Dealing with array bounds.

Revision en2, by EugeneJudo, 2021-09-19 19:27:56

There are many methods of ensuring that you don't write beyond your arrays bounds, i'm wondering which others have found to be best.

For instance, I just went through a simple problem which was solvable via recursion, and handled bounds like this:

bool inBounds(vector<vector<int>> &G,int i,int j){
return (i >= 0) && (j >= 0) && (i < G.size()) && (j < G[0].size());
}

void fill(vector<vector<int>> &G,int i,int j){
if(!inBounds(G,i,j) || !G[i][j]) return;
G[i][j] = 0;
fill(G,i-1,j); fill(G,i+1,j);
fill(G,i,j-1); fill(G,i,j+1);
}


Instead of this, I could have handled bounds like this:

void fill(vector<vector<int>> &G,int i,int j){
if(!G[i][j]) return;
G[i][j] = 0;
if(i-1 >= 0) fill(G,i-1,j);
if(i+1 < G.size()) fill(G,i+1,j);
if(j-1 >= 0) fill(G,i,j-1);
if(j+1 < G[0].size()) fill(G,i,j+1);
}


but the second approach seems more susceptible to bugs! i.e. it relies on picking the right bound to check, instead of blanket checking every one of them.

There's also a third approach I sometimes use, which here would be to modify G in the following way

void preprocess(vector<vector<int>> &G){
G.insert(G.begin(),vector<int>(G[0].size()+2,0));
for(int i=1;i<G.size();i++){
G[i].insert(G[i].begin(),0);
G[i].push_back(0);
}
G.push_back(vector<int>(G[0].size(),0));
}

void fill(vector<vector<int>> &G,int i,int j){
if(!G[i][j]) return;
G[i][j] = 0;
fill(G,i-1,j); fill(G,i+1,j);
fill(G,i,j-1); fill(G,i,j+1);
}


Then no bounds checking is needed! As long as we never start at an edge, since our preprocessing adds a boundary of 0's around G, and part of the problem already involved terminating when we hit a 0.

Are there other method/tricks for making this process easier? It's usually not that big of a consideration, but when a bug appears because of it, it can sometimes lead to 30 minutes of fruitless debugging. How do you handle this?

#### History

Revisions

Rev. Lang. By When Δ Comment
en2 EugeneJudo 2021-09-19 19:27:56 4 Tiny change: 'ause of it can so' -> 'ause of it, it can so'
en1 EugeneJudo 2021-09-19 19:24:45 2060 Initial revision (published)