Arrays

**mcgeady** · 01-18-2005

Code:

#include <stdio.h>
#define NO_OF_MONTHS 12
main()
{
    int days[NO_OF_MONTHS] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    int month;
        
    printf("Please enter a month: ");
    
    do
    {
        scanf("%d", &month);
    }
    while (month < 1 || month > 12);
    
    printf("\nThe number of days in month %d is %d\n", month, days [month-1]);
}

Would anyone be able explain the line in bold to me please? What it looks like to me is that the second printf will be shown if the number entered is lower than 1 or higher than 12. I'm obviously wrong though.

Thanks in advance.

**kermit** · 01-18-2005

You have a do while loop - the line in bold is saying 'while the number entered for month is less than 1 OR greater than 12, scanf() a number' Basically, because it is a do while loop, scanf() will happen at least once, prompting the user for a number. Then. if the number is greater than one, or less than 12, the program will carry on with the next printf() statement. Otherwise, if the number entered is less than 1 or greater than 12, scanf() will run again, and the user will be expected to enter another number within the range.

~/

**mcgeady** · 01-18-2005

Thanks, thats very clear but I entered the number 13 and it skipped to the next line, I then entered a number in the range of 1-12 and it skipped a line again (and keeps doing so until i exit the cmd prompt).

**mcgeady** · 01-18-2005

and just to add to my last post. I took the do-while out for experimentation and its obviously ok when entering a number between 1-12 but when i enter a number outside this range it returns a random long number. What makes it decide which number to return when its outside of the range?

**quzah** · 01-18-2005

The code works just like it should. It's waiting for you to enter the correct number. If you actually want it to prompt for the number, move the first printf line inside the loop:

Code:

do
{
    printf("Please enter a month: ");
    scanf("%d", &month );
...

Oh, and this is C, so main does not automaticly return an integer for you, so add a return statement.

Quzah.

**mcgeady** · 01-18-2005

Ah, its working fine now. Thanks guys.

**hk_mp5kpdw** · 01-18-2005

Originally Posted by mcgeady

and just to add to my last post. I took the do-while out for experimentation and its obviously ok when entering a number between 1-12 but when i enter a number outside this range it returns a random long number. What makes it decide which number to return when its outside of the range?

It goes exactly where you tell it to in order to get the number regardless of whether or not the location it is even valid. Consider a system where ints are 32 bits (4 bytes), your array could look like:

Code:

var      address
-------- -----------
days[0]  0x000000A0
days[1]  0x000000A4
days[2]  0x000000A8
...
...
...
days[10] 0x000000C8
days[11] 0x000000CC

Now, the only valid indexes for the array are obviously 0-11 and represented in the range of address' given above. If you try to output the contents of days[12] for example, the program will happily go and try to get whatever 4 bytes are stored at address 0x000000D0 and convert that into an int and then display it. Who knows what that might result in depending on what happens to be stored at that location? There could be a bunch of chars or a double or a pointer to something at that spot. Likewise, if you were to try and output the contents of days[-1] it would go and get the 4 bytes at memory address 0x0000000C and convert that to an int and display it.

It is up to you the programmer to ensure such things do not happen.

**Kleid-0** · 01-18-2005

Code:

#include <stdio.h>
#define NO_OF_MONTHS 12
int main() 
{
    int days[NO_OF_MONTHS] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    int month;

    printf("Please enter a month: ");

   do
   {
      scanf("%d", &month);
   } while (month < 1 || month > 12);
   /*
   If the month is higher than the max OR the month is lower than the minmum, keep asking the user for another month that WORKS
   */

    printf("\nThe number of days in month %d is %d\n", month, days [month-1]);
  return 0;
}

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