How to initialize a char*[] array?

Question

int main(){
int n;
cin>>n;
char *str[40];
str=new char [10000][40]; //I don't know how to initialize this array
std::map<char*,int> system;
int i,j;
//solving Question 4C @ Codeforces
for(i=0;i<n;i++){
    cin>>str[i];
}
for(i=0;i<n;i++){
    int count=0;
    for(std::map<char*,int>::iterator iter=system.begin();iter!=system.end();iter++){
        if(strcmp(iter->first,str[i])==0){
            count=++iter->second;
        }
        else if(strcmp(str[i],iter->first)>0){
            break;
        }
    }
    if(count==0){
        system[str[i]]=1;
        cout<<"OK"<<endl;
    }
    else{
        char *strint;
        strint=new char[5];
        strint=convert(count);
        strcat(str[i],strint);
        cout<<str[i]<<endl;
    }
}

}

Question: http://codeforces.com/problemset/problem/4/C

I am looking for a way that I could put the char array as an element of the map to help solving the scenario, however, I ended up with either screwing up the map.insert function when I declare str[n][40] as str, system as , (Could somebody please explain why map.insert(str[i],1) won't work in this case? It would be sweet to clarify the concepts) or in this case, fails to initialize the char*str[40] array.

I wonder what is the legit way to do this?

so you don't like std::string then ;-) ? You code currently leaks like a colander. — Bathsheba
– Bathsheba, Commented May 19, 2016 at 11:05
Using pointers as keys in a std::map is usually not what you want to do. Why don't you use std::vector and std::string? There's no need to program like it's 1969 any more. — molbdnilo
– molbdnilo, Commented May 19, 2016 at 11:17
I am used to char[] as my school secondary school taught me C, and I have just recently transferred to C++. Perhaps I have overlooked the benefits of using string and vector. :P — haleyk
– haleyk, Commented May 19, 2016 at 12:05

Sam Varshavchik · Accepted Answer · 2016-05-19 11:09:52Z

4

char *str[40];
str=new char [10000][40]; //I don't know how to initialize this array

str is an array. You can assign something to values in an array, but arrays themselves cannot be assigned to. You are probably trying to do:

char *str[40];

for (size_t i=0; i<40; ++i)
   str[i]=new char [10000];

Still, in modern C++ one rarely needs to do new and delete in the first place; instead modern C++ code uses containers. Speaking of containers:

std::map<char*,int> system;

This is not going to lead to anything good. A std::map literally compares key by their values. So:

char *foo1=new char[40];

strcpy(foo1, "foo");

system[foo1]=0;

char *foo2=new char[40];

strcpy(foo2, "foo");

auto iter=system.find(foo2);

Do you think that find() here, will find this element? Of course not, because it's a different pointer value.

Although maps that use raw pointers as keys are valid C++, it's not very obvious how to use them correctly.

In conclusion, you should spend more time studying and learning how to use various containers. Here, std::vector should be used instead of futzing around with new and delete. And std::string should be used instead of char * as the map key. You'll be surprisd to learn that the end result will be smaller and easier to read, and understand.

answered May 19, 2016 at 11:09

Sam Varshavchik

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1 Comment

haleyk Over a year ago

Thanks for the reply, I shall look further into the string container.

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