Copying and comparing individual element from 2D array to another array in C

Question

char first_array[5][4] = {"aaa","bbb","ccc","ddd","eee"};
char second_array[1][4];

How would I copy, for example, the third element in first_array ("ccc") and save it to second_array?

The syntax below is clearly wrong, but this is what I'm asking for:

second_array[0] = first_array[2];

Also, after copying, I also want to know how to compare elements in the two arrays. Again, the syntax below might be wrong, I'm just explaining what I'm trying to do:

if(second_array[0] == first_array[2]){ printf("yes"); } //should print yes

strncpy(second_array[0], first_array[2], 4); or perhaps memcpy()? — Cyclonecode
– Cyclonecode, Commented Mar 13, 2015 at 23:14

Iharob Al Asimi · Accepted Answer · 2015-03-13 23:20:35Z

2

You can't assign to arrays in c, you can fill arrays with some library functions like strcpy(), so

second_array[0] = first_array[2];

would be

strcpy(second_array[0], first_array[2]);

you must however ensure that the destination array fits the number of characters you are copying to it.

If you try to compare two strings in c, you can't do it through the == operator, because strings in c are arrays of char which contain a sequence of non-nul characters followed by a nul character, so if you write this

if (second_array[0] == first_array[2])

even when you succeeded at copying the data, the result will be most likely false, because you are not comparing the contents of the arrays, but their addresses, so to compare them correctly there is also a function strcmp() then the correct way of comparing the strings is

if (strcmp(second_array[0], first_array[2]) == 0)

The functions above require you to include the string.h header, and also that the passed strings are strings in the c sense, i.e what I described above.

answered Mar 13, 2015 at 23:20

Iharob Al Asimi

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2 Comments

01001000 01101001 Over a year ago

Great! strncpy and strcmp is exactly what I need

M.M Over a year ago

@0100100001101001 be careful with strncpy as it does not generate a null-terminated string in some cases. Instead check the buffer sizes and use strcpy/memcpy; or use snprintf.

baum · Accepted Answer · 2015-03-14 00:06:31Z

-1

I was recently trying to do this, as well: it is not possible to do this sort of direct assignment in C.

When you write first_array[0], the compiler will read that as an address which points to the first element (character) of first_array[2], not the entire string. When you run the assignment, if it were to work, it would only set the first character.

The easiest way is to use strncpy or memcpy (or a loop to cycle through the string.

edited Mar 14, 2015 at 0:06

answered Mar 13, 2015 at 23:14

baum

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6 Comments

Iharob Al Asimi Over a year ago

Did you ask this on SO?

Cyclonecode Over a year ago

Why would a loop be necessary?

baum Over a year ago

We have a 2D array; this direct assignment isn't possible. A loop is necessary if you use strong copy.

Cyclonecode Over a year ago

@baum - Not sure what you mean by strong copy, however a loop is not required since you only need to use strcpy or strncpy to get the desired result.

Iharob Al Asimi Over a year ago

Curious, why Unfortunately?

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