Scanf() not waiting for an integer input

Question

The second time I call scanf in this program, the function doesn't wait for user input. I know with chars you have to leave a space before the format specifier (which I have tried) but can't figure out whats happening with this int. Thanks!

// Program to calculate the number of elasped days between two dates.

#include <stdio.h>

struct date
{
    int day;
    int month;
    int year;
};

int valueofN (struct date d);
int f(int year, int month);
int g(int month);

int main (void)
{
    struct date date1, date2;

    printf("Enter the first date - dd:mm:yyyy : ");
    scanf("%i:%i:%i", &date1.day, &date1.month, &date1.year);

    printf("Enter the second date - dd:mm:yyyy : ");
    scanf("%i:%i:%i", &date2.day, &date2.month, &date2.year);

    // The line above here is where the problem lies ^^^^

    long int N1 = valueofN (date1);
    long int N2 = valueofN (date2);

    printf("Number of elasped days = %li\n", (N2 - N1));

    return 0;
}

// Functions

int valueofN (struct date d)
{
    int N;
    return N = (1461 * (f(d.year, d.month) / 4) + 153 * (g(d.month) / 5) +d.day);
}

int f(int year, int month)
{
    if (month <= 2)
        return (year - 1);
    else
        return year;
}

int g(int month)
{
    if (month <= 2)
        return (month + 13);
    else
        return (month + 1);
}

Answer 1

This is due to format specifier %i.

You may not observe any difference in behaviour %d and %iWhen used with a printf(), but when used with scanf the significance is obvious, that %i takes an integer value as integer value with decimal, hex or octal type, based on some prefix, such that if it begins with 0x its taken a Hex, when preceded with 0, its taken as an Octal value. When you use %d in a scanf, it assume the base 10.

So, you need to replace %i with %d.

Additionally, make a habit of standard library reading man pages of the functions that you are planning to use. For example, scanf, section "Conversions" states that,

i

Matches an optionally signed integer; the next pointer must be a pointer to int. The integer is read in base 16 if it begins with 0x or 0X, in base 8 if it begins with 0, and in base 10 otherwise. Only characters that correspond to the base are used.

Also in section "Return Value", it states,

Return Value These functions return the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.

The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOF is also returned if a read error occurs, in which case the error indicator for the stream (see ferror(3)) is set, and errno is set indicate the error.

Getting the return values from scanf, and addition some error handling for unexpected return will save you a lot of pain going ahead.

Additional Inputs:

1. Its usually preferred/recommended to name your function with verb (or an action) rather than anything that makes no sense.

For example,

int valueofN (struct date d);
int f(int year, int month);
int g(int month);

If I am reviewing your code, I don't know what this function does. Going ahead, name functions and other variables in such a way that it makes sense.

2. Use compatible data types

For example,

long int N1 = valueofN (date1);
long int N2 = valueofN (date2);

N1 and N2 are of the data type long int while valueofN() is returning an int. Its a different data type, there is no immediate harm with this code, but there is a potential harm in this practice going ahead.

3. Some improvement

This

int valueofN (struct date d)
{
    int N;
    return N = (1461 * (f(d.year, d.month) / 4) + 153 * (g(d.month) / 5)+ d.day);
}

could have been

int valueofN (struct date d)
{
    return (int)(1461 * (f(d.year, d.month) / 4) + 153 * (g(d.month) / 5) +d.day);
}

4. Some more improvement: You should always validate the data entered by user (unless you don't know the lower and higher bound values). For example, user can enter 50 for month. It isn't validated. Handle that at the input stage itself. Make that a habit.