Pointer syntax and incrementation - c++

Question

#include <iostream>
#include <cstdlib>
using namespace std;
void reverse(char* str){
char *end = str;
char tmp;
if(str){
cout << "hello" << endl;
while(*end){
cout << end << endl;
++end;
}
--end;
while (str < end){
tmp = *str;
*str++ = *end;
*end-- = tmp;
}
}
}
int main(){
char str[] = "helloyouarefunny";
string input = str;
reverse(str);
for(int i = 0; i < input.length(); i++) {
cout << str[i];
}
}
Is if(str){} equivalent to if(str == NULL){}?
What does while(*end){} mean and what is it exactly doing? I think I have a general understanding that the while loop will continue to be executed as long as it does not "see" a '\0'. But I am not sure what is exactly going on with this line of code.
Given that if(str){} is an equivalent statement to if(str == NULL){}, what would you pass into a function to make str = NULL? For example, in my main(){}, I tried to do char str[] = NULL, thereby, attempting to pass a NULL so that it wouldn't go inside the code if(str == NULL){}. But I get an error saying I cannot make this declaration char str[] = NULL. So my question is why am I getting this error and what can I pass through the reverse() function in order to make the code inside of if(str){} not execute? I hope this question made sense.
And the code ++end is doing pointer arithmetic correct? So every time it is incremented, the address is moving to the address right next to it?
I'm a little confused while(str < end){}. What is the difference between just str and *str? I understand that cout << str << endl; has to do with overloading the operator << and therefore prints the entire string that is passed through the argument. But why, when I cout << *end << endl;, it only prints the character at that memory address? So my question is, what's the difference between the two? Is it just dereferencing when i do *str? I might actually be asking more than that question in this question. I hope I don't confuse you guys >_<.

Answer 1

Is if(str){} equivalent to if(str == NULL){}?
No, if(str){} is equivalent to if(str != NULL){}
What does while(*end){} mean and what is it exactly doing?
Since the type of of end is char*, while(*end){} is equivalent to while (*end != '\0'). The loop is executed for all the characters of the input string. When the end of the string is reached, the loop stops.
Given that if(str){} is an equivalent statement to if(str == NULL){}
That is not correct. I did not read rest of the paragraph since you start out with an incorrect statement.
And the code ++end is doing pointer arithmetic correct? So every time it is incremented, the address is moving to the address right next to it?
Sort of. The value of end is incremented. It points to the next object that it used to point to before the operation.
I'm a little confused while(str < end){}
In the previous while loop, end was incremented starting from str until it reached the end of the string. In this loop, end is decremented until it reaches the start of the string. When end reaches str, the conditional of the while statement evaluates to false and the loop breaks.
Update
Regarding
what would you pass into a function to make str = NULL?
You could simply call
reverse(NULL);
I tried to do char str[] = NULL;
str is an array of characters. It can be initialized using couple of ways:
// This is what you have done.
char str[] = "helloyouarefunny";
// Another, more tedious way:
char str[] = {'h','e','l','l','o','y','o','u','a','r','e','f','u','n','n','y', '\0'};
Notice the presence of an explicitly specified null character in the second method.
You cannot initialize a variable that is of type array of chars to to NULL. The language does not allow that. You can initialize a pointer to NULL but not an array.
char* s1 = NULL; // OK
reverse(s1); // Call the function
s1 = malloc(10); // Allocate memory for the pointer.
strcpy(s1, "test") // Put some content in the allocated memory
reverse(s1); // Call the function, this time with some content.

Answer 2

These are pretty standard C programming idioms.
No, in fact if (str) ... is equivalent to if (str != NULL) ...
C character strings are null terminated, meaning that "Hello" is represented in memory as the character array {'H', 'e', 'l', 'l', 'o', '\0'}. As with pointers, the 0 or NULL value is considered false in a logical expression. Thus while (*end) ... will execute the body of the while loop so long as end has not reached the null character.
N/A
Correct - this advances to the next character in the string, or to the null terminator.
This is the reverse algorithm. After the first loop, end points to one past the end of the string and str points to the beginning. Now we work these two pointers toward each other, swapping characters.

Answer 3

1/2) In C and C++, whatever is in the if or while is evaluated as a boolean. 0 is evaluated to false while any other value is evaluated to true. Given that NULL is equivalent to 0, if(str) and if(str != NULL) do the same things.
Likewise, while(*end) will only loop so long as the value end is pointing to does not evaluate to 0.
3) If you pass a char pointer to this function, it could be the null pointer (char *str = 0), so you're checking to make sure str is not null.
4) Yes, the pointer is then pointing to the next location in memory until eventually you find the null at the end of the string.
5) Perhaps your confusion is based around the fact that the code is missing parenthesis, the loop should look like:
while (str < end){
tmp = *str;
*(str++) = *end;
*(end--) = tmp;
}
So that the two pointers will continue to make there way towards eachother until crossing paths (at which point, str will no longer be less than end)