Thread: const char* clarification

Aha, this clears it up! Thank you for insight

Thanks, Salem. I didn't know assigning character arrays to pointers was possible. What I've always done was use strcpy to assign them.

Originally Posted by catacombs

Thanks, Salem. I didn't know assigning character arrays to pointers was possible. What I've always done was use strcpy to assign them.

i thought it was me being thick i didnt want to take over someone elses post though or appear to be arguing with salems explanation.

im sure in the past i have had a char *name[10] and when i tried to write name[0] = "thomas" it wouldn't let me something about assignment to an array

Originally Posted by cooper1200

im sure in the past i have had a char *name[10] and when i tried to write name[0] = "thomas" it wouldn't let me something about assignment to an array

I'm guessing you had char name[10]; and then tried to do name = "thomas";, which isn't allowed because name is an array, and you can't assign to an array.

nope definatly had *name in the end i had to use malloc and then strcpy it across

cant find the exact code but it was something along the lines of this with my first attempt at linked lists which i abandoned under your advice of keeping it simple with one data type.

Code:

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

typedef struct PERSON
{
    int age;
    char *name[20]
}PERSON;

int main()
{
    PERSON first_record;

    first_record.age = 34;
    first_record.name = "greg";
    return 0;
}

this produces the errors as follows

Code:

||=== Build: Debug in array of pointer to strings (compiler: GNU GCC Compiler) ===|
/home/ben/Documents/coding/array of pointer to strings/main.c|8|warning: no semicolon at end of struct or union|
/home/ben/Documents/coding/array of pointer to strings/main.c||In function ‘main’:|
/home/ben/Documents/coding/array of pointer to strings/main.c|15|error: assignment to expression with array type|
/home/ben/Documents/coding/array of pointer to strings/main.c|12|warning: variable ‘first_record’ set but not used [-Wunused-but-set-variable]|
||=== Build failed: 1 error(s), 2 warning(s) (0 minute(s), 0 second(s)) ===|

updated code to get rid of the warnings

Code:

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

typedef struct PERSON
{
    int age;
    char *name[20];
}PERSON;

int main()
{
    PERSON first_record;

    first_record.age = 34;
    first_record.name = "greg";
    printf("persons age is %d\n", first_record.age);
    return 0;
}

found the code where i had to use strcpy etc

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
typedef struct Address
{
    int door_number;
    char *street;
    char *city;
    struct Address *next;
}Address;
 
void add_address(Address *first, Address *current);
void print_list(Address *first);
void delete_list(Address *first);
 
int main()
{
    Address *first = NULL, *current = NULL;
 
    add_address(first, current);
    add_address(first, current);
    add_address(first, current);
    add_address(first, current);
    add_address(first, current);
 
    print_list(first);
    delete_list(first);
 
    return 0;
}
void add_address(Address *first, Address *current)
{
    char my_str[20];
    Address *p_new_element;
 
    //get a new block of memmory and test the pointer isnt null
    p_new_element = malloc(sizeof(Address));
    if (!p_new_element)
    {
        printf("memmory allocation error");
        return;
    }
    //get door number and assign it to the new elements member 'door_number'
    printf("Please enter the door number: ");
    scanf(" %d", &p_new_element->door_number);
 
    //get street name and assign it to my_str
    printf("Please enter street name: ");
    scanf(" %s", my_str);
    //create memory for the street name and assign the pointer address->street to point to it
    p_new_element->street = malloc(strlen(my_str) + 1);
    //copy contents of my_str into the memmory location pointed to by address->street
    strcpy(p_new_element->street, my_str);
 
    //get city name
    printf("Please enter city name: ");
    scanf(" %s", my_str);
    //create memory for the city name and assign the pointer address->city to point to it
    p_new_element->city = malloc(strlen(my_str) + 1);
    //copy contents of my_str into the memmory location pointed to by address->city
    strcpy(p_new_element->city, my_str);
 
    if (first == NULL) // no list yet so make p_new_element the head
    {
        first = p_new_element;
        first->next = NULL;
        current = first;
        return;
    }
    if (first->next == NULL) //only one element in the list
    {
        first->next = p_new_element;
        current = p_new_element;
        current->next = NULL;
        return;
    }
    //more than 2 elements
    current->next = p_new_element;
    current = p_new_element;
    current->next = NULL;
}
 
void print_list(Address *first)
{
    int count = 1;
    Address *temp = first;
 
    while (temp)
    {
        printf("address no %d is door number %d street %s city %s\n", count, temp->door_number,
                                                                       temp->street, temp->city);
        count++;
        temp = temp->next;
    }
}
 
void delete_list(Address *first)
{
    int count = 1;
    Address *temp = NULL;
 
    printf("freeing memmory....\n");
    while (first)
    {
        temp = first->next; // set temp to point at the next element in the list
        printf("freeing element %d's street name\n", count);
        free(first->street); //free the memmory block that holds firsts street name
        printf("freeing element %d's city name\n", count);
        free(first->city); //free the memmory block that holds firsts city name
        printf("freeing struct %d's memmory\n", count);
        free(first); //free the memmory block that holds the first element
        first = temp; // set first to point at the next memmory block;
        count++;
    }
    printf("finished freeing all allocated memmory\n");
}

here is the link to the entire thread error subscripted value is neither an array nor pointer nor vector

Originally Posted by cooper1200

updated code to get rid of the warnings

Code:

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

typedef struct PERSON
{
    int age;
    char *name[20];
}PERSON;

int main()
{
    PERSON first_record;

    first_record.age = 34;
    first_record.name = "greg";
    printf("persons age is %d\n", first_record.age);
    return 0;
}

You declare name (in struct PERSON) to be an array of 20 pointers to char. From your usage of name, that doesn't seem to be the correct type.

and as i said to catacombs whom im sure you will agree understands more and is a better programmer than me i thought i was being thick but he mentioned he always used strcpy hence my post about needing to use strcpy in the above example

incidently isn't p_new_element->city a pointer of type char as p is in the original post so by that reasoning surely i could of just said p_new_element->city = my_str;