Concatenating two 2d char arrays - c

Question

I wrote a small program to combine two 2d arrays. Here is the code:
#define MAX 7
int main(void) {
int i, j;
char *array1[] = {"Welt,", "bist", "du"};
char *array2[] = {"noch", "zu", "retten?"};
char final[MAX][MAX];
for(i = 0; i < 3; i++) {
// initialize ith names element with first name
strcpy(final[i], array1[i]);
}
for(j = 0; j < 3; j++) {
// concatenate the last name to the firstname+space string
strcat(final[i], array2[j]);
}
for (i = 0; i != 6; i++) {
printf("%s", final[i]);
}
return EXIT_SUCCESS;
}
I get really strange output like:
Welt,bistbistdunochzuretten?uretten?en?
while what I want is this:
Welt,bistdunochzuretten
As you can see it is not completely wrong. There should not be a space between the words.
How can I fix my code?

Answer 1

The problems were that in the second for you were doing strcat(final[3], array2[j]);, because i was 3 at that point and in the final for you were trying to print from final[0] to final[5], when you only had defined final[0] to final[3] (where on final[0] to final[2] you had the names, and in final[3] you had all the last names concatenated which also exceeded the limit of characters), and without printing them in a new line it was hard to tell which string was what.
Try this.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 7
int main(void) {
int i,j;
char *array1[] = {"Welt","bist","du"};
char *array2[] = {"noch","zu","retten?"};
char final[MAX][MAX];
for(i=0;i<3;i++)
strcpy(final[i], array1[i]); //To initialize ith names element with first name
for(j=0;j<3;j++)
strcat(final[j],array2[j]); //Concatanate the last name to the firstname+ space string
for (i = 0; i < 3; i++)
printf("%s\n", final[i]);
return EXIT_SUCCESS;
}

Answer 2

There are several problems with your code:
The constant MAX is not large enough for your data. The string "retten?" contains seven characters plus one terminating byte. As such, MAX must be at least 8, otherwise you get undefined behavior.
Your second loop contains uses the wrong index into final[i]. See point 3. for corrected versions.
The use of strcat() is wrong, you should be using strcpy() just like in the first loop. Together with point 2., your second loop should either look like this:
for(j = 0; j < 3; i++, j++) { //add increment for i
strcpy(final[i], array2[j]);
}
or like this:
for(j = 0; j < 3; j++) {
strcpy(final[3 + j], array2[j]); //derive the index from j
}
Regarding Point 1, I always advise against using any compile time constants like MAX. My experience is that these are just bugs that are waiting to strike. Someday, someone will have a use case that exceeds the limit, and your program goes boom. I always allocate buffers to fit the strings that I need to store, leaving the available RAM as the only limit to my code. To this end, functions like strdup() and asprintf() are extremely handy because they already do the allocation for me.
Regarding Point 2, you should try to declare all your loop variables right inside the initialization statement. Like so:
for(int i = 0; i < 3; i++) {
// initialize ith names element with first name
strcpy(final[i], array1[i]);
}
That way you don't run the danger of inadvertently using the loop variable after the loop / forgetting the initialization, etc. because your compiler will complain about the unknown variable.