Suppose we have char *a ="Mission Impossible";
If we give cout<<*(a+1), then the output is i.
Is there any way to change this value, or this is not possible?
4 Answers
Yes, there are several ways to do this, but you have to make a copy of the string first because if you didn't, you'd be modifying memory you're not allowed to (where string literals are stored).
const char* a = "Mission Impossible"; // const char*, not char*, because we can't // modify its contents char buf[80] = {}; // create an array of chars 80 large, all initialised to 0 strncpy(buf, a, 79); // copy up to 79 characters from a to buf cout << *(buf + 1); // prints i buf[1] = 'b'; cout << *(buf + 1); // prints b *(buf + 1) = 't'; cout << buf[1]; // prints t That said, if this exercise is not for learning purposes, it is highly recommended that you learn and use std::string rather than C-style strings. They are superior in almost every way and will result in far less frustration and errors in your code.
Comments
char a[] = "Mission Impossible"; a[1] = 'x'; String literals cannot be modified. Typically they are placed a section of the binary that will be mapped read-only, therefore writing to them generates a fault. (This is implementation-defined behavior, but this happens to be the most common implementation these days.)
By declaring the string as a character array it is writable. The other alternative would be to duplicate the string literal into heap memory, either through malloc, new, or std::string.
Comments
No, the char* a is actually read-only and if you try to modify the content you will get undefined behavior. You should ideally declare a as const char*.
Comments
The simplest way to change that is doing *(a+1)='value_you_want';
This will change the content of a pointer (your case pointer is a+1) to the value you set.
5 Comments
fge
I suppose you meant
*(a+1)?
Theocharis K.
@fge isn't & supposed to modify the content of a pointer? If no then this is what I meant :)
Ernest Friedman-Hill
This will work if the data is definitely mutable; if it might be a string constant like this, it's not a good idea, though.
ruakh
Even if you correct the
& to * as fge says, that's not legal code; you're not allowed to alter characters from character literals. It will work on some platforms and compilers, but not on others -- and it can have drastically bad effects.Theocharis K.
Thanks for the correction guys. I didn't know about that until now.
const char* a = "mission possible"What do you expect the output to be? what do you want to change?