How to Initialize 2D Vector or an object in class

help
,
1

What is the correct way to initialize any object (2D vector in this case)?

Getting compilation error: " no match for ‘operator!=’ "


class Solution {
public:
    
    vector<vector<int>> *dp;
    
    Solution(){
        dp = new vector<vector<int>>(5001,vector<int>(501,-1));
    }

    int f(int c,int indx,const vector<int> &v){
        if(c==0)return 1;
        if(indx>=v.size())return 0;
        if(c<0)return 0;
        if(dp[c][indx]!=-1)return dp[c][indx];//ERROR HERE:  " no match for 'operator!=' "
        int ans=f(c,indx+1,v)+f(c-v[indx],indx,v);
        dp[c][indx]=ans;
        return ans;
    }

    int change(int amount, vector<int>& coins) {
        return f(amount,0,coins);
    }
};
2

Instead of dp[c][indx] use (*dp)[c][indx] or equivalently dp[0][c][indx] because dp is a pointer.

I would also suggest not using a pointer and omitting the new keyword when initializing.

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3 
class Solution {
public:
    
    vector<vector<int>> dp;
    
    Solution(){
        dp.assign(5001,vector<int>(501,-1));
    }

    int f(int c,int indx,const vector<int> &v){
        if(c==0)return 1;
        if(indx>=v.size())return 0;
        if(c<0)return 0;
        if(dp[c][indx]!=-1)return dp[c][indx];//ERROR HERE:  " no match for 'operator!=' "
        int ans=f(c,indx+1,v)+f(c-v[indx],indx,v);
        dp[c][indx]=ans;
        return ans;
    }

    int change(int amount, vector<int>& coins) {
        return f(amount,0,coins);
    }
};
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4

Thanks! That helps.

One more thing:

class Solution {
public:
    //1st one works, 2nd one gives error, why?
    vector<vector<int>> dp = vector<vector<int>>(5001,vector<int>(500,-1));//works
    vector<vector<int>> dp(5001,vector<int>(500,-1));
    //error : "expected identifier before numeric constant vector<vector<int>> dp(5001,vector<int>(500,-1));"

};
5

Is “dp.assign(5001,vector<int>(501,-1));” same as “dp = vector<vector>(5001,vector(500,-1));” ?

6

Why can’t I just assign It from the constructor while declaring?
like this: vector<vector<int>> dp(5001,vector<int>(500,-1));

7

No idea why that happens. Seems to be something with declaring it inside the class. I copy pasted the exact same line inside main and it compiles.

EDIT: I found this.
vector<vector<int>> dp{5001,vector<int>(500,-1)}; works.

1 Like
8

Thanks!
What is the reason for this? Or is it just how the syntax is…?

9

A comment on the same post

1 Like
10

I think both are the same.

1 Like