What is the difference between these two?

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C++

*returnColumnSizes=malloc(sizeof(int)*numRows);
 *(*returnColumnSizes+i)=i+1;// This statement is in loop

and

C++

*returnColumnSizes = (int*)malloc(sizeof(int)*numRows);
*(returnColumnSizes)[i]=i+1;//This statement is also in the loop same as first one

What I have tried:

There is so much confusion, I was unable to find the difference in that.
What is returnColumnSizes here?

Posted 23-Jun-21 3:49am

_-_-_-me

Updated 23-Jun-21 3:56am

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CPallini 23-Jun-21 10:04am

"What is returnColumnSizes here"
What you have declared it to be.

_-_-_-me 23-Jun-21 10:16am

Here returnColumnSizes is a double pointer .
It is like this:
int ** returnColumnSizes;

_-_-_-me 24-Jun-21 8:36am

*returnColumnSizes = (int**)malloc(sizeof(int*) * numRows);
*(*returnColumnSizes+i)=i+1;// This statement is in loop

and

*returnColumnSizes = (int**)malloc(sizeof(int*) * numRows);
*(returnColumnSizes)[i]=i+1;//This statement is also in the loop same as first one

What is the difference between these two?
Thank you !

1 solution

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Richard MacCutchan · Answer 1 · 2021-06-23T03:56:00

Solution 1

I am guessing that returnColumnSizes is a pointer to an array of int types, based on the parameter passed to malloc. Not sure what the second lines are supposed to be doing, other than storing some value somewhere.

Posted 23-Jun-21 3:56am

Richard MacCutchan

Comments

_-_-_-me 23-Jun-21 10:19am

Thank you!
Here it is declared like this:
The declaration is this,
int **returnColumnSizes;

Richard MacCutchan 23-Jun-21 10:32am

To be honest I find more than one level of indirection too much for my tiny brain. There are simpler ways of creating and using arrays in C.

Richard MacCutchan 23-Jun-21 10:39am

Then I think it should really be:

*returnColumnSizes = (int**)malloc(sizeof(int*) * numRows); // see how spaces make things clearer?

But as I say, multiple levels of indirection - phooey!

jeron1 23-Jun-21 13:35pm

I'd have to agree.

_-_-_-me 24-Jun-21 0:04am

*returnColumnSizes = (int**)malloc(sizeof(int*) * numRows);
*(*returnColumnSizes+i)=i+1;// This statement is in loop

and

*returnColumnSizes = (int**)malloc(sizeof(int*) * numRows);
*(returnColumnSizes)[i]=i+1;//This statement is also in the loop same as first one

What is the difference between these two?
Thank you !

Richard MacCutchan 24-Jun-21 4:01am

No idea, and I am not going to try and understand them. But they are (IMHO) examples of bad programming, since it is difficult to figure out what you are trying to do.

_-_-_-me 24-Jun-21 8:38am

Okay :(