4 replies to this topic

[AdvMutant]

Hey.
Recently, on my way back home from school, I was thinking general thoughts about programming. One thing I was wondering is how to create a string array in C. Since a strings in C look like the following -

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char *str1;//Pointer
char str2[];//Pointer
char str3[256];//Array

The solution came up naturally - a two dimensional array.

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char *strArray[5];//5 strings in an array

I managed it to figure out how to use such arrays, but it's still confusing me - How do multi-dimensional arrays work? If I write "char [50] [2]", does the compiler declare 100 characters? Because the array arithmetic doesn't apply here. If I write the following code -

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char *str[5];//5 strings in an array
char *p_str, *p_str2;

str[0] = "This is string number 1.";
str[1] = "This is string number 2.";

p_str = str[0]; p_str2 = str[0] + strlen(str[0]) + 1;
puts(p_str);
puts(++p_str);
puts(p_str2);

This is the output -

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This is string number 1.
his is string number 1.
This is string number 2.

Why does the program output "his is string number 1." instead of "This is string number 2."?

[Ancient Dragon]

char* str[5] declares 5 pointers, not 5 strings. Each of those pointers can be set to point to random memory locations which may or may not be contiguous. In the example you posted, the two strings are not contiguous, so its not possible to get to the second string from the memory address of the first. The only reason that seemed to work in your example is by pure chance, not because it will always work like that.

puts(++p_str) skipped the first character because that's what you told it to do -- increment the pointer then display the contents of the string from that point to the end of the string.

>>If I write "char [50] [2]", does the compiler declare 100 characters?
Yes, but if you intend to have 2 strings with room for 50 character each you would declare it as char str[2][50];

[AdvMutant]

Now I'm even more confused. Yes, char *str[5] declares 5 pointers.
If I haven't initialized the pointers to point a variable, and gave them a value, they point to a random location?
I forgot that pointers aren't like variables, so an array of pointers can still be contiguous. But what if I want them to? Should I use the malloc() function?

[Ancient Dragon]

In an array of pointers, the pointers themself are in contiguous memory but not the items they point to. The only way to insure that a 2d array of strings are in contiguous memory is to declare them like this: char sarray[5][100]; which declares an array of 5 strings, each can hold a maximum of 100 characters including the NULL terminator.

[AdvMutant]

So, the only way I can make sure that the items pointer arrays point to are contiguous, is pointing them to an array?