以下代码在C中做什么？

Question

I saw the following code from somewhere:

while(*i++ = *j++)
{
}

but what is this code doing? what is the meaning of it?

Answer 1

It copies elements from an array (or a pointer to an array) called j to one called i . It does this until it finds a value (from j ) which is equivalent to zero.

This is a common idiom for copying C-style, null-terminated strings; it could also be used to copy an array of integers terminated by a sentinel zero.

In case the size of j can be known in advance, it might be better to use memcpy() . And in case the size of j cannot be known in advance, it is likely the code is unsafe, because the proper size to allocate for i cannot be known either.

Answer 2

Most likely, if i and j are char*, then it copyes null-terminated string j into memory that starts at i. You might wanna keep in mind that i and j itself changes ( i += strlen(j) ) so code above also breaks the pointers to a strings.

Answer 3

It copies the data pointed to by j, to the array pointed to by i, and continues until a value of 0 has been copied. It is perhaps used to copy a null-terminated string. To be even more clever, you can use

while(*i++ = *j++);

Answer 4

*j++ derefrences the pointer, increments its value.

*i++ = *j++ assigns the old value of *j to *i, then *i++ increments this value and saves it for use the next time

while(*i++ = *j++)

is executed.

---

If i and j are char[], then

while(*i++ = *j++)

is copying characters from j[] to i[] until NULL character is reached.

Answer 5

In addition to the other answers, while(*i++ = *j++){} is a less readable, more compact and more dangerous way of writing

*i = *j;

while(*i != 0)
{
  i++;
  j++;
  *i = *j;
}

The two cases will generate exactly the same machine code .

Answer 6

It primarily depends on what i and j are. One case can as be follows.

Assume that i and j are two pointers to a character string, then the value of i and j will get simultaneously increased, and assign the value of *j as *i , whenever the value of j becomes 0 ie, \\0 the loop will exit after assigning that 0 to *i.

Obviously this can be used to copy the content of j to i .