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3:21 AM
I made a program that calculates the standard deviation of a list of numbers:
λ1|:∑$L/; £mean_function
@std_dev:data|
	¥data ¥mean_function ⍎
	¥data $ - ĵ ¥mean_function ⍎ √
;
It's a lambda and a normal function
Example usage:
⟨16|34|29|80|65|77|91|58|67|40⟩ @std_dev; `The standard deviation of the data is: `, , `\n`,
And golfed:
λ:∑$L/;£f@s:d|¥d¥f⍎¥d$-ĵ¥f⍎√;
⟨16|34|29|80|65|77|91|58|67|40⟩ @s; `The standard deviation of the data is: `, , `\n`,
^ they both produce the same result
With an average built-in and context vars, it becomes @s:1|nÑn$-ĵÑ√;
If it's a full program, it becomes Ñ?-ĵÑ√
I prefer the first one though for it's readability
Oh, and
 

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