Using structures with pointers in C with an array

Question

I have the following code that produces a warning stating "warning: initialization makes pointer from integer without a cast".

If I change the datatype *eleNum to char then it compiles cleanly. What am I doing wrong to cause the compilation to complain about *eleNum being an integer?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct ptElements {
    int  *eleNum;
    char *eleSym;
    char *eleNme;
    char *elePrd;
    char *eleGrp;
} Elements;

Elements database[] = { 
    { 1, "H",  "Hydrogen", "1", "1"  },  
    { 2, "He", "Helium",   "1", "18" },
    { 3, "Li", "Lithium",  "2", "1" },
    { 4, "Be", "Berylium", "2", "2" },
    { 5, "B",  "Boron",    "2", "13" }
};

int main() {
    int     i;  
    int     limit;

    limit = sizeof(database) / sizeof(Elements);

    for (i = 0; i < limit; i++)
        printf("%3s - %-3s %-20s %2s %2s\n", database[i].eleNum, database[i].eleSym, database[i].eleNme, database[i].elePrd, database[i].eleGrp);

    return 0;
}

struct-point-02.c:14:7: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
     { 1, "H",  "Hydrogen", "1", "1"  },
       ^
struct-point-02.c:14:7: note: (near initialization for ‘database[0].eleNum’)
struct-point-02.c:15:7: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
     { 2, "He", "Helium",   "1", "18" },
       ^
struct-point-02.c:15:7: note: (near initialization for ‘database[1].eleNum’)
struct-point-02.c:16:7: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
     { 3, "Li", "Lithium",  "2", "1" },
       ^
struct-point-02.c:16:7: note: (near initialization for ‘database[2].eleNum’)
struct-point-02.c:17:7: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
     { 4, "Be", "Berylium", "2", "2" },
       ^
struct-point-02.c:17:7: note: (near initialization for ‘database[3].eleNum’)
struct-point-02.c:18:7: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
     { 5, "B",  "Boron",    "2", "13" }
       ^
struct-point-02.c:18:7: note: (near initialization for ‘database[4].eleNum’)

Just use an integer when you should? int eleNum, it's an integer, not a pointer! — Matthieu Brucher
– Matthieu Brucher, Commented Nov 4, 2018 at 23:15

Jonathan Leffler · Accepted Answer · 2018-11-05 00:41:43Z

Main answer

Either use int eleNum in the structure, or use char *eleNum and change the initializer to use "1" for Hydrogen.

You currently print eleNum with %s — that's completely wrong (printing integer pointer as if it was a character pointer). Use %3d if eleNum becomes an int; use %3s if it becomes a char *.

If it remains an int *, you'll need to provide something like (int []){ 1 } (a C99 or later compound literal) as the initializer, and *database[i].eleNum or database[i].eleNum[0] in the printf() statement to print it, and use %3d.

#include <stdio.h>

typedef struct ptElements
{
    int   eleNum;
    char *eleSym;
    char *eleNme;
    char *elePrd;
    char *eleGrp;
} Elements;

Elements database[] =
{ 
    { 1, "H",  "Hydrogen", "1",  "1" },  
    { 2, "He", "Helium",   "1", "18" },
    { 3, "Li", "Lithium",  "2",  "1" },
    { 4, "Be", "Berylium", "2",  "2" },
    { 5, "B",  "Boron",    "2", "13" }
};

int main(void)
{
    int limit = sizeof(database) / sizeof(Elements);

    for (int i = 0; i < limit; i++)
    {
        printf("%3d - %-3s %-20s %2s %2s\n", database[i].eleNum,
               database[i].eleSym, database[i].eleNme, database[i].elePrd,
               database[i].eleGrp);
    }
    return 0;
}

Tangential information about the Periodic Table

I note that all the elements now have one-letter or two-letter symbols; the 3-letter symbols like Uub have all been replaced now with 'real' names. (See WebElements as one source of information — there are undoubtedly many others, such as Wikipedia Periodic Table of Elements.)

Back in 2010, I had an SQL table of elements with entries (the N indicates 'not stable' or 'radioactive'):

INSERT INTO elements VALUES(112, 'Uub', 'Ununbium',      277.0000, 'N');
INSERT INTO elements VALUES(113, 'Uut', 'Ununtrium',     284.0000, 'N');
INSERT INTO elements VALUES(114, 'Uuq', 'Ununquadium',   289.0000, 'N');
INSERT INTO elements VALUES(115, 'Uup', 'Ununpentium',   288.0000, 'N');
INSERT INTO elements VALUES(116, 'Uuh', 'Ununhexium',    293.0000, 'N');
INSERT INTO elements VALUES(118, 'Uuo', 'Ununoctium',    294.0000, 'N');

The first names for these were were added in mid-2010. By 2016, I had data equivalent to this (I added the period and group information in between the dates):

INSERT INTO elements VALUES(112, 'Cn',  'Copernicium',   285.1800, 7, '12', 'N');
INSERT INTO elements VALUES(113, 'Nh',  'Nihonium',      286.1800, 7, '13', 'N');
INSERT INTO elements VALUES(114, 'Fl',  'Flerovium',     289.1900, 7, '14', 'N');   -- Yes, Fl and Mc have the same weight
INSERT INTO elements VALUES(115, 'Mc',  'Moscovium',     289.1900, 7, '15', 'N');
INSERT INTO elements VALUES(116, 'Lv',  'Livermorium',   293.2000, 7, '16', 'N');
INSERT INTO elements VALUES(117, 'Ts',  'Tennessine',    293.2100, 7, '17', 'N');
INSERT INTO elements VALUES(118, 'Og',  'Oganesson',     294.2100, 7, '18', 'N');

It intrigues me to have lived through the 'completion' of the periodic table, at least for the first seven periods of it.

(You can find information about a hypothetical Ununennium (element 119, Uue) and Unbinilium (element 120, Ubn) on Wikipedia, if you're curious. There's also Beyond element 118: The next row of the Periodic Table.)

Jonathan, many thanks for your solution(s) they worked perfectly. Thank you for taking the time to respond it is greatly appreciated.

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