How to create an array of strings when there isn't a fixed length of items or characters. I'm new to pointers and c in general and I couldn't understand the other solutions posted on here so my solution is posted below. Hopefully it helps someone else out.
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Are you looking for a jagged array (array of arrays of different length), or X*Y fixed size array? And are you looking for 2-dimensional array of strings(==pointers to char), or are you looking for array of pointers to arrays of strings(==pointers to char), or are you looking for 3-dimensional array of char with no pointers in the array?hyde– hyde2012-10-28 13:11:47 +00:00Commented Oct 28, 2012 at 13:11
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i was looking for a way to store strings in an array and access them like string1 = array[1] etc when i didn't know the number of strings nor their length. is that pointers to character pointers?Cadell Christo– Cadell Christo2012-10-28 13:15:27 +00:00Commented Oct 28, 2012 at 13:15
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Ok, so you are actually looking for a single array of strings (=char pointers). Thinking of it as a 2D array is probably not helpful (even if it is true from a certain point of view).hyde– hyde2012-10-28 13:22:09 +00:00Commented Oct 28, 2012 at 13:22
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The OP's wanting a 1d-array of pointers to character. So it might be a good idea to adjust the title of the question accordingly.alk– alk2012-10-28 15:26:07 +00:00Commented Oct 28, 2012 at 15:26
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3 Answers
char **twod_array = NULL;
void allocate_2darray(char ***source, int number_of_slots, int length_of_each_slot)
{
int i = 0;
source = malloc(sizeof(char *) * number_of_slots);
if(source == NULL) { perror("Memory full!"); exit(EXIT_FAILURE);}
for(i = 0; i < no_of_slots; i++){
source[i] = malloc(sizeof(char) * length_of_each_slot);
if(source[i] == NULL) { perror("Memory full!"); exit(EXIT_FAILURE);}
}
}
// sample program
int main(void) {
allocate_2darray(&twod_array, 10, 250); /*allocate 10 arrays of 250 characters each*/
return 0;
}
4 Comments
Geoff Montee
Your
source pointer will be lost when the allocate_2darray function returns. You should change the argument type to char*** and pass it as &twod_array. Also, since you are allocating uniformly sized arrays, this solution wouldn't be very good if the lengths of each string can vary widely.
Aniket Inge
you have realloc for varying lengths. I don't understand the source pointer being lost, you're passing in the pointer.
Geoff Montee
yes, you are passing in the pointer by value. The original pointer is not being modified.
twod_array will be NULL after the function is returned. You need to pass a pointer to the pointer itself, hence char***. I know, it is confusing.Geoff Montee
see the code that I posted in my answer. That is the kind of thing that I am talking about.
Simply makes an array from the argv items bar the first item.
char **dirs = NULL;
int count = 0;
for(int i=1; i<argc; i++)
{
int arraySize = (count+1)*sizeof(char*);
dirs = realloc(dirs,arraySize);
if(dirs==NULL){
fprintf(stderr,"Realloc unsuccessful");
exit(EXIT_FAILURE);
}
int stringSize = strlen(argv[i])+1;
dirs[count] = malloc(stringSize);
if(dirs[count]==NULL){
fprintf(stderr,"Malloc unsuccessful");
exit(EXIT_FAILURE);
}
strcpy(dirs[count], argv[i]);
count++;
}
2 Comments
Geoff Montee
You should change
int arraySize = (count+1)*sizeof(dirs); to int arraySize = (count+1)*sizeof(char*);. Semantically, your code is a bit incorrect, since dirs is a char** but you are allocating char*. In practical terms, it shouldn't make a difference either way, since sizeof(char**) == sizeof(char*).
Cadell Christo
you're right i've edited it in it makes more sense that way because dirs is an array of pointers
Yours is close, but you are allocating the main array too many times.
char **dirs = NULL;
int count = 0;
dirs = malloc(sizeof(char*) * (argc - 1));
if(dirs==NULL){
fprintf(stderr,"Char* malloc unsuccessful");
exit(EXIT_FAILURE);
}
for(int i=1; i<argc; i++)
{
int stringSize = strlen(argv[i])+1;
dirs[count] = malloc(stringSize);
if(dirs[count]==NULL){
fprintf(stderr,"Char malloc unsuccessful");
exit(EXIT_FAILURE);
}
strcpy(dirs[count], argv[i]);
count++;
}
1 Comment
Cadell Christo
yes this would be more appropriate for the argv case i was really just using argv as an example to show how to do it if we didn't know argc