Question about char arrays

This is a discussion on Question about char arrays within the C++ Programming forums, part of the General Programming Boards category; I've only used strings in my programs but for class I was instructed to use character arrays which I am ...

  1. #1
    Cheesy Poofs! PJYelton's Avatar
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    Question about char arrays

    I've only used strings in my programs but for class I was instructed to use character arrays which I am completely unfamiliar with. Basically I need to create random sentences by pulling verbs, nouns, etc from an array of char arrays. For some reason though, if I pull the last word in the array I get gibberish. Like for example in the line return articles[rand()%5] if the random number is 4, my sentence prints random characters for the word, even though there does exist a word at array location 4. The only way I've been able to solve it is to extend the arrays to size six instead of five. Can someone explain to me why I get gibberish when I try to return the last word in the array? MSVC++ by the way.

    Code:
    #include<cstring>
    #include<iostream>
    #include<ctime>
    
    using namespace std;
    
    void printSentence(void);
    
    char* returnArticle(void);
    char* returnNoun(void);
    char* returnVerb(void);
    char* returnPreposition(void);
    
    
    int main()
    {
    	srand(time(NULL));
    
    	for (int x=0; x<20; x++)
    	{
    		printSentence();
    	}
    
    	return 0;
    }
    
    void printSentence()
    {
    	// create the sentence by pulling random verbs, articles, prepositions, and nouns
    	char sentence[50]="";
    	char period[2]=".";
    	char space[2]=" ";
    	strcat(sentence, returnArticle());
    	strcat(sentence, space);
    	strcat(sentence, returnNoun());
    	strcat(sentence, space);
    	strcat(sentence, returnVerb());
    	strcat(sentence, space);
    	strcat(sentence, returnPreposition());
    	strcat(sentence, space);
    	strcat(sentence, returnArticle());
    	strcat(sentence, space);
    	strcat(sentence, returnNoun());
    	strcat(sentence, period);
    
    	sentence[0]=toupper(sentence[0]);
    
    	cout<<sentence<<endl;
    }
    
    char* returnArticle()
    {
    	char articles[5][5]={"the", "a", "one", "some", "any"};
    	return articles[rand()%5];
    }
    
    char* returnNoun()
    {
    	char nouns[5][5]={"boy", "girl", "dog", "town", "car"};
    	
    	return nouns[rand()%5];
    }
    
    char* returnVerb()
    {
    	char verbs[5][8]={"drove", "jumped", "ran", "walked", "skipped"};
    
    	return verbs[rand()%5];
    }
    char* returnPreposition()
    {
    	char prepositions[5][6]={"to", "from", "over", "under", "on"};
    
    	return prepositions[rand()%5];
    }

  2. #2
    ...
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    you are returning a pointer to a variable that is local to that function. when that function dies, that variable dies with it, but the pointer still points to where the variable used to be.

    the only reason it seems to work most of the time is that most of that array still exists in its normal state after the function dies (memory doesnt get reset after variables die, its just ignored). when its messing up is when something overwrites that area of memory partially, so you get whatever is in the memory pointed at by that pointer, which now looks like gibberish.

    there are a couple ways you can solve it. you can make all of the character arrays in the functions static so they dont die, or you can create the variables outside of the function and pass them in. how you do it is up to you.
    Last edited by ...; 10-20-2003 at 04:02 PM.
    I came up with a cool phrase to put down here, but i forgot it...

  3. #3
    and the hat of wrongness Salem's Avatar
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    Code:
    char *articles[5]={"the", "a", "one", "some", "any"};
    Should fix that problem
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  4. #4
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    youre still returning a pointer to invalid memory. it may work most of the time, but its still going to cause problems occasionally.
    I came up with a cool phrase to put down here, but i forgot it...

  5. #5
    and the hat of wrongness Salem's Avatar
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    return "hello";
    is not returning a pointer to invalid memory. String constants have global scope - the fact that you have an array of them makes no difference.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  6. #6
    the hat of redundancy hat nvoigt's Avatar
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    Though your compiler won't complain if you leave it out, it's really one of the things that might help you when you least expect it:

    char* returnArticle(void);

    should be

    const char* returnArticle(void);

    because you are returning a pointer to a string constant.
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

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