available for operations which use the numeric prefix argument for some other purpose.

Numeric prefixes are specified the same way as always in Emacs: Press a sequence of META-digits, or press ESC followed by digits, or press C-u followed by digits. Some commands treat plain C-u (without any actual digits) specially.

You can type ~ (`calc-num-prefix`) to pop an integer from the top of the stack and enter it as the numeric prefix for the next command. For example, C-u 16 p sets the precision to 16 digits; an alternate (silly) way to do this would be 2 RET 4 ^ ~ p, i.e., compute 2 to the fourth power and set the precision to that value.

Conversely, if you have typed a numeric prefix argument the ~ key pushes it onto the stack in the form of an integer.

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