CODEWITHSUNDEEP
c++loopsvariables
user4375224
user4375224

Reputation:

Iteration to create variables

I was just curious to know if there is way to create a new variable with a new name every time a loop is executed.

For example:

#include <iostream>
int main()
{
    for (int x = 1 ; x<=5 ; x++)
       int a_x;
    return 0;
}

5 new variables should be created with names a_1, a_2, ..., a_5

The above code just shows what I am looking for and is not the answer.

Is this possible without using arrays?

Upvotes: 4

Views: 16249

Answers (3)

pmr
pmr

Reputation: 59811

While many will assume that this is impossible, it can be achieved with the preprocessor. It is necessary that the loop count is known at compile time. Here I use the Boost Preprocessor Library. The example for PP_REPEAT does almost exactly what you want.

#include <boost/preprocessor/repetition/repeat.hpp>

#define DECL(z, n, text) text ## n = n;

int main()
{
  BOOST_PP_REPEAT(5, DECL, int a_) // expands to int a_0 = 0; int a_1 = 1; ...
  return 0;
}

Please remember: this is certainly not what you want. You probably want to use an array. Do only use this if you are absolutely certain that you need it.

Upvotes: 1

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171117

No, there is no way to do what you've outlined (directly). Here are several possible alternatives:

  1. First off, if you do not need the "variables" to be accessible outside the loop, just use a normal local variable:

    for (int x = 1; x <= 5; ++x) {
  int a = whatever;  // This will be freshly redeclared & reinitialised in each iteration
}

  2. If the bounds of the iteration are known at compile time, you can use an array:

    std::array<int, 5> a;
for (int x = 0; x < a.size(); ++x) {
  a[x];
}

  3. If the bounds are only known at runtime, use a dynamic array:

    std::vector<int> a(the_runtime_size);
for (int x = 0; x < a.size(); ++x) {
  a[x];
}

  4. If you really need individual variables for some reason (and you know the number at compile time), you could resort to preprocessor tricks with Boost.Preprocessor. But that is far above beginner level:

    #include <boost/preprocessor>

#define DECLARE_MY_VARIABLE(z, idx, name) \
  int BOOST_PP_CAT(name, BOOST_PP_CAT(_, idx));

BOOST_PP_REPEAT(5, DECLARE_MY_VARIABLE, a)

    The code above will expand to:

    int a_0; int a_1; int a_2; int a_3; int a_4;

    You could of course take this several steps further, to have each of the variables of a different type, or name them by names instead of by indices. It will just require more macro magic.

    Disclaimer: Do NOT use this approach unless you very clearly know you need it. Even then, reconsider twice before you actually do that. And if you still do, document it heavily. Stuff like this should generally be hidden deep inside a library under a nice & clean user-friendly interface.

Upvotes: 5

Bathsheba
Bathsheba

Reputation: 234665

No you can't do that in C++.

The best thing to do in this case would be to create an array of ints and use the for loop to populate them.

int a_x[5];
for (int x = 1 ; x<=5 ; x++)
       a_x[x - 1] = /*ToDo - something*/

Note that

  1. arrays are zero-based: can you see how I've used x - 1. The normal thing to do would be to rebase x in the for loop though: for (int x = 0 ; x < 5; ...

  2. arrays are not initialised. You must populate the contents.

Upvotes: 3

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