~ / insert / expression-registerlesson 7/7

Do the maths in place: <C-r>=

Let Vim evaluate the arithmetic and type the answer for you.

?how it works

<C-r>= opens a small = prompt at the bottom of the screen. Type an expression, press <CR>, and its value is inserted at the cursor as if you had typed it. <Esc> on the prompt cancels. Perfect for back-of-the-envelope maths you'd otherwise do on your phone.

Two rules catch everyone out:

Whole numbers stay whole: 7/2 is 3. Make either side a float to get decimals: 7.0/2 is 3.5. A float needs digits on both sides of the dot.
Floats print with up to six decimal places: 1.0/3 gives 0.333333.

The = prompt is a little command line, so <C-r> works there too. That makes a slick trick for an expression that is already in your text: cut it (say with C), then <C-r>=<C-r>"<CR> evaluates what you just cut and puts the answer in its place.

Real Neovim accepts any Vimscript expression on this prompt, functions included. This editor understands arithmetic only: numbers, + - * / % and parentheses.

⌨keys

`<C-r>=`	evaluate an expression and insert the result
`<C-r>{reg}`	on the = prompt: paste a register into the expression
`<CR>`	on the = prompt: evaluate and insert (<Esc> cancels)

Nin neovim

Neovim colours the = prompt with its own expression parser while you type (:h expr-highlight), so mistakes like a missing bracket stand out before you press <CR>.

3 drills

Divide without a calculator: Finish the line with the decimal value. Type the division on the = prompt and let Vim work it out (remember the .0).
Replace the sum with its answer: Swap the arithmetic for its result without doing the maths yourself: cut it with C, then <C-r>=<C-r>"<CR>.
Answers with decimals: Same trick, but these answers have decimals. The .0 (or any decimal) in each expression makes Vim use floating point.

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