Pointers Homework

[littleprogrammer]

Hello ! I started to learn to work with pointers today. I wanna make some exercices, but I don't know what should they sound.


I write, I experiment things and I like how they work. But I want to test my knowlage with some homework.

Can you provide me some exercices ? :)

[helios]

1. Write a function that returns the size of the string pointed to by a passed pointer. Assume the pointer is valid. the prototype for the function is
int strlen(const char *);
Remember that strings are null-terminated. Don't count the null character in the returned size.

2. What's wrong with this code?

char *string="hello";
string[0]='A';

3. What's the value of x at the end of this code? Warning: a compiler's output is not a reliable to confirm the answer.

const int a=15;
int *b=(int *)&a;
*b=20;
int x=a;

[littleprogrammer]

2. The bug is that string points to something which doesn't exist in memory. It's the same with

int *pointer = 1;

3. x is 15

I'll make the first exercise tonight. Are 2 and 3 right ?

[helios]

Both answers are wrong.

[Disch]

Warning: a compiler's output is not a reliable to confirm the answer.

Doesn't that, by definition, mean the answer you're thinking of is wrong?

Way to throw curveballs in the beginner's forum XD

EDIT: nevermind -- I know what answer you're looking for now. Haw.

[helios]

When you want to test corner cases, you don't test under normal conditions.

[Zhuge]

What is that code all about? And your answers make little sense.

[helios]

Oh, God! I can see forever!

http://web.mit.edu/tarvizo/Public/fractal-wrongness.jpg

[olredixsis]

@helios
could you post the correct answer tomorrow? I want to see if my answers are all correct :)

[SimpleButPerfect]

1.

int strlen(const char * str)
{
    int strlength = 0;
    while(str[strlength] != '\0')
           strlength++;
    return strlength;
}

3. x = 20

edit: not sure of 2.. still thinking about it but what i could get off the top of my head is, probably a char * acts like a string literal so it is constant? I am still a c++ beginner, not used to those char * yet. :D

[helios]

1. Can you do it without using the offset operator?

3. Still wrong.

[SimpleButPerfect]

1.

int strlen(const char * str)
{
     int strlength = 0;
     while(*(str + strlength) != '\0')
             strlength++;
     return strlength;
}

Is number 2 correct then? =o

3. I really can't find out, first you specify a constant "a" = 15, then you create a pointer to it "b" and then you dereference "b" and change its value which as far as i know changes the value of "a" then you assign "a" to the value of "x", if I am wrong please correct me :)

[helios]

1.

*(str + strlength)

That's in the definition of "offset operator".

3. The correct answer can be found in these fora.

[SimpleButPerfect]

1. My last shot, I can not think of another function now..

int strlen(const char * str)
{
int length = strcspn(str, "\0");
return length;
}

3. I will start searching, thanks for giving such an example :)

[helios]

strcspn

Now, now, don't be silly. If I was to allow that, you may as well return strlen(p);.

[SimpleButPerfect]

Oops i went off the scope..
Well, then I can't.

[helios]

Consider this:

int strlen(const char *s){
    char *p=s;
    for (;*p;p++);
    return p-s;
}

[SimpleButPerfect]

Wow, i actually did something like this but it didn't work for some reason, now i found out :D

int strlen(const char* str)
{
	int len = 0;
	char * szSt = const_cast<char*>(str);
	while(szSt != "\0")
	{
		szSt = (szSt + 1);
		len++;
	}
	return len;
}

It should have been

int strlen(const char* str)
{
	int len = 0;
	char * szSt = const_cast<char*>(str);
	while(*szSt)
	{
		szSt = (szSt + 1);
		len++;
	}
	return len;
}

[helios]

Oops. Line 2 above should be const char *, not just char *.

[firedraco]

分かりにくいなぁ！