Can declarations and conditions be separated in while loops?

Question

I'm trying to write something like this in C++20 mode.

I see that C++17 can do this for if conditionals.

char *o = ch;
char *a = ch;
while (o = strstr(o, "||"), a = strstr(a, "&&");
            o != nullptr && a != nullptr)
{
}

Is there any comparable syntax for while loops doing assignment?

`if(o != nullptr && a != nullptr) break;` is much more readable. — SuperStormer, May 12 '21 at 05:05
The problem I'm having is if `o` or `a` is a `nullptr` it exits the `while` loop. — ZeroPhase, May 12 '21 at 05:14
@ZeroPhase what do you *want* it to do? What are you trying to accomplish? — Remy Lebeau, May 12 '21 at 05:15
I'm trying to do work in `while`, until `o` and `a` are nullptrs — ZeroPhase, May 12 '21 at 05:15
Perhaps you want `o != nullptr || a != nullptr`, which would run the loop until *both* `o` and `a` are nullptrs? — James R., May 12 '21 at 05:20
Apparently [they considered it](https://stackoverflow.com/questions/59985550/while-statement-with-initializer) but chose not to. `if` and `for` can have initialization statements, `while` cannot. Shrug. — Nathan Pierson, May 12 '21 at 05:37
assignment inside while loop = for loop: `for (; o = strstr(o, "||"), a = strstr(a, "&&"); o != nullptr && a != nullptr) {}` — 康桓瑋, May 12 '21 at 07:16

score 2 · Answer 1 · answered May 12 '21 at 08:01

2

You might use for loop

for (auto* o = strstr(ch, "||"), a = strstr(ch, "&&"); // init-statement
     o && a; // Condition
     /*o = strstr(o, "||"), a = strstr(a, "&&")*/) // iteration_expression
{
// ...
}

answered May 12 '21 at 08:01

Jarod42

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It's just frustrating that it's not consistent in C++ currently. I'm sure in C++ 27 we'll have while loops with my syntax. Thanks. – ZeroPhase May 12 '21 at 08:19

score 0 · Answer 2 · answered May 15 '21 at 17:00

C++ already supports compact assign and compare expressions, since assigning returns the updated value. What you describe can be written as:

char const *o = ch;
char const *a = ch;
    
while ((o = strstr(o, "||")) != nullptr && 
       (a = strstr(a, "&&")) != nullptr) { 
  /*...*/ 
}

or more succintly (since comparing with nullptr is just checking a truthy value):

char const *o = ch;
char const *a = ch;
    
while ((o = strstr(o, "||")) && (a = strstr(a, "&&"))) { 
  /*...*/ 
}

the only caveat is that due to short-circuiting, the second assignment only happens when the first condition evaluates to true. Here's an example.

ZeroPhase · Answer 3 · 2021-05-12T05:37:26.813

-1

Ended up going with.

char *o = strstr(ch, "||");
char *a = strstr(ch, "&&");
while (o != nullptr || a != nullptr)
{
    o += 2;
    if (o) o = strstr(o, "||");
    if (a) a = strstr(a, "&&");
}

That other syntax would have been more succinct for this. Wish C++ would allow that syntax for all loops and conditionals.

edited May 12 '21 at 05:37

answered May 12 '21 at 05:24

ZeroPhase

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4
21

1

What's the deal with `o += 2;`? If `o` is null to begin with, then what? And if by incrementing it it points to garbage, then what? – acraig5075 May 12 '21 at 05:57
Have to increment `o += 2` past `||` in the string or it keeps looping forever. I can put in the condition when needed. – ZeroPhase May 12 '21 at 07:10

Can declarations and conditions be separated in while loops?

3 Answers3