When you click RUN<\/i>, DrRacket evaluates the expressions in the definitions area and shows their result in the\u00a0interactions area<\/i>. Then, DrRacket, your faithful servant, awaits your commands at the\u00a0prompt<\/i>\u00a0(>). The appearance of the prompt signals that DrRacket is waiting for you to enter additional expressions, which it then evaluates like those in the definitions area:
\n> (+ 1 1)
\n2<\/li>\n<\/ul>\nEnter an expression at the prompt, hit the \u201creturn\u201d or \u201center\u201d key on your keyboard, and watch how DrRacket responds with the result. You can do so as often as you wish:
\n> (+ 2 2)
\n4
\n> (* 3 3)
\n9
\n> (- 4 2)
\n2
\n> (\/ 6 2)
\n3
\n> (sqr 3)
\n9
\n> (expt 2 3)
\n8
\n> (sin 0)
\n0
\n> (cos pi)
\n#i-1.0<\/p>\n
Take a close look at the last number. Its \u201c#i\u201d prefix is short for \u201cI don\u2019t really know the precise number so take that for now\u201d or an\u00a0inexact number<\/i>. Unlike your calculator or other programming systems, DrRacket is honest. When it doesn\u2019t know the exact number, it warns you with this special prefix. Later, we will show you really strange facts about \u201ccomputer numbers,\u201d and you will then truly appreciate that DrRacket issues such warnings.<\/p>\n
By now you might be wondering whether DrRacket can add more than two numbers at once, and yes, it can! As a matter of fact, it can do it in two different ways:
\n> (+ 2 (+ 3 4))
\n9
\n> (+ 2 3 4)
\n9<\/p>\n
The first one is\u00a0nested arithmetic<\/i>, as you know it from school. The second one is\u00a0BSL arithmetic<\/i>; and the latter is natural, because in this notation you always use parentheses to group operations and numbers together.<\/p>\n","protected":false},"excerpt":{"rendered":"
![\"\"](\"https:\/\/www.booksofall.com\/wp-content\/uploads\/2023\/05\/img_6458c2cc3e187.png\")
<\/p>\n