Simple asci drawing algorithm

**heatblazer** · 09-16-2016

Hello, it`s not exactly C/C++ related even though written in C/C++. It`s an algo to draw a triangle but I am missing one draw iterration.

Code:

static void set_char_at(char* mat, int cols, int x, int y, char c)
{
    mat[x*cols+y] = c;
}

int  test3()
{
    int n;
    cin >> n;
    char* mat = new char[(n*n)]; // newlines
    for(int i=0; i < n*n; i++) {
        mat[i] = ' ';
    }


    for(int i=0; i <= n; i++) {
        for(int j=0; j < i; j++) {
            set_char_at(mat, n, i, -j, '*');
        }
        set_char_at(mat, n, i, 0, '\n');
    }

    cout << mat;

}

This outputs the desired triangle, but not the last line, which shall be long equally to the cin input (5) let`s say.

**Salem** · 09-16-2016

I would check your subscripting, because whenever I see 0 and <= in a loop, I'm thinking buffer overruns.

Valgrind thinks so as well.

Code:

$ g++ -g main.cpp
$ valgrind ./a.out 
==11395== Memcheck, a memory error detector
==11395== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==11395== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==11395== Command: ./a.out
==11395== 
5
==11395== Invalid write of size 1
==11395==    at 0x4008E9: set_char_at(char*, int, int, int, char) (main.cpp:5)
==11395==    by 0x400994: main (main.cpp:20)
==11395==  Address 0x5ab60d9 is 0 bytes after a block of size 25 alloc'd
==11395==    at 0x4C2E80F: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11395==    by 0x400928: main (main.cpp:12)
==11395== 
==11395== Invalid write of size 1
==11395==    at 0x4008E9: set_char_at(char*, int, int, int, char) (main.cpp:5)
==11395==    by 0x4009B7: main (main.cpp:22)
==11395==  Address 0x5ab60d9 is 0 bytes after a block of size 25 alloc'd
==11395==    at 0x4C2E80F: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11395==    by 0x400928: main (main.cpp:12)
==11395== 
==11395== Invalid read of size 1
==11395==    at 0x4C30F74: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11395==    by 0x4F49228: std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==11395==    by 0x4009CE: main (main.cpp:25)
==11395==  Address 0x5ab60d9 is 0 bytes after a block of size 25 alloc'd
==11395==    at 0x4C2E80F: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11395==    by 0x400928: main (main.cpp:12)
==11395== 

    
   *
  **
 ***
==11395== Invalid read of size 1
==11395==    at 0x52375D2: _IO_default_xsputn (genops.c:455)
==11395==    by 0x52351F6: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1352)
==11395==    by 0x522A6FA: fwrite (iofwrite.c:39)
==11395==    by 0x4F48EC5: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==11395==    by 0x4F49236: std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==11395==    by 0x4009CE: main (main.cpp:25)
==11395==  Address 0x5ab60d9 is 0 bytes after a block of size 25 alloc'd
==11395==    at 0x4C2E80F: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11395==    by 0x400928: main (main.cpp:12)
==11395== 
****
==11395== 
==11395== HEAP SUMMARY:
==11395==     in use at exit: 72,729 bytes in 2 blocks
==11395==   total heap usage: 4 allocs, 2 frees, 74,777 bytes allocated
==11395== 
==11395== LEAK SUMMARY:
==11395==    definitely lost: 25 bytes in 1 blocks
==11395==    indirectly lost: 0 bytes in 0 blocks
==11395==      possibly lost: 0 bytes in 0 blocks
==11395==    still reachable: 72,704 bytes in 1 blocks
==11395==         suppressed: 0 bytes in 0 blocks
==11395== Rerun with --leak-check=full to see details of leaked memory
==11395== 
==11395== For counts of detected and suppressed errors, rerun with: -v
==11395== ERROR SUMMARY: 5 errors from 4 contexts (suppressed: 0 from 0)

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