3

I was trying to make the connection between python string and C character array assignment to string literals. For example:

char* word="Hello";
        word="Now";

assigns to the character String "Hello" placed in read only memory location. And now a reassignment of word to "Now" means a now the character array is assigned a memory location corresponding to "Now".

In python even numbers (and obviously strings) seem to work like that with a being assigned a memory location with value 2 and then reassigned a memory location with value 3. 

 a=2
 a=3

This contrasts with C where for almost all variable assignments the variable contains the value it is assigned. Am I making a good comparison here?

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2 Answers 2

Reset to default
3

In C, char * is the type "pointer to character." A pointer is like a "handle" that gives you access to the value itself. When you assign word = "Now";, you are changing the handle, not the characters of the string. Both "Hello" and "Now" still exist as groups of bytes in constant storage.

Python hides more of what it's doing, but internally its built-in string references essentially act like pointers. So your observation is correct up to a point.

The big difference in strings of these languages is that in Python all strings are immutable. C lets you manipulate (by assignment) characters within a (non-constant) string. For example:

char word [] = "Hello";
// word is now the name of a 6-character (including final null) mutable string
word[0] = 'h';
// Now word has the value "hello".

In Python, you would have to create a completely new string:

word = 'h' + word[1:]

Here the bytes that make up the new string are different than those of word.

You are right that Python has "reference semantics". Everything behaves as though you are manipulating a handle rather than a value.

>>> a=[1]
>>> b=a
>>> a
[1]
>>> b
[1]
>>> a[0]=5
>>> a
[5]
>>> b
[5]

Here the assignment b = a copied a handle or reference to the list. The list itself is not copied. This is evident because changing the first element of a also changed b.

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5 Comments

I don't see any changes in your output of a and b. They both print [1]. Can you clarify what you mean by change?
@Joe Baltimore: I fixed the example. I think Gene had a typo. It is a fine example tho. +1
@JoeBaltimore sorry I made and editing error. Thanks for fixing it.
Still if I use char* word="Hello" instead of char word[]="Hello", it becomes immutable like python where I cannot modify anything.
In all ways that matter this is correct. Literal strings are defined constant by the language. Note though that compilers don't have to put literal strings in read-only storage, so word[0] = 'h' might still work. But it's not correct C.
0

char * means word is a pointer variable. The address of the string is placed in the variable, the characters in the string don't move. For C all assignments move a value into the variable, not almost all. In this case an address is the value being moved.

Python is a little more complex. It binds the variable to the value which may cause other operations to occur behind the scenes, like freeing the old value.

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1 Comment

Yes, but still it is special because an immutable string is created separately in the memory and then the address is given to char* (not char var[] which makes it possible to change the string contents) variable using the same statement. It is not the case for assignment of other types like int or even char* when it is not assigned to a string.

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