Use pointer for multidimension array

This is a discussion on Use pointer for multidimension array within the C++ Programming forums, part of the General Programming Boards category; I have learnt how to use pointer for multidimension array. this is my example: Code: int trace [3][4]; int (*ptr)[4] ...

  1. #1
    hqt
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    Question Use pointer for multidimension array

    I have learnt how to use pointer for multidimension array. this is my example:
    Code:
    int trace [3][4];
    int (*ptr)[4] =trace;
    After that,I don't know how to use pointer ptr to process elements of trace array.
    who can teach me, please.

    thanks for tons

  2. #2
    Registered User manasij7479's Avatar
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    >int(*ptr)[4] =trace;
    This... AFAIK... is useless.
    You can already use trace as a pointer.
    "trace[i][j]" gives the required element, so does " *(trace+i*4+j) "

  3. #3
    hqt
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    Yes, I know this way too. there three ways same same like that to touch trace[i][j]:
    Code:
    1) *(*(trace +i)+j)
    2)*(trace[i]+j)
    3)*(trace+i)[j]
    these ways easy to understand why. but the above example, in my book say that, but they don't say how to use this way
    I think will exist a method to use this

    @: manasij: I see that you like to use slang to talk today, I have learnt word: asap: as soon as possible. and now, new word:afaik

  4. #4
    spurious conceit MK27's Avatar
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    afaik = as far as I know. You can find most of these by googling "acronym afaik".

    I agree with manasij7479 that using a pointer here is generally more awkward than using the index, but there are times when a pointer is easier or tidier. So if you need an example of how to walk a matrix that way:

    Code:
    #include <iostream>
    
    using namespace std;
    
    int main(void) {
    	int ray[3][3] = { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9} },
    		(*p1)[3] = ray, *p2 = p1[0];
    
    	while (p1 - ray < 3) {
    		if (p2 - *p1 == 3) {
    			cout << endl;
    			p1++;
    			p2 = p1[0];
    		} else {
    			cout << *p2 << ' ';
    			p2++;
    		}
    	}
    
    	return 0;
    }
    It is very important when doing this that the pointers are correctly typed!

    Some things to notice: p1 is effectively the same type as "ray" (technically, ray is an array while p1 is just a pointer, but ray implicitly converts to a pointer in context). However, p2 is not the same type as p1, and so when doing arithmetic with addresses, you must dereference (via *) p1 to make them equivalent, as on line 10 (just as you must dereference p2 on line 15 to get a value).
    Last edited by MK27; 11-11-2011 at 08:08 AM.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  5. #5
    hqt
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    Question

    Oh, thanks very much to help me solve my problem. I have understand your code. It likes this idea:
    Code:
    int trace[4];
    int ptr*=trace; 
    // So: (ptr+1)=trace[1]; (ptr+2)=trace[2];
    And, I have an other question bulding from above problem:
    I have this code:
    Code:
       int trace[3][4];
       int (*ptr)[3]=trace;
    the complier will notice this error:cannot convert 'int (*)[4]' to 'int (*)[3]' in initialization.
    I know C++ associate demension of array with type of variable, and int(*)[4] and int(*)[3] cannot convert together. But, why this line:
    Code:
     int(*ptr)[3]=trace
    , C++will know type of trace is int(*)[4]. Can you teach me more, please.

    thanks
    Last edited by hqt; 11-11-2011 at 08:36 AM.

  6. #6
    C++ Witch laserlight's Avatar
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    An array is converted to a pointer to its first element. So, when you have an array of 3 arrays of 4 ints, it is converted to a pointer to an array of 4 ints, i.e., int (*)[4]. This is different from, and incompatible with, a pointer to an array of 3 ints.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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