Questions on C strings

Question

I am new to C and I am very much confused with the C strings. Following are my questions.

Finding last character from a string

How can I find out the last character from a string? I came with something like,

char *str = "hello";
printf("%c", str[strlen(str) - 1]);
return 0;

Is this the way to go? I somehow think that, this is not the correct way because strlen has to iterate over the characters to get the length. So this operation will have a O(n) complexity.

Converting char to char*

I have a string and need to append a char to it. How can i do that? strcat accepts only char*. I tried the following,

char delimiter = ',';
char text[6];
strcpy(text, "hello");
strcat(text, delimiter);

Using strcat with variables that has local scope

Please consider the following code,

void foo(char *output)
{
   char *delimiter = ',';
   strcpy(output, "hello");
   strcat(output, delimiter);
}

In the above code,delimiter is a local variable which gets destroyed after foo returned. Is it OK to append it to variable output?

How strcat handles null terminating character?

If I am concatenating two null terminated strings, will strcat append two null terminating characters to the resultant string?

Is there a good beginner level article which explains how strings work in C and how can I perform the usual string manipulations?

Any help would be great!

Answer 1

1. Last character: your approach is correct. If you will need to do this a lot on large strings, your data structure containing strings should store lengths with them. If not, it doesn't matter that it's O(n).

2. Appending a character: you have several bugs. For one thing, your buffer is too small to hold another character. As for how to call strcat, you can either put the character in a string (an array with 2 entries, the second being 0), or you can just manually use the length to write the character to the end.

3. Your worry about 2 nul terminators is unfounded. While it occupies memory contiguous with the string and is necessary, the nul byte at the end is NOT "part of the string" in the sense of length, etc. It's purely a marker of the end. strcat will overwrite the old nul and put a new one at the very end, after the concatenated string. Again, you need to make sure your buffer is large enough before you call strcat!

Answer 2

1. O(n) is the best you can do, because of the way C strings work.
2. char delimiter[] = ",";. This makes delimiter a character array holding a comma and a NUL Also, text needs to have length 7. hello is 5, then you have the comma, and a NUL.
3. If you define delimiter correctly, that's fine (as is, you're assigning a character to a pointer, which is wrong). The contents of output won't depend on delimiter later on.
4. It will overwrite the first NUL.

You're on the right track. I highly recommend you read K&R C 2nd Edition. It will help you with strings, pointers, and more. And don't forget man pages and documentation. They will answer questions like the one on strcat quite clearly. Two good sites are The Open Group and cplusplus.com.