# Homework Help: A C program to determine the most common birthday

Tags:
1. Mar 27, 2017

### toforfiltum

1. The problem statement, all variables and given/known data
Write a program that opens a file of the users choice that contains a list of birthdays. Extract from this file two things: (1) the date with the most common birthday (all of them) and (2) the month with the most people born. We will not test for a tie in either of these statistics. The file tested will always be in the format:
mm/dd/yyyy

2. Relevant equations
bday1.txt file is uploaded, second one is too large but of same format.

3. The attempt at a solution
Ok, at first I tried to come up separate array of counters for the month and day, thinking that the highest for both would give me the most common birthday. But that logic is flawed. I may have lesser months or days but a higher occurrence of the particular same month and day.

So, I realise I must create a 2D array counter for birthdays and months, and another array for months.
I hope my logic here is right so far.

Therefore, this is my code:
Code (C):

#include <stdio.h>

int main ( void )
{
int month [12] = {0}, m_d [12][31] = {0},i = 0, flag, store_m = 0, store_d = 0, count_month [12] = {0}, count_m_d [12][31] = {0}, trash;
int largest_m, largest_bd, a, j;

for ( i = 0; i < 12; i++ )
{
month [i] = i+1;
}

for ( i = 0; i < 31; i++ )
{
for ( int j = 0; j < 31; j++ )
{
m_d [i][j] = j+1;
}
}

flag = scanf("%d/%d", &store_m, &store_d );

while ( flag != EOF )
{
for ( i = 0; i < 12; i++ )
{
if ( store_m == month [i] )
{
count_month [i]++;
for ( int j = 0; j < 31; j++ )
{
if ( store_d  == m_d [i][j] )
{
count_m_d [i][j]++;
}
}
}
}

scanf("/%d", &trash );
flag = scanf("%d/%d", &store_m, &store_d );

}

largest_m = count_month [0];

for ( int i = 0; i < 12; i++ )
{
if ( largest_m < count_month [i] )
{
largest_m = count_month [i];

}
}
largest_m = month [i];

largest_bd = count_m_d [0][0];

for ( a = 0; a < 12; a++ )
{
for ( j = 0; j < 31; j++ )
{
if ( largest_bd < count_m_d [a][j] )
{
largest_bd = count_m_d [a][j];

}
}
}
largest_bd = m_d [a][j];
a = month [a];

printf("Most common birthday: \n");
printf("%2d/%2d\n", a, largest_bd );

printf("Most common birthday month: \n");
printf("%d", largest_m );

return 0;
}

The code could compile but during runtime, it has segmentation fault. Using UNIX to run this file, I could redirect input from stdin to the file using '<'. I'm suspecting the fault lies in a = month [a]? Because that's the value that I want, not the total count, but I need the count to determine the month. Am I doing this right?

#### Attached Files:

• ###### bday1.txt
File size:
45 bytes
Views:
27
2. Mar 27, 2017

### Staff: Mentor

You access m_d with a first index beyond 11.

I don't understand what you want to do in this step. Where is the point of m_d if you just want m_d[i][j] to be j+1?

count_m_d is all you need. count_m is optional. Everything else is unnecessary.

3. Mar 27, 2017

### .Scott

1) As mfb said, your month[j] is always equal to j, and your m_d[ i][j] is always j, so why use those arrays.
2) You have bounds checked store_m and store_d by comparing each to every valid value. But why not simply make sure that each is greated than or equal to 1 and less than or equal to 12 or 31?
3) Once you have bound check store_m and store_d, you can increment count_month[store_m-1] and count_m_d[store_m-1][store_d-1]. Or, if you don't like subtracting the 1's, dimension your arrays by 13 and 32.

Last edited by a moderator: Mar 27, 2017
4. Mar 27, 2017

### wle

Well just at a glance: 1) you're writing to array elements that don't exist, and 2) you create two arrays that are presumably meant to count how many times each month and each day/month pair appear in the file, but then you have two loops at the start of your program in which you do month [i] = i+1; and m_d [i][j] = j+1; -- why? If you want to count them then surely you want the counters to start at zero, no? Then at the end just scan both arrays to find which month and which day/month pair have the highest counts.

You're probably meant to open and read from the file from C, which you can do with standard library functions.

This means you need to read the filename from somewhere. The normal way to do this would be to read the filename as a command-line argument, so you would run your program from the terminal like this:
Code (Text):
\$ ./my_program my_file.txt
and the program would try to open "my_file.txt" (if it exists). C provides an easy and standard way to access the command-line arguments your program was called with (if any); googling for "C command line arguments" or "argc argv" should turn up explanations.

Just make sure to check for errors if you do this (i.e., if the program was run without a command line argument or there's an error attempting to open the file) and do reasonable things in these cases (e.g., read from standard input by default and/or quit with an error message).

Last edited: Mar 27, 2017
5. Mar 27, 2017

### Staff: Mentor

Unrelated to the current segfault, but as the problem will come up anyway later:
What do you expect this code to do?
Follow its execution step by step: What will it actually do? In particular, what will the value of "i" be when the last line is executed? And what do all the other lines do?

Same problem for the days.