Is it possible to index arrays using strings or characters? [duplicate]

Not in c it's not possible, only integers are allowed.

Associative containers can be built with a simple struct like

struct double_map_item {
    char *key;
    double value;
};

take a look at bsearch()'s manual page example for a simple method to find items through their key values.

Of course, this is a very simplistic possible implementation, for a more complete and robust implementation you should read about hash tables.

Regarding this comment, the type of a character constant in c is int and so

array['x'] = value;

is valid, but note that this is not really useful because,

1. You can only use a single character.
2. You will probably use an array with a size larger than needed.
3. You only have a very limited set of possible indices.

NOTE: following your own comment, there is a chance that you saw c++ code. In c++ you can overload the [] operator so that it takes const char * as a parameter and then use a hash table or any other method to find the matching element for the given key.

But in c, it is not possible.

If one were to implement such a container in c++, the following example illustrate how to proceed, please note that this is just for illustration of the underlying concept, you should use std::map instead,

#include <iostream>
#include <map>

class Array {
public:
    Array();
    double operator[](const char *const key);
    void insert(const char *const key, double value);

private:
    std::map<const char *, double> m_items;
};

Array::Array()
{
}

double
Array::operator[](const char *const key)
{
    return m_items[key];
}

void
Array::insert(const char *const key, double value)
{
    m_items.insert(std::pair<const char *, double>(key, value));
}

int
main(void)
{
    Array items;

    items.insert("SSN_1", 2.0);
    items.insert("SSN_2", 3.0);
    items.insert("SSN_3", 2.0);

    std::cout << items["SSN_1"] << std::endl;
    return 0;
}