How can mixed data types (int, float, char, etc) be stored in an array?

You can make the array elements a discriminated union, aka tagged union.

struct {
    enum { is_int, is_float, is_char } type;
    union {
        int ival;
        float fval;
        char cval;
    } val;
} my_array[10];

The type member is used to hold the choice of which member of the union is should be used for each array element. So if you want to store an int in the first element, you would do:

my_array[0].type = is_int;
my_array[0].val.ival = 3;

When you want to access an element of the array, you must first check the type, then use the corresponding member of the union. A switch statement is useful:

switch (my_array[n].type) {
case is_int:
    // Do stuff for integer, using my_array[n].ival
    break;
case is_float:
    // Do stuff for float, using my_array[n].fval
    break;
case is_char:
    // Do stuff for char, using my_array[n].cvar
    break;
default:
    // Report an error, this shouldn't happen
}

It's left up to the programmer to ensure that the type member always corresponds to the last value stored in the union.

Use a union:

union {
    int ival;
    float fval;
    void *pval;
} array[10];

You will have to keep track of the type of each element, though.