C++ – Declaring and Initializing Variable in a For Loop

c++declarationfor-loop

Can I write simply

for (int i = 0; ...

instead of

int i;
for (i = 0; ...

in C or C++?

(And will variable i be accessible inside the loop only?)

Best Answer

It's valid in C++.

It was not legal in the original version of C.
But was adopted as part of C in C99 (when some C++ features were sort of back ported to C)
Using gcc

gcc -std=c99 <file>.c

The variable is valid inside the for statement and the statement that is looped over. If this is a block statement then it is valid for the whole of the block.

for(int loop = 0; loop < 10; ++loop)
{
    // loop valid in here aswell
}

// loop NOT valid here.

Related Solutions

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.

C++ For-Loop – Structure with Multiple Variable Initialization

Try without the int before col.

for (int i = 0, col = 0; i < values.size(); i++, col++)

Best Answer

Related Solutions

C – Variables Declared Inside a Loop

C++ For-Loop – Structure with Multiple Variable Initialization

Related Question