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POBinary searching a 2D array to find all the indexes of the key
 

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  1. POBinary searching a 2D array to find all the indexes of the key
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    <p>I am trying to binary search a 2D array which will give me the position of every key in the array, my code is</p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; using namespace std; vector&lt;pair&lt;int,int&gt; &gt; up; vector&lt;pair&lt;int,int&gt; &gt; down; void binsearchd(int **A,int row,int low,int high,int key) { int mid=(low+high)/2; if(low==high&amp;&amp;A[row][mid]!=key)//!(A[row][mid]&lt;=key&amp;&amp;A[row][mid-1]&lt;=key&amp;&amp;A[row-1][mid] &lt;=key&amp;&amp;A[row+1][mid]&gt;key&amp;&amp;A[row][mid+1]&gt;key)) return; if(A[row][mid]==key)//&lt;=key&amp;&amp;A[row][mid-1]&lt;=key&amp;&amp;A[row-1][mid]&lt;=key&amp;&amp;A[row+1][mid]&gt;key&amp;&amp;A[row][mid+1]&gt;key) { down.push_back({row,mid}); return; } if(A[row][mid]&gt;key) binsearchd(A,row,low,mid,key); else if(A[row][mid]&lt;key) binsearchd(A,row,mid+1,high,key); else { binsearchd(A,row,low,mid,key); binsearchd(A,row,mid+1,high,key); } return; } void searchd(int **A,int lowi,int lowj,int highi,int highj,int key) { if(lowi==highi) { binsearchd(A,lowi,lowj,highj,key); return; } int midi=(lowi+highi)/2,midj=(lowj+highj)/2; if(A[midi][midj]==key)//&lt;=key&amp;&amp;A[midi][midj-1]&lt;=key&amp;&amp;A[midi-1][midj]&lt;=key&amp;&amp;A[midi+1][midj]&gt;key&amp;&amp;A[midi][midj+1]&gt;key) down.push_back({midi,midj}); if(A[midi][midj]&gt;key) { searchd(A,lowi,lowj,midi,highj,key); searchd(A,midi+1,lowj,highi,midj,key); } else if(A[midi][midj]&lt;key) { searchd(A,midi+1,lowj,highi,highj,key); searchd(A,lowi,midj+1,midi+1,highj,key); } else { searchd(A,lowi,lowj,midi,highj,key); searchd(A,midi+1,lowj,highi,highj,key); binsearchd(A,midi,lowj,highj,key); } return; } int main() { int row,col; while(cin&gt;&gt;row&gt;&gt;col&amp;&amp;row){ int **A=new int*[row]; for(int i=0;i&lt;row;++i) { A[i]=new int[col]; for(int j=0;j&lt;col;++j) cin&gt;&gt;A[i][j]; } int query; cin&gt;&gt;query; while(query--) { int d; cin&gt;&gt;d; //searchu(A,1,1,row+1,col+1,u); searchd(A,0,0,row,col,d); int max=-999999999; //for(vector&lt;pair&lt;int,int&gt; &gt;::iterator itr=up.begin();itr!=up.end();++itr) //{ for(vector&lt;pair&lt;int,int&gt; &gt;::iterator itr2=down.begin();itr2!=down.end();++itr2) { cout&lt;&lt;itr2-&gt;first&lt;&lt;" "&lt;&lt;itr2-&gt;second&lt;&lt;endl; } up.resize(0); down.resize(0); } for(int i=0;i&lt;row+1;++i) delete A[i]; delete A; } } </code></pre> <p>the problem is it gives segmentation fault on array</p> <pre><code>4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 </code></pre> <p>key=4 and also in most of the time, I tried debugging found the reason of seg fault was binary searching a row 4, which is greater than the no. of row.. but I cant find out how that row can appear because my code divides the row in exactly 2 halves each time and thats how it shoudnt be able to cross the boundary.. Any hint??</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USPaul R
    1. USTamim Addari
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
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      1. PTAnswer
    1. VO
      singulars
      1. POBinary searching a 2D array to find all the indexes of the key
      1. This table or related slice is empty.
      1. VTUpMod
    1. COHave you tried checking your arithmetic to make sure it's all sound? You could do this with debugging or printing your index variables.
      singulars
      1. POBinary searching a 2D array to find all the indexes of the key
      1. USGWW
    2. COI do not think it is a problem here but you should have `mid=low+(high-low)/2` instead of `mid=(low+high)/2` to avoid overflows
      singulars
      1. POBinary searching a 2D array to find all the indexes of the key
      1. USAman Deep Gautam
    3. COThere must be a wrong .. But I have tried for 2 days all night and day.. but couldnt find that..
      singulars
      1. POBinary searching a 2D array to find all the indexes of the key
      1. USTamim Addari
 

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