Thread: File update with fseek and printf an arr of strutc.

open file for reading
read all
close it
open file for writing
write all
close it
check the result

Originally Posted by mnd22

open the file with "r+b" then fseek at position 0 to update the file. r+b will allow you to update the binary file on any position.

Shall fseek be before or after fread? I believe I did exactly what You said but I see no progress...

Excuse me that, but I'm newbie with this...

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>
 
int main()
{
    int a=0,b=0,e=0,h=0,i=0,j=0,x=0;
    int hm,nb;
    char c,d;
 
    FILE *p;
    p=fopen("test.txt","r+b");
 
    struct store
    {
        char title[100];
        char author[50];
        char genre[25];
        char year[10];
        char pages[10];
        char price[10];
        int howmany;
 };
 
    store base[50];
    for(i=0;i<50;i++)
    {
        strcpy(base[i].title,"(null)");
        strcpy(base[i].author,"(null)");
        strcpy(base[i].genre,"(null)");
        strcpy(base[i].year,"(null)");
        strcpy(base[i].pages,"(null)");
        strcpy(base[i].price,"(null)");
        base[i].howmany=0;
    }
    fread(base,sizeof(store),50,p);
 
    do
    {
        printf("\tMENU\n");
        printf("\t1. Quick add book.\n"); /*READY*/
        printf("\t4. Inventory.\n");
        printf("\t5. Exit.\n"); /*READY*/
 
        char tmp[100];
        fgets(tmp,100,stdin);
        sscanf(tmp,"%c",&c);
 
        switch(c)
        {
            case '1': /*adds just one book - DONE*/
                hm=0;
                system("cls");
                while(strcmp(base[e].title,"(null)")!=0)
                {
                    e++;
                    if(e==50)
                        break;
                }
                if(strcmp(base[e].title,"(null)")==0)
                {        
                    printf("What is the title of the book?\n");
                    fgets(base[e].title,sizeof(base[e].title),stdin);
                    for(j=0;j<50;j++)
                        for(x=0;x<50;x++)    
                            if(x!=j && strcmp(base[x].title,base[j].title)==0)
                            {
                                if(strcmp(base[x].title,"(null)")==0)
                                    break;
                                else if(strcmp(base[j].title,"(null)")==0)
                                    break;
                                else
                                {
                                printf("This book is already in our data base! Press ENTER to continue.\n");
                                getchar();
                                strcpy(base[e].title,"(null)");
                                base[j].howmany=base[j].howmany+1;
                                hm=1;
                                system("cls");
                                break;
                                }
                            }
                            else
                            {
                            }
                    if(hm==0)
                    {
                        printf("What is the author of the book?\n");
                        fgets(base[e].author,sizeof(base[e].author),stdin);
                        printf("What kind of genre is that?\n");
                        fgets(base[e].genre,sizeof(base[e].genre),stdin);
                        printf("What year the book was released?\n");
                        fgets(base[e].year,sizeof(base[e].year),stdin);
                        printf("How many pages does the book count?\n");
                        fgets(base[e].pages,sizeof(base[e].pages),stdin);
                        printf("What is the price?\n");
                        fgets(base[e].price,sizeof(base[e].price),stdin);
                        base[e].howmany=1;
                        printf("Book(s) added succesfully! Press ENTER to return to the menu.\n");    
                    }
                    else
                        break;
                }
                else
                {
                    printf("Our database is full! Press ENTER to continue.\n");
                    getchar();
                    system("cls");
                }                
                getchar();
                system("cls");
                break;
 
     
            case '4': /*inventory*/
                system("cls");
                for(b=0;b<10;b++)
                    printf("Title: %sAuthor: %sGenre: %sYear: %sPages: %sPrice: %sQuantity:%d\n\n",base[b].title,base[b].author,base[b].genre,base[b].year,base[b].pages,base[b].price,base[b].howmany);
                break;
                 
 
            case '5':
                system("cls");
                printf("Thank You! Press ENTER to leave");
                getchar();
                break;
 
            default:
                system("cls");
                printf("Error, press any key to back to the main menu");
                getchar();
                system("cls");
        }    
    }while(c!='5');
fseek(p,0,SEEK_SET);
    fwrite(base,sizeof(store),50,p);
    fclose(p);

@edit: Ou, it works for now. But still not sure about solution. Feels so unsolid

But what about second problem? Printing it?

I commend you for using fgets(), and not scanf. Not sure if line 48 is really necessary though, there's probably a more elegant way to eat the '\n' without making a temp..

However, you should put your struct array in the heap, not the stack. The reason being is that the struct alone takes up 209 bytes, it probably gets word-aligned up to 212, and you're generating fifty of them. That's 10.6 kb you're using of stack memory, which is generally a bad practice. The (surprisingly simple) solution, is to make your main struct a pointer, and call malloc() at the beginning, and free() at the end. The same can be done with the arrays inside, by changing them to char pointers and allocating/freeing them dynamically with the main struct.

Every time someone deletes <conio.h>, someone saves a kitten. (seriously though, do it, conio is outdated and useless)

Why do you have so many counter variables at the beginning? Not only are you zeroing them twice (once in the declaration and once in the loop), you can reuse one variable for all the counters, provided they're not nested. The only time you'll need multiple counters is for nested loops, which you have a couple (two for's inside a do-while). Also, the variables "h", "a", "nb", and "d" are unused (and named horribly).

Lines 61-86 are really clunky, and there's probably a much better way to do it. Currently, you're doing 2500 iterations, and calling strcmp three times during each iteration. Using braces would really help the structure of the code, and might remove the reason for allocating 4 bytes for "hm", a seemingly magic-number repository that is only zero or one.

Edit: I decided I'd paste it into GCC and try it out, turns out that forgetting the "struct" keyword creates tons of errors. Change "store base" into "struct store base".

A better solution is to replace this:

Code:

    struct store
    {
        char title[100];
        char author[50];
        char genre[25];
        char year[10];
        char pages[10];
        char price[10];
        int howmany;
 };
  
    store base[50];
    for(i=0;i<50;i++)
    {
        strcpy(base[i].title,"(null)");
        strcpy(base[i].author,"(null)");
        strcpy(base[i].genre,"(null)");
        strcpy(base[i].year,"(null)");
        strcpy(base[i].pages,"(null)");
        strcpy(base[i].price,"(null)");
        base[i].howmany=0;
    }

With this:

Code:

    struct store
    {
        char title[100];
        char author[50];
        char genre[25];
        char year[10];
        char pages[10];
        char price[10];
        int howmany;
 } base[50] = { { 0 } };

Now you can get rid of all of those strcmp checks and simply do:

Code:

if( base[ x ].title[ 0 ] == 0 )
{
    ... this is unused
}

Unless this is some kind of embedded system, I wouldn't worry about 10k of stack space in something so trivial.


Quzah.

Originally Posted by memcpy

Edit: I decided I'd paste it into GCC and try it out, turns out that forgetting the "struct" keyword creates tons of errors. Change "store base" into "struct store base".

Note: This may also be an indication that the Compiler is doing an "C++" compile instead of "C" compile. In C++, the struct keyword is optional.

Code:

void chomp( char * s )
{
    s[ strcspn( s, "\n" ) ] = '\0';
}

Quzah.