C – Declare Several Variables in a For Loop

c++declarationfor-looploops

I thought one could declare several variables in a for loop:

for (int i = 0, char* ptr = bam; i < 10; i++) { ... }

But I just found out that this is not possible. GCC gives the following error:

error: expected unqualified-id before 'char'

Is it really true that you can't declare variables of different types in a for loop?

Best Answer

You can (but generally shouldn't) use a local struct type.

for ( struct { int i; char* ptr; } loopy = { 0, bam };
      loopy.i < 10 && * loopy.ptr != 0;
      ++ loopy.i, ++ loopy.ptr )
    { ... }

Since C++11, you can initialize the individual parts more elegantly, as long as they don't depend on a local variable:

for ( struct { int i = 0; std::string status; } loop;
      loop.status != "done"; ++ loop.i )
    { ... }

This is just almost readable enough to really use.

C++17 addresses the problem with structured bindings:

using namespace std::literals::string_literals;

for ( auto [ i, status ] = std::tuple{ 0, ""s }; status != "done"; ++ i )

Related Solutions

C Programming – Using Two Variables in a For Loop

The problem is the test condition:

loop_1 >= (offset - 190),loop_2 <= (190 + offset + 2)

This does not check both parts. (Well, it does, but only the result of the second part is used.)

Change it to

(loop_1 >= (offset - 190)) && (loop_2 <= (190 + offset + 2))

if you want both conditions to be checked.

C – Variables Declared Inside a Loop

Variables are not really "created" or "destroyed". They are concepts at the abstraction level of the programming language. The compiler is not required to have a one to one mapping between a variable and memory addresses. In practice, most of the time, stack space for local variables is allocated at once at the beginning of the function, so it won't make a difference in performance.

Note that, C++, unlike C, which doesn't have a notion for constructors, supports object construction and destruction, so if you were to define a variable of a class type in a for loop, like the following,

class MyClass { 
    public: MyClass() { cout << "hello world" << endl; }
};
//...
for (int i = 0; i < 10; ++i) {
   MyClass m;
}

you'd call its constructor every time, effectively printing "hello world" ten times. This is very different from C declarations and should not be confused with it.

Best Answer

Related Solutions

C Programming – Using Two Variables in a For Loop

C – Variables Declared Inside a Loop

Related Question