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I'm trying to get random expressions displaying. For that, I'm starting with addition, multiplication, substraction and division. When trying out this code

 HoldForm[c e d] /. c -> RandomInteger[{1, 15}] /. 
 d -> RandomInteger[{1, 15}] /. 
 e -> RandomChoice[{"-", "+", "/", "*"}]

, I sometimes get indeed expressions as "11 "+" 6" as expected, but I also get

 14 "-"

or

 "-" 9.

Why is that?

Perhaps anyone has another idea for making randomised arithmetic expressions? Thanks, once again!

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2 Answers 2

up vote 4 down vote accepted

Try this

HoldForm[op@##] & @@ RandomInteger[{1, 15}, 2] /. 
 op -> RandomChoice[{Plus, Subtract, Times, Divide}]
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Thank you for your help! One more question: why doesn't this work: HoldForm[Plus @@ ##] &@{a, b} ? Thanks for all information! –  Gabriel Sep 9 '13 at 9:27
    
@Gabriel FullForm of a + b is Plus[a, b], not Plus@@{a, b} –  ybeltukov Sep 9 '13 at 12:17

Here is a subtly different form of what ybeltukov proposed:

RandomChoice[{Plus, Subtract, Times, Divide}] /. op_ :>
    HoldForm[op@##] & @@ RandomInteger[{1, 15}, 2]

You'll notice that the rule is used in reverse. I call this the injector pattern. Here is serves an important role of localizing the Symbol op. You could also use HoldPattern[op] -> . . . to do the same.

Here is another option:

Function[op, HoldForm[op@##] & @@ RandomInteger[{1, 15}, 2]] /@ 
  RandomChoice[{Plus, Subtract, Times, Divide}, 10]

enter image description here

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