Error in scanf() - c

Question

first of all, I've got a logical error in my code. Well, this is the code
#include <stdio.h>
int main()
{
long i,j,t;
scanf("%ld",&t);
long n[t],d[t][t];
for(i = 0; i < t;i++){
scanf("%ld",&n[i]);
for(j = 0; j < n[i] ;j++){
scanf("%ld",&d[j][i]);
}
}
for(i = 0; i < t;i++){
for(j = 0; j < n[i] ;j++){
printf("%ld ",d[j][i]);
}
printf("\n");
}
return 0;
}
And I input the data
2
4
25 20 30 90
3
45 50 55
And the result is
25 20 30 90
45 50 55
Well, that's what I expected. However, when the input become like this
3
5
12 67 89 34 56
6
34 56 78 90 12 34
7
12 34 89 23 56 78 89
The result become like this
12 34 89 23 56 78 89
12 67 89 34 56 4206692 7 2293472 1982002386 16 3 2293344 2293408 0 2293552 0 0 4
198585 8918456 1982106837 1982010910 8918456 2293640 0 0 1985286516 2009576437 0
0 2293664 2009323341 2293740 2147348480 0
34 56 78 90 12 34 4199405 1982595752 8 12 2293424 2 2 1982356412 2147348480 2293
608 2147348480 1 -1 297753669 1982010784 1982015505 4199044 0 0 2147348480 21473
48480 0 0 0 7273647 2009576392 0 0 0 1 0 20 52 0 0 438759246 736 -214797894 1420
760826203 2272 852421325 3108 944791496 4028 -1322777276 4988 9 1 1 1204 7168 4
2 152 11832 7 1 40 12316 1682469715 1 140 44 0 0 0 2 0 7209065 5701724 6029427
12 34 89 23 56 78 89
Well, the simple question, why the output become like the above?? When I input above 2, the same result will be happened. Any possible answers and links if you don't mind it?? Thanks

Answer 1

You are writing outside your 2D array in many cases, sometimes you don't get errors, but that's just by chance.
You determine the size of the 2D array by the number of arrays to be inputted, but you also determine the size of the inner arrays at the same time:
scanf("%ld",&t);
long n[t],d[t][t];
So for example, let's take the first example:
2 >> create array n[2], and array d[2][2]
4 >> number of values to d[0]
25 20 30 90 >> d[0][1] = 25 d[0][2] = 20 you access d[0][3] and d[0][4] but you are not allowed to do that.
3 >> number of values to d[1]
45 50 55 >> d[1][0] = 45 d[1][1] = 50 you access d[1][2] but you are not allowed to do that

Answer 2

You build a matrix with size t*t, then fill in rows with more or less elements.
If you fill a row with too few elements, the rest remain uninitialized, and you get strange numbers. It's OK in your case, because you don't print these elements.
If you fill a row with too many elements, the excess overlaps into the next row. It may also exceed the whole matrix and corrupt your stack.
I guess this is what's going on - your n array is overrun, and your code goes crazy.

Answer 3

I believe that you can use malloc.
#include <stdio.h>
#include <stdlib.h>
int main()
{
long i,j,t;
printf("Rows : ");
scanf("%ld",&t);
long *n;
long **d;
n = (long* )malloc(sizeof(long) * t); // add malloc
d = (long** )malloc(sizeof(long *) * t); // add malloc
for(i = 0; i < t;i++){
printf("Column : ");
scanf("%ld",&n[i]);
d[i] = (long* )malloc(sizeof(long) * n[i]); //add malloc
if(d[i] == NULL)
printf("ERROR\n");
for(j = 0; j < n[i] ;j++){
scanf("%ld", &d[i][j]); // change from &d[j][i]
}
}
printf("\n\n");
for(i = 0; i < t;i++){
for(j = 0; j < n[i] ;j++){
printf("%ld ",d[i][j]); // change from d[j][i]
}
printf("\n");
}
return 0;
}
Well, the simple question, why the output become like the above?? When
I input above 2, the same result will be happened. Any possible
answers and links if you don't mind it?? Thanks
because you allocate less memory than used.
scanf("%ld", &d[j][i]); you have to exchange the "i" and "j".