Is there a way to replace string ">" with > in an 'if' condition?

Question

I came across the below use case, but I could not find a proper solution. Is there a way to replace string "<" or ">" with condition < or > in an if condition?

Example:

  string condition = "<";
  if (10 condition 8)   // Here I want to replace condition with <
  {
      // Some code
  }

I don't want to do it like:

if ("<" == condition)
{
   if (10 < 8)
   {
   }
}
else if (">" == condition)
{
   if (10 > 10)
   {
   }
}

And my condition will change during run time. I am just searching for a simple way if exist apart from above.

Use case: The user will give some query like below:

input: 10 > 9   =>  output: true
input: 10 < 7   =>  output: false

Basically I need to parse this query, as I have these 3 words (10, >, 9) as strings, and somehow I want to convert string ">" or "<" to actual symbol > or <.

You can do `#define lessThan <` and then write `if (10 lessThan 8)`. This is probably useless but you didn't mention an actual use case either. — melpomene, Jun 17 '16 at 22:08
@melpomene your suggestion is irrelevant, as `lessthen` is just the identifier name he use for the operand... `#define` will result in pre-compile time and won't change between runs. — Tomer W, Jun 17 '16 at 22:12
@TomerW Changes between runs wasn't part of the requirements. Besides, why would it be called `lessThan` if it could represent other operations than `<`? — melpomene, Jun 17 '16 at 22:13
Are there other string representations of the standard operators that you want to be able to process? — R Sahu, Jun 17 '16 at 22:15

score 5 · Answer 1 · answered Jun 17 '16 at 22:04

5

You can map the string to a standard library comparator functor such as std::less via a std::map or a std::unordered_map.

answered Jun 17 '16 at 22:04

Cheers and hth. - Alf

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Yes, but this involves using the preprocessor, so you couldn't use the value of a string as a dynamically set operator like in the asker's example. This is an interesting strategy to consider, though, as it could make things look a little easier to read. – Robert Columbia Jun 17 '16 at 22:10
3

The preprocessor does not need to be used for this. – Peter Jun 17 '16 at 22:14
4

@RobertColumbia: Sorry, I don't understand how you arrive at your conclusion. There's no preprocessor involved. – Cheers and hth. - Alf Jun 17 '16 at 22:14

score 5 · Answer 2 · edited May 23 '17 at 10:27

5

You can't create a new operator in C++ (Can I create a new operator in C++ and how?). I can see where you are coming from with this idea, but the language just doesn't support that. You can, however, create a function that takes two operands and a string "argument" and returns the appropriate value.

bool CustomCompare(int operand1, int operand2, string op)
{
    if (op == "<")
    {
        return operand1<operand2;
    }
    if (op == ">")
    {
        return operand1>operand2;
    }
    if (op == "_")
    {
        return DoTheHokeyPokeyAndTurnTheOperandsAround(operand1, operand2);
    }
}

edited May 23 '17 at 10:27

Community

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answered Jun 17 '16 at 22:09

Robert Columbia

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As I told, I have thought about It but looking if there is a another way. just like token pasting operators – NDestiny Jun 17 '16 at 23:26

score 3 · Answer 3 · edited Jun 17 '16 at 22:14

3

std::function<bool(int,int)> comparator = std::less;
if(comparator(10, 8))
{
    //some code
}

See Also:

edited Jun 17 '16 at 22:14

melpomene

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answered Jun 17 '16 at 22:11

Mooing Duck

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score 0 · Answer 4 · edited Oct 30 '16 at 12:59

If you are a C++ Ninja, and you are very stubborn to get it working just the way you wish, there is a way, but it is advanced and complicated.

I mostly write POSIX code for VxWorks, but I guess it can be done for any other.

Let's say you have: string myExpression = "mySize > minSize";

"Rebuild the String as a C code" (save to file, use ccppc, gcc, gpp, whatever toolchain you have.)
You need to link it with your code, at least to get ELF relocations for mySize & minSize (I think it can be done using app-load, if you customize your ld command.
Load the code using ld
Jump to the new address you loaded your code to.

All that said, I would not recommend you to do that:

Very complicated.
Not the most stable, and very bug/error prone.
Can lead to major vulnerabilities "Hacker" style, and proper sanitation is required.

The only pro I can see, is that this method supports everything C has to offer out of the box! BitWise, +-/*^!, even functions as pow(), and such.

A little bit better is:

To compile a function as:

`"bool comparer_AB(int a, int b) { return a " + operator + "}"`

and then call comparer_AB();.

score 0 · Answer 5 · answered Jun 18 '16 at 02:22

#include <functional>
#include <map>
#include <string>

int main()
{
  using operation = std::function<bool(int,int)>;
  std::map<std::string, operation> mp = 
    {{"<", std::less<int>()}, 
     {">", std::greater<int>()}};

  int x = 5; 
  int y = 10;
  std::string op = "<";

  bool answer = mp[op](x, y);
}

Is there a way to replace string ">" with > in an 'if' condition?

5 Answers5