Recently I had an Interview in C. The interviewer has asked me to explain
how to access particular element in 2D array using double pointer. I gave the answer as *(*(a+i)+j), where a is a double pointer, i is number of rows and j is number of columns. Later he asked me to explain using an example. I am confused at *(a+i) as it gives value instead of address and adding to j gives some junk value.
Can anyone please explain.
Add a comment
|
2 Answers
Remember that in a 1D array a[i] equals *(a+i). And there are no 2D arrays in C, just arrays of arrays.
So a[i][j] is actually equal to *(*(a+i)+j).
If the type of a is int**, then the type of (a+i) is still int**, you need to dereference it. Type of *(a+i) is int* and the type of *(*(a+i)+j) is int.
About the interview question, no matter that a is a double pointer, you should still use the [] notation. The alternative is too cumbersome:
int **a = ...;
int x = a[i][j];
2 Comments
user694733
"there are no 2D arrays in C" is somewhat incorrect. Double pointer
int **a; can be used as 2D array with array of arrays approach, but int a[N][M]; is true 2D array.
rodrigo
@user694733: Well, that's a matter of definition.
int a[N][M] can be seen as a 2D array or as an array of N values of type int[M], that is an array of arrays. Other languages that have true 2D arrays would use a[i,j] instead of a[i][j].The a[i][j] element looks like
a[i][j]=*(*(a+i)+j)
So it is a double pointer because it first determines the row and then the column, as you can see the 2d matrix looks like
----- _________________
|___| ---> |___|____|____|___|
|___| ---> |___|____|____|___|
|___| ---> |___|____|____|___|
|___| ---> |___|____|____|___|
actually you can see that a one dimensional which holds the address of another one dimensional matrix. So the first vertical matrix data type is int** because it holds the address another one dimensional which is int*
