Why is user input skipped when using fgets in this code?

Question

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

typedef struct
{
    char* title;
    float price;
} Book;

void Display(Book[], int);

int main()
{
    int n;
    printf("How many books would you like to create?\n");
    scanf("%d", &n);

    Book *books = malloc(n * sizeof(*books));

    if (books == NULL)
    {
        printf("ERROR: Out of memory. \n");
        return 1;
    }

    for (int i = 0; i < n; i++)
    {
        books[i].title = malloc(50);

        if (books[i].title == NULL) {
                printf("No memory\n");
                return 1;
            }

        printf("Please enter a title for book %d\n", (i+1));

        fgets(books[i].title, 50, stdin);

        if ((strlen(books[i].title) > 0) && (books[i].title[strlen (books[i].title) - 1] == '\n'))
            books[i].title[strlen (books[i].title) - 1] = '\0';

        printf("Please enter a price for book %d\n", (i+1));
        scanf("%f", &books[i].price);
    }

    Display(books, n);

    for(int i = 0; i < n; i++)
        free(books[i].title);

    free(books);

    return 0;
}

void Display(Book list[], int size)
{
    for (int i = 0; i < size; i++)
    {
        printf("Title: %s, price: %.2f$. \n", list[i].title, list[i].price);
    }
}

Hey guys I'm trying to pick up C but I've been stuck on this problem for a while. I'm trying to make a program which ask the user how many books he wants and then proceeds to ask for a title (including whitespace) and a price for each book. whenever I run this code, this is the result:

How many books would you like to create?
2
Please enter a title for book 1
Please enter a price for book 1
45.4
Please enter a title for book 2
Please enter a price for book 2
245.6
Title: , price: 45.40$. 
Title: , price: 245.60$.

why does the title input get skipped?

Don't just `malloc` arbitrary amounts of memory, allocate what you *need*. — tadman, Jun 07 '20 at 23:54

score 0 · Accepted Answer · answered Jun 08 '20 at 01:54

Your problem is this call:

scanf("%d", &n);

Once scanf() finishes parsing the number, it leaves unused input in the buffer. So the newline that the user typed after the number is what your:

fgets(books[i].title, 50, stdin);

gets as input. An empty line. If all your inputs were only fgets() or only scanf() there might be a clean way to deal with this. However, to get you moving forward in your development, you can do:

scanf("%d%*c", &n);
scanf("%f%*c", &books[i].price);

Which will eat the newline for you. However, this is not a complete fix, as the user could type an extra space after the number, and then return, and you're back in the same boat. Just a quick fix to get you moving forward, and then you can go back and address this properly after some research on Google.

Why is user input skipped when using fgets in this code?

1 Answers1