String in structure gets deleted

Question

I'm working on the last exercise of the "Think like a computer scientist, C version" book and I have some trouble with one particular point.

The exercise consists of making a small game, where the computer picks a random value between 0 and 20 and then asks me to guess the number.

After that, the computer counts the number of tries I made and, if I get a better score than the previous party, I need to store my name and the number of tries in a structure.

My problem is the following: When I restart the game, the string value, player_name, in the structure gets somehow deleted but player_score is still there.

First, I made a "call by value" function to create the structure and then a tried with a "call by reference" but getting the same results.

I think I tried everything I could with my actual knowledge for now; so, if someone could check my code and give me some tips about what's wrong I would much appreciate it!

//HEADERS
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define FALSE 0
#define TRUE 1

//TYPEDEF STRUCTS
typedef struct
{
    int player_score;
    char *player_name;
} HS_Player;

//FUNCTION PROTOTYPES
int Random_Value(void);
int Get_User_Choice(void);
int Check_Result(int computer, int my_choice);
int Try_Again(int game_result, int computer);
void Player_Infos(HS_Player *player_p, int score); 
int Game_Restart(void);

//MAIN 
int main(void)
{
    int end_game;
    int high_score_value = 100;
    HS_Player player;

    while (end_game != TRUE)
    {
        int computer_number = Random_Value();
        printf("Guess the number between 0 et 20 chosen by the computer.\n");

        int your_number = Get_User_Choice();
        int result_game = Check_Result(computer_number, your_number);   
        int tries_to_win = Try_Again(result_game, computer_number);

        printf("Number of tries: %i\n", tries_to_win);  
        if (tries_to_win < high_score_value)
        {
            Player_Infos(&player, tries_to_win );
            high_score_value = player.player_score;
        }
        printf("Highest score: %i   By: %s\n", player.player_score, player.player_name);
        printf("\n");

        end_game = Game_Restart();

    }
    return EXIT_SUCCESS;
}

//Random_Value FUNCTION
int Random_Value(void)
{
    srand(time(NULL));
    int x = rand();
    int y  = x  % 20;
    return y;
}

//Get_User_Choice FUNCTION
int Get_User_Choice(void)
{
    int success, x;
    char ch;

    printf("Your Guess:\t");
    success = scanf("%i", &x);
    while (success != 1)
    {
        printf("Your input is not a number. Please try again:\t");
        while ((ch = getchar()) != '\n' && ch != EOF);
        success = scanf("%i", &x);
    }
    if (x < 0 || x > 20)
    {
        printf("Your input must be between 0 and 20. Please try again.\n");
        Get_User_Choice();
    }
    return x;
}

//Check_Result FUNCTION
int Check_Result(int computer, int my_choice)
{
    int check_result;

    if (my_choice < computer)
    {
        printf("Computer number is larger!\n");
        check_result = FALSE;
    }
    else if (my_choice > computer)
    {
        printf("Computer number is smaller!\n");
        check_result = FALSE;
    }
    else if (my_choice == computer)
    {
        printf("It's a Match! You chose the same number than the computer.\n");
        printf("\n");
        check_result = TRUE;
    }
    return check_result;
}

//Try_Again FUNCTION
int Try_Again(int game_result, int computer)
{
    int tries_befor_success = 1;
    while (game_result != TRUE)
    {
        int your_number = Get_User_Choice();
        game_result = Check_Result(computer, your_number);
        tries_befor_success++;
    }
    return tries_befor_success;
}

//Player_Infos FUNCTION
void Player_Infos(HS_Player *player_p, int score)
{
    char new_name[80];
    printf("Congrats! Your made a new high score.\n");
    printf("What's your name ?\t");
    scanf("%s", new_name);
    printf("\n");

    player_p->player_score = score;
    player_p->player_name = new_name;
}

//Game_Restart FUNCTION
int Game_Restart(void)
{
    int quit_value;
    printf("Quit Game ?\n");
    printf("Press 'y' to quit or any other keys to continue.\n");
    fflush(stdin);
    char quit_game = getchar();
    printf("\n");

    if (quit_game == 'y')
    {
        quit_value = TRUE;
    }
    else
    {
        quit_value = FALSE;
    }
    return quit_value;
}

Answer 1

The problem is that, in your Player_Infos function, you are assigning the address of a local array to the char* player_name pointer member of the passed structure. When that function ends, the local array it used will be deleted and the pointer in the structure will be invalid. (In the case of the player_score, you don't have that problem, because the given value is copied to the structure member.)

