Struct 2d array of strings problem

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Hello guys.I have a problem and I do not know the reason 100%.
I give a parameter in main function(for example: ./code 3 ) ,so I make an array of struct of 3 people (struct contains first , last name and age of each human).
I want to store information to a 2d array ,for example:

Nick     my_last_name     22
George   his_last_name    20
Anna     her_last_name    18

but I receive an error(maybe I don't receive space for array[i][j]).I don't know.
So, I want your help.

Thanks in advance.

What I have tried:

C++

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

#define N 3


struct person
{
    char *first_name;
    char *last_name;
    int age;
};


int main(int argc , char **argv)
{
    int number = atoi(argv[1]);
    struct person *human = (struct person *)malloc(sizeof(struct person) * number);

    if(human == NULL)
    {
        perror("ERROR 1");
        exit(1);
    }
    char **array = (char **)malloc(sizeof(char *) * number);
    if(array == NULL)
    {
        perror("ERROR 2");
        exit(1);
    }

    for(int i = 0; i < number; i++)
    {   
        array[i] = (char *)malloc(sizeof(char) * N);
        human[i].first_name = (char *)malloc(sizeof(char) * 30);
        if(human[i].first_name == NULL)
        {
            perror("ERROR 3");
            exit(1);
        }
        printf("Give first name of human #%d" , i+1);
        scanf("%s" , human[i].first_name);

        human[i].last_name = (char *)malloc(sizeof(char) * 30);
        if(human[i].last_name == NULL)
        {
            perror("ERROR 4");
            exit(1);
        }
        printf("Give last name of human #%d" , i+1);
        scanf("%s" , human[i].last_name);


        printf("Give age of human #%d" , i+1);
        scanf("%d" , &human[i].age);

 
    }
    for(int i = 0; i < number; i++)
    {
        for(int j = 0; i < 30; j++)
        {
            
            if(j == 0)
            {
                strcpy(&array[i][j] , human[i].first_name);
            }
            if(j == 1)
            {
                strcpy(&array[i][j] , human[i].last_name);
            }
            if(j == 2)
            {
                sprintf(&array[i][j] , "%d" , human[i].age);    // int to str
            }
        }
    }
    
    for(int i = 0; i < number; i++)
    {
        for(int j = 0; i < N; j++)
        {
            printf("%s\t" , array[i][j]);
        }
        printf("\n");
    }

    for(int i = 0; i < number; i++)
    {
        free(array[i]);
    }
    free(array);
    return 0;
}

Posted 18-Jan-21 10:15am

Nick_is_asking

Updated 18-Jan-21 20:52pm

CPallini

v4

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Comments

Patrice T 18-Jan-21 17:32pm

And you plan to tell the error message ?
And input ?

David Crow 18-Jan-21 21:55pm

In the second and third for(int i...) loops, the inner for(int j...) loop is terminating on some i value. Is that intentional?

If you know that first_name and last_name are going to be 30 characters, why not make them so rather than dynamically allocate the memory? It would make the code that much easier to read, maintain, and fix.

2 solutions

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OriginalGriff · Answer 1 · 2021-01-18T11:12:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
That, together with the error message you are getting should help you at least"home in" on where you have a problem.

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

CPallini · Answer 2 · 2021-01-18T20:52:00

Your code doesn't handle properly the monodimensional nature of the array pointer.
Try

C

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

#define N 3

struct person
{
    char *first_name; // field '0'
    char *last_name; // field '1'
    int age;         // field '2'
};

// this maps the pair {item, field} into the linear space of 'array'
#define INDEX(item, field)  (item) * N + (field)

int main(int argc , char **argv)
{
    if ( argc < 2)
      exit(1);

    int number = atoi(argv[1]);
    if (number < 1 || number > 1024)
      exit(1);

    struct person *human = (struct person *)malloc(sizeof(struct person) * number);

    if(human == NULL)
    {
        perror("ERROR 1");
        exit(1);
    }
    char **array = (char **)malloc(sizeof(char *) * (number * N)); // you need N fields per item
    if(array == NULL)
    {
        perror("ERROR 2");
        exit(1);
    }
    for(int i = 0; i < number; i++)
    {
        for (int f=0; f<N; ++f)
        {
          array[INDEX(i,f)] = (char *)malloc(sizeof(char) * 30);
        }
        human[i].first_name = (char *)malloc(sizeof(char) * 30);
        if(human[i].first_name == NULL)
        {
            perror("ERROR 3");
            exit(1);
        }
        printf("Give first name of human #%d" , i+1);
        scanf("%s" , human[i].first_name);

        human[i].last_name = (char *)malloc(sizeof(char) * 30);
        if(human[i].last_name == NULL)
        {
            perror("ERROR 4");
            exit(1);
        }
        printf("Give last name of human #%d" , i+1);
        scanf("%s" , human[i].last_name);


        printf("Give age of human #%d" , i+1);
        scanf("%d" , &human[i].age);
    }
    for(int i = 0; i < number; i++)
    {
      strcpy( array[INDEX(i,0)], human[i].first_name);
      strcpy( array[INDEX(i,1)], human[i].last_name);
      sprintf( array[INDEX(i,2)] , "%d" , human[i].age);    // int to str
    }

    for(int i = 0; i < number; i++)
    {
        for(int f = 0; f < N; f++)
        {
            printf("%s\t" , array[INDEX(i,f)]);
        }
        printf("\n");
    }

    for (int i = 0; i < number * N; ++i)
    {
      free( array[i] );
    }
    free(array);
    free(human);

    return 0;

}

Struct 2d array of strings problem

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