C Programming – Variable Not Declared in For Loop Condition Still Works

c++for-loopinitializationvariables

I wrote the below code to find the sum of all digits in C, and when I compiled and ran it, it was successful.
But, only later I realized that i had not entered any value for the variable 'n' in the for loop's condition.
I'm confused on how this program works, even when there is no value assigned to the condition variable.
I would like to be clarified of the same.

#include<stdio.h>
void main()
{
int no,a,b,n,sum=0;
printf("Enter the number to be added");
scanf("%d",&no);
for(int i=0;i<n;i++)
    {
     a=no%10;
     b=no/10;
    sum=a+sum;
    no=b;
    }   
printf("The sum is %d",sum);
}

Best Answer

I'm confused on how this program works

Well, "works" is a very poor observation / decision here. This is undefined behavior.

You're attempting to use the value of an automatic local variable n while it is indeterminate. This invokes the UB.

To quote the C11 standard, chapter §6.7.9

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. [...]

So, in your case, n meets the criteria described above, and hence the content is indeterminate.

Now, after that, in case you try to use a variable while it holds indeterminate value and either

does not have the address taken
can have trap representation

the usage will lead to undefined behavior. That is exactly the case here.

That said, for a hosted environment, the conforming signature of main() is int main(void), at least.

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C – For Loop Missing Initialization

The for statement works like:

for (initialization; test-condition; update)

And any or all of those three can be omitted (left blank). So:

for (;;) is an infinite loop¹ equivalent to while (true) because there is no test condition. In fact, for (int i=0; ;i++) would also be an infinite loop¹.
for ( ; *s != '\0'; s++) is a loop with no initialization. s will point to the beginning of (probably) a string and is incremented until it reaches the null character '\0' denoting end-of-string. This essentially means loop through all characters of the string s

¹ The loop will still be interrupted if there's a break statement in the loop body, or a call to exit(), etc...

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

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