There are several ways around this; one would be to use the strdup() function to make a copy of the local char new_name[80]; array – but that is really overkill, and you would need to manage (i.e. free()) that allocated string whenever you make a modification.

A simpler way is to make the player_name member an actual array of char and then use strcpy() to copy the local array into that member.

Better, still, with the player_name member defined as char [80], you can read directly into that (in the function), and avoid the local array completely:

typedef struct
{
    int player_score;
    char player_name[80];
} HS_Player;

//...

void Player_Infos(HS_Player *player_p, int score)
{
    printf("Congrats! Your made a new high score.\n");
    printf("What's your name ?\t");
    // Read directly. Limit input to 79 chars (allowing room for null terminator).
    scanf("%79s", player_p->player_name); 
    printf("\n");
    player_p->player_score = score;
}

Also, just as a "style" tip, you may want to change the member names to just score and name, as the "player" part is implied by the structure type-name itself.

Answer 2

This issue you are having is that you are associating the player name pointer to a variable that goes out of scope when you leave the "player_Infos" function. What you probably would want to do is define the name as a character array in your structure and then use the "strcpy" call in your function instead. Following is a couple of code snippets illustrating that point.

//TYPEDEF STRUCTS
typedef struct
{
    int player_score;
    char player_name[80];
} HS_Player;

Then, in your function, use the "strcpy" call.

//Player_Infos FUNCTION
void Player_Infos(HS_Player *player_p, int score)
{
    char new_name[80];
    printf("Congrats! Your made a new high score.\n");
    printf("What's your name ?\t");
    scanf("%s", new_name);
    printf("\n");

    player_p->player_score = score;
    strcpy(player_p->player_name, new_name);
    //player_p->player_name = new_name;
}

When I tested that out, I got a name to appear in the terminal output.

Computer number is smaller!
Your Guess: 4
It's a Match! You chose the same number than the computer.

Number of tries: 8
Highest score: 4   By: Craig

FYI, you will need to include the "string.h" file.

Give that a try.

Answer 3

Name Update

The reason your player.player_name is not getting updated is because you can't assign a string this way in C. When doing player_p->player_name = new_name; you're actually saving in player_p->player_name the memory address of new_name.
Instead, what you want to achieve, is to copy each character of new_name to player_p->player_name and in order to achieve this, you have to change the type of prlayer_name field from char* player_name to char player_name[80], then assign it using, for example, strcpy():

#include <string.h>

// [...]

//TYPEDEF STRUCTS
typedef struct
{
    unsigned int player_score;
    char player_name[80];
} HS_Player;

// [...]

//Player_Infos FUNCTION
void Player_Infos(HS_Player *player_p, int score)
{
    char new_name[80];
    printf("Congrats! Your made a new high score.\n");
    printf("What's your name ?\t");
    scanf("%s", new_name);
    printf("\n");

    player_p->player_score = score;
    strcpy(player_p->player_name, new_name);
}

Data Persistence

To make data (players info) persistent over multiple runs, you have to save the content of the struct to a file.

Example

int Save_Score(char* filename, HS_Player* player)
{
    FILE* file = fopen(filename, "w");
    if (file == NULL)
    {
        fprintf(stderr, "\nAn error occurred while opening the file\n");
        return -1;
    }

    if (fprintf(file, "%d %s", player->player_score, player->player_name) < 0)
        return -1;

    fclose(file);

    return 0;
}

int Load_Score(char* filename, HS_Player* player)
{
    FILE* file = fopen(filename, "r");
    if (file == NULL)
    {
        fprintf(stderr, "\nAn error occurred while opening the file\n");
        return -1;
    }

    if (fscanf(file, "%d %79s", &player->player_score, player->player_name) < 0)
        return -1;

    fclose(file);

    return 0;
}