Vector with char Arrays as Elements

December 7, 2023 3 minute read

How to Store char arrays in a Vector

Frequently I need to handle several strings separated by line breaks as single testcase. For example, consider inputs as follows:

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//strings with maximum length of 50 are given:
apple
banana
strawberry

Let’s say that I want to get each string with cin and store into a vector, but use of string library is forbidden. What could I do to do this?

After some trials I searched for vector with char array as element. There was a StackOverFlow post, dating back to 2010.

        
//Question - wanna do following but not working. What can I do?

char test [] = { 'a', 'b', 'c', 'd', 'e' };
vector<char[]> v;
v.push_back(test);

which I thought was the very thing I wanted to do. I was under impression that it would work for me by simply modifying test [] to {'apple', 'banana', 'strawberry'}.

When I struck by below answer with the most upvotes.

You cannot store arrays in vectors (or in any other standard library container). The things that standard library containers store must be copyable and assignable, and arrays are neither of these.

vector<char []> Cannot Be Declared

Many similar posts and question/answers read that having a vector to store arrays is not possible, but I tried anyway.

The very first approach was like this. Considering I normally define an char array like below,

        
      
char word[] = "apple";

to define vector with elements with word[] form, I defined a vector.

        
      
std::vector<char[]> words;

and then, I used cin to receive individual input strings and then pushed into my words vector.

        
      
for (int i=0; i<3; ++i) {
    char word[] = {0};
    cin >> word;
    words.push_back(word);
}

But this did not work. Actually, I couldn’t even define words vector due to compile error. It is technically forbidden to define vector with arrays as its elements.

Declare and Handle vector<char*>

Possible workaround was to define vector<char*>. By this, I define a vector which stores pointers pointing each char array I want to store.

First, it technically works to declare a vector like this.

        
      
std::vector<char*> words;

Now I can push_back pointers pointing to apple, banana, and strawberry.

        
      
#define MAXLEN 50
for (int i=0; i<3; ++i) {
    char *word = new char [MAXLEN];
    cin >> word;
    words.push_back(word);
}

I could check this concept worked well by pulling out elements one by one.

        
      
#define MAXLEN 50
for (int i=0; i<3; ++i) {
    cout << words[i] << "\n";
}

// execution result
apple
banana
strawberry

Use of new keyword is important. Actually, below what I tried for the first time than above solution.

        
      
#define MAXLEN 50
for (int i=0; i<3; ++i) {
    char word[MAXLEN] = {0};
    cin >> word;
    words.push_back(word);
}

Above code outputs strawberry three times, maybe because cin >> word just works to change the target string pointed by word pointer. As push_back above simply inserted same pointer into the vector, and all the elements in words vector might have been all the same. As strawberry was the last string, word pointer was pointing strawberry when the for loop ended, and all three elements in words must have been pointing this single string.

        
      
#define MAXLEN 50
for (int i=0; i<3; ++i) {
    cout << words[i] << "\n";
}

// execution result
strawberry
strawberry
strawberry

As a conclusion, if you want to declare a vector to handle strings but you cannot use string library, you can define vector<char*> and then store pointers pointing to each of the strings as elements of this vector.

Using `std::string`

This way is recommended in most posts / online Q&A threads.

        
      
char word = "word";
std::vector<std::string> v;
v.push_back(word);

To use string class, include <string> header first.

        
      
#include <string>

Remind different usage of cin and getline: cin receives an input until the stream hits a whitespace. getline receives an input until the stream its \n. Getline can be used in conjunction with cin like getline(cin, str). Used with getline, input is not broken by whitespace and whole incoming line of characters are inserted to string str.

How to Declare `string`s

You can declare new string by some different ways.

Declare empty string
```
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std::string a;
```

Declare with initialization by using raw string literal

        
std::string a = "apple";
std::string b("banana");

Declare by copying other string

        
// if b was "apple", a declared as follows is also apple
std::string a(b);

Declare by referring char array

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char a = {'a','p','p','l','e'};
std::string b(a); // b is apple

Use of new keyword

        
      
std::string a = new std:string("apple");

Member Functions

a is a string object.

a.at(idx): Returns idx-th letter with out-of-range check.
a.at[idx]: Returns idx-th letter without out-of-range check.
a.front(): Returns the very first letter of string.
a.back(): Returns the last letter of string.
a.length(): Returns the length of a given string.
a.size(): Returns the length of a given string.
a.max_size(): Returns the maximum length of string reachable.
a.capacity(): Returns memory size of given string.
a.resize(n): Resize given string into length of n. Append 0 to lenghten and remove tail-side elements to shorten.
a.resize(n, 'a'): Same with .resize(n) but appended letter is changed from 0 to ‘a’.
a.append(b): Concatenate string a with string b.
a.append(b, n, m): Same with above but appended entity is changed to substring b[n,n+m]
a.clear(): Remove all characters stored in a
a.erase(): Remove all characters stored in a
a.erase(n,m): Remove substring a[n,n+m] from a
a.push_back('b'): Append character at the end of string a
a.pop_back(): Remove last character from string a
a.find(b): Try to find substring b from string a. If found, the lowest idx which satisfies a[idx] == b[idx] is returned.
a.find(b, n): Same with a.find(b) but source string is a substring of a starting from index n.
a.find_first_of('b'): Return smallest idx such that a[idx] = ‘b’.
a.find_last_of('b'): Return largest idx such that a[idx] = ‘b’.
to_string(num): Return a string converted from a number num.
to_stoi(str): Return an number converted from a string str.

Comparison Operation

Two strings can be compared each other using comparision operators (＜, ＞, ≤, ≥, ==, !=). Strings are compared based on their order in dictionary. The earlier a string comes in dictionary, the smaller it is considered in comparison operation.

        
string a = "apple";
string b = "banana";

cout << (a < b); // True
cout << (a > b); // False

References

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