如何声明一个空char *并动态增加大小？

Question

Let's say I am trying to do the following (this is a sub problem of what I am trying to achieve):

int compareFirstWord(char* sentence, char* compareWord){
      char* temp; int i=-1;
      while(*(sentence+(++i))!=' ') { *(temp+i) = *(sentence+i); }
      return strcmp(temp, compareWord); }

When I ran compareFirstWord("Hi There", "Hi"); , I got error at the copy line. It said I was using temp uninitialized. Then I used char* temp = new char[]; In this case the function returned 1 and not 0. When I debugged, I saw temp starting with some random characters of length 16 and strcmp fails because of this.

Is there a way to declare an empty char* and increase the size dynamically only to length and contents of what I need ? Any way to make the function work ? I don't want to use std::string .

Answer 1

In C, you may do:

int compareFirstWord(const char* sentence, const char* compareWord)
{
    while (*compareWord != '\0' && *sentence == *compareWord) {
        ++sentence;
        ++compareWord;
    }
    if (*compareWord == '\0' && (*sentence == '\0' || *sentence == ' ')) {
        return 0;
    }
    return *sentence < *compareWord ? -1 : 1;
}

With std::string , you just have:

int compareFirstWord(const std::string& sentence, const std::string& compareWord)
{
    return sentence.compare(0, sentence.find(" "), compareWord);
}

Answer 2

temp is an uninitialized variable.

It looks like you are attempting to extract the first word out of the sentence in your loop.

In order to do it this way, you would first have to initialize temp to be at least as long as your sentence.

Also, your sentence may not have a space in it. (What about period, \\t, \\r, \\n? Do these matter?)

In addition, you must terminate temp with a null character.

You could try:

int len = strlen(sentence);
char* temp = new char[len + 1];
int i = 0;

while(i < len && *(sentence+(i))!=' ') {
   *(temp+i) = *(sentence+i); 
   i++;
}
*(temp+i) = '\0';
int comparable = strcmp(temp, compareWord);
delete temp;
return comparable;

Also consider using isspace(*(sentence+(i))), which will at least catch all whitespace. In general, however, I'd use a library, or STL... Why reinvent the wheel...