Accessing a 2D array in C but no column index

Question

if i have a 2D array

int A[3][2] = { {3,4}, {5,6}, {0,-5} }

what will be the result if I try to print the element A[1]? will it be garbage value? or will it become a logical error?

Technically C doesn't have multi-dimensional array. What you have with A is an array of arrays of int. So A[1] is an array (of two int values). If the compiler will complain or if it's a logical error depends on the context, what you do with the array. — Some programmer dude
– Some programmer dude, Commented Nov 26, 2021 at 13:13
so if I try to print the value of A[1], what would be the result of it? garbage? — Terry Tan
– Terry Tan, Commented Nov 26, 2021 at 13:19
@Someprogrammerdude, technically C does not have negative numbers but only unary minus followed by numerical literal — tstanisl
– tstanisl, Commented Nov 26, 2021 at 13:19
@TerryTan, it depends on how you try to print it. There are ways that would produce perfectly valid, well-defined results, and ways that would produce undefined behavior. — John Bollinger
– John Bollinger, Commented Nov 26, 2021 at 13:20
Note also that the program producing undefined behavior does not necessarily mean it would yield output that you would recognize as "garbage". — John Bollinger
– John Bollinger, Commented Nov 26, 2021 at 13:22

Some programmer dude · Accepted Answer · 2021-11-26 13:23:05Z

1

Assuming you do something like

printf("%d\n", A[1]);

then that will be undefined behavior. That's because the array A[1] will decay to a pointer to its first element (i.e. in that context A[1] will decay to &A[1][0]), and the %d format specifier is wrong for printing pointers.

However

printf("%p\n", (void *) A[1]);

will be perfectly fine, as %p is the correct format specifier to print void * pointers.

And as noted in a comment, just because you have undefined behavior that doesn't mean the program will crash or result it "garbage" (even though the (possibly truncated) pointer might look like that when printed as a decimal integer).

answered Nov 26, 2021 at 13:23

Some programmer dude

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