Array of Structures:No matching function to call to and taking user input

Question

I have written a code for Knapsack problem.The code is working perfectly when i hardcode the inputs but now i want to take the inputs from the user in the array of structures but it is showing error.How can i achieve this? original code:

#include<iostream>
#include<algorithm>
using namespace std;
struct Item{
    int value;
    int weight;
    Item(int value,int weight):value(value),weight(weight){

    }
};

bool cmp(struct Item a,struct Item b){
    float r1=(float)a.value/a.weight;
    float r2=(float)b.value/b.weight;
    //cout<<r1<<" "<<r2<<endl;
    return r1>r2;
}

void knapsack(int m,int n,struct Item arr[]){
    sort(arr,arr+n,cmp);
   /* for(int i=0;i<n;i++){ //new sorted array
        cout<<arr[i].value<<" "<<arr[i].weight<<" ";
    }*/
    cout<<endl;
    float result[n];
    for(int i=0;i<n;i++){
        result[i]=0.0;
    }
    int rem=m;
    int j=0;
    for(j=0;j<n;j++){
        if(arr[j].weight>rem){
            break;
        }
        else{
            result[j]=1;
            rem=rem-arr[j].weight;
        }
    }
    if(j<n){
        result[j]=(float)rem/arr[j].weight;
    }
    for(int k=0;k<n;k++){
        cout<<result[k]<<" ";
    }
}

int main(){
    struct Item arr[]={{25,18},{24,15},{15,10}};
    knapsack(20,3,arr);
    return 0;
}

Now i want to take user input in the array of structures but it shows an error "no matching function to call to"

#include<iostream>
#include<algorithm>
using namespace std;
struct Item{
    int value;
    int weight;
    Item(int value,int weight):value(value),weight(weight){

    }
};

bool cmp(struct Item a,struct Item b){
    float r1=(float)a.value/a.weight;
    float r2=(float)b.value/b.weight;
    //cout<<r1<<" "<<r2<<endl;
    return r1>r2;
}

void knapsack(int m,int n,struct Item arr[]){
    sort(arr,arr+n,cmp);
   /* for(int i=0;i<n;i++){ //new sorted array
        cout<<arr[i].value<<" "<<arr[i].weight<<" ";
    }*/
    cout<<endl;
    float result[n];
    for(int i=0;i<n;i++){
        result[i]=0.0;
    }
    int rem=m;
    int j=0;
    for(j=0;j<n;j++){
        if(arr[j].weight>rem){
            break;
        }
        else{
            result[j]=1;
            rem=rem-arr[j].weight;
        }
    }
    if(j<n){
        result[j]=(float)rem/arr[j].weight;
    }
    for(int k=0;k<n;k++){
        cout<<result[k]<<" ";
    }
}

int main(){
    struct Item arr[10];
    int n,m;
    cin>>m>>n;
    for(int i=0;i<n;i++){
        cin>>arr[i].value>>arr[i].weight;
    }
    knapsack(m,n,arr);
    return 0;
}

Answer 1

You have provided parameterized constructor Item(int value,int weight), so compiler won't generate default constructor by itself.

You have to define the default constructor explicitly to make the below statement work since it needs default constructor.

struct Item arr[10]; 

But the below line of code uses the parameterized constructor you have provided.

struct Item arr[]={{25,18},{24,15},{15,10}};

Answer 2

Check below code:

struct Item{
    int value;
    int weight;
    Item(int value,int weight):value(value),weight(weight){

    }
};

There is no default constructor. So the below line fails:

struct Item arr[10];

But the following line works:

struct Item arr[]={{25,18},{24,15},{15,10}};

Solution^[1]: Provide a default constructor or remove the explicit constructor.

---

^{[1] There may be more problems in the code, but I looked into the one you pointed in your question.}