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### 4.13 Incomplete Objects

When `(` or `[` is typed to begin entering a complex number or vector, respectively, the effect is to push an incomplete complex number or vector onto the stack. The `,` key adds the value(s) at the top of the stack onto the current incomplete object. The `)` and `]` keys “close” the incomplete object after adding any values on the top of the stack in front of the incomplete object.

As a result, the sequence of keystrokes `[ 2 , 3 RET 2 * , 9 ]` pushes the vector ‘`[2, 6, 9]`’ onto the stack. Likewise, `( 1 , 2 Q )` pushes the complex number ‘`(1, 1.414)`’ (approximately).

If several values lie on the stack in front of the incomplete object, all are collected and appended to the object. Thus the `,` key is redundant: `[ 2 RET 3 RET 2 * 9 ]`. Some people prefer the equivalent `SPC` key to `RET`.

As a special case, typing `,` immediately after `(`, `[`, or `,` adds a zero or duplicates the preceding value in the list being formed. Typing `DEL` during incomplete entry removes the last item from the list.

The `;` key is used in the same way as `,` to create polar complex numbers: `( 1 ; 2 )`. When entering a vector, `;` is useful for creating a matrix. In particular, `[ [ 1 , 2 ; 3 , 4 ; 5 , 6 ] ]` is equivalent to `[ [ 1 , 2 ] , [ 3 , 4 ] , [ 5 , 6 ] ]`.

Incomplete entry is also used to enter intervals. For example, `[ 2 .. 4 )` enters a semi-open interval. Note that when you type the first period, it will be interpreted as a decimal point, but when you type a second period immediately afterward, it is re-interpreted as part of the interval symbol. Typing `..` corresponds to executing the `calc-dots` command.

If you find incomplete entry distracting, you may wish to enter vectors and complex numbers as algebraic formulas by pressing the apostrophe key.

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