the declare scope of loop

Question

Can anyone explain to me, why the below codes have different output?

void GenerateMatrix(int mat[][MaxSize],int ran[],const int rows,const int cols) { int i,k=0; while (i<rows) { int j=0; while (j<cols) { mat[i][j]=ran[k]; j,k++; } i++; } }

and the correct one

void GenerateMatrix(int mat[][MaxSize],int ran[],const int rows,const int cols) { int k=0; for (int i=0; i<rows;i++) { for (int j=0; j<cols;j++) { mat[i][j]=ran[k]; k++; } } }

the ran[] is an array declared in main()

int main() { srand(time(NULL)); int a[10]; for (int i=0;i<10;i++) { a[i]={(rand() % 20-0+1)+1}; cout<<a[i]<<endl; } . . . }

the first one always output strange random numbers, not in the range of random numbers I set for a[] in main(). The second one is correct. I have change the scope of declaration of k in the first one function, still the same. I'm just wondering witch part result in this?

What do you intend j,k++; to do? Did you debug to observe whether the variables change as you intended? — Yunnosch
– Yunnosch, Commented May 25, 2021 at 6:32
int i, k = 0 is equivalent to int i; int k = 0 so i is uninitialized when you are accessing it, triggering undefined behavior. You probably meant int i = 0, k = 0. — CherryDT
– CherryDT, Commented May 25, 2021 at 6:36
Also j,k++ is equivalent to k++ (assuming the evaluation of j doesn't have side-effects), not a very useful statement. — CherryDT
– CherryDT, Commented May 25, 2021 at 6:37
Thanks, I had always thought I can declared two integers at the same time..., and is there a better way instead j,k++ if I want to increase both counter variables at the same time? — drew
– drew, Commented May 25, 2021 at 6:52

Yunnosch · Accepted Answer · 2021-05-25 06:46:53Z

You seem to be following a "spoken English" approach to using the ,, like "do something to this list of variables". In this reading, the initialisation failure (where it does create a list other than you mean; spotted by CherryDT) and the incrementation failure (caused by misuse as comma operator; spotted by two commenters, including me) can be "read".
I propose to change that by reading up on the "comma operator" and reviewing some tutorials on other uses of ",".
Consider reducing/avoiding the use altogether, it is too dangerous for your coding habits.

@CherryDT not in all the problematic cases. Also, I believe that referring to it that way is not helpful to OP. But I will add it for completeness. Thanks.
I just meant it's hard to google a literal comma. But you are right, in case of the initialization issue it has nothing to do with that operator.
Unrelated: yes, you are correct. Kudos for being the first one to point this out.

Some programmer dude · Accepted Answer · 2021-05-25 06:42:04Z

In the first function you have two errors:

The statement
```
int i,k=0; 
```
is equivalent to
```
int i; int k=0; 
```
This means that i is uninitialized and will have an indeterminate value.

You need to explicitly initialize it as well:
```
int i = 0, k = 0; 
```
The second error is the statement:
```
j,k++; 
```
This is equivalent to
```
j; k++; 
```
That is, it evaluates the left-hand side of the comma operator (i.e. j) and then throws away the result. Then it evaluates and returns the result of the right-hand side (i.e. k++).

You need to increment both variables:
```
j++; k++; 
```

Or you can use the for loops which are more compact, still as readable, and harder to make such mistakes as in the first function.

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