How to pass a struct array into a function

Question

I want to pass a struct array into a function and I keep getting an error. I'm not sure how to do it in general as I was never taught. I've done some googling but I can't find anything. Can you guys help me out and explain why this is not working? Here's the code of the entire program (it's small, just a test program):

 #include <stdio.h>
 #include <stdlib.h>
 #define hour 60


int getNum(void);
int distance(int start, int end , int flight_time[5], int flight_layover[5]);

int main(void)
{
    int user_start=-1;
    int user_end=-1;
    int travel_time=0;

struct flight 
{
    int flight_time;
    int flight_layover;
};

struct flight flights[5]=
{
        {4 * hour + 15, 1 * hour + 20},
        {3 * hour + 58, 0 * hour + 46},
        {3 * hour + 55, 11 * hour + 29},
        {2 * hour + 14, 0 * hour + 53},
        {3 * hour + 27, 0 * hour + 0}
};

printf ("Hello sir. Please enter you starting city:\n");
user_start=getNum();
user_start--;

printf ("Good. Now enter the city you would like to end it:\n");
user_end=getNum();
user_end--;

travel_time = distance(user_start,user_end, flights[5].flight_layover,          flights[5].flight_time);

printf ("The total travel time from %d to %d is %d.",user_start, user_end, travel_time);

return 0; 
}



int distance(int start, int end , int flight_time[5], int flight_layover[5])
{
    int total_mins=0;
    int i=0;
    for (i=end+1;i--;i=start)
    {
        total_mins=total_mins + flight_time[i] + flight_layover[i];
    }

    return total_mins;
}


int getNum(void)
{
    char record[121] = {0}; 
    int number = 0;
    fgets(record,  121, stdin);
    if(sscanf_s(record, "%d", &number) != 1 )
    {
        number  = -1;
    }
    return  number;
}

I made a mistake when I put int flight_time[5] and int flight_layover[5]. I need the entire array in there, not just 5 (index 5, or 4 to be more precise). — Robolisk
– Robolisk, Commented Nov 10, 2011 at 21:06
Be aware that a parameter declaration int flight_time[5] is exactly equivalent to int *flight_time; the 5 is ignored. Suggested reading: section 6 of the comp.lang.c FAQ. — Keith Thompson
– Keith Thompson, Commented Nov 10, 2011 at 21:12

AusCBloke · Accepted Answer · 2011-11-10 21:40:37Z

2

Your distance function is all wrong. Since you have a struct that contains flight_time and flight_layover, you want to pass an array of these structs to the function, not an array of each of those int values. ie.

int distance(int start, int end, struct flight flights[])
{
    int total_mins=0;
    int i=0;
    for (i=end+1;i--;i=start)
    {
        total_mins=total_mins + flights[i].flight_time + flights[i].flight_layover;
    }

    return total_mins;
}

Changed the function signature and the line inside the for loop.

Then where you call distance, you can change the call to:

distance(user_start,user_end, flights)

That'll pass a pointer to the start of your array (flights), and user_start and user_end will specify the bounds of the array to be used to calculate the distance.

Also note that you could be accessing indexes out of the flights array's bounds. I especially don't understand why you have i=end+1, perhaps you wanted i = end - 1?

You also have the condition and decrement part of your for loop reversed, this is how I think it should be:

for (i = start; i <= end; i++)

EDIT: Obviously your function prototype needs to be updated, and your structs should be declared before they're used anywhere (including before the prototypes). Here's the whole code with the modifications I've mentioned.

Also, be careful with your getNum function, since it can return -1 upon error and you'll cause undefined behaviour if you're accessing elements before your array. You should also check the user input to ensure that the start and end values are between 1 and 5, and are decremented so that they're valid array indexes between 0 and 4.

Also, you can use the += operator to add to the current value of a variable, ie

total_mins=total_mins + flights[i].flight_time + flights[i].flight_layover;

can be changed to:

total_mins += flights[i].flight_time + flights[i].flight_layover;

EDIT2: One other thing. By convention, constant #defines should be in upper-case, ie.

#define HOUR 60 as opposed to #define hour 60.

edited Nov 10, 2011 at 21:40

answered Nov 10, 2011 at 21:08

AusCBloke

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7 Comments

Robolisk Over a year ago

yeah I thought that if I had the for loop start at end it would decremment (i--) and it would skip one. I also forogt to mention, that the struct flight (I don't have error checking yet), the way it works is I have to have the flight citys displayed and where the user can go to from each city. So, the way it works, is city 1 can only go to city 2, and city 2 can only go to city 3. I just wanted to make sure it was adding up the mins correctly of the total travel time.

Robolisk Over a year ago

Also, putting stuct flight flights[] gave me lots of errors. The way you did.

AusCBloke Over a year ago

@Robsta you need to change the function prototype also and move the struct declarations, let me upload the full code.

AusCBloke Over a year ago

@Robsta, there you go, it'll all compile now. Also note that I changed sscanf_s to sscanf for it to compile.

AusCBloke Over a year ago

@Robsta if you already downloaded the updated code, do it again. I just realised that end in your distance() for loop was inclusive, so it should in fact be: for (i = start; i <= end; i++)

|

K-ballo · Accepted Answer · 2011-11-10 21:32:44Z

0

I believe you want something like this:

int distance(int start, int end , struct flight flights[5])
{
    int total_mins=0;
    int i=0;
    for (i=end+1;i--;i=start)
    {
        total_mins=total_mins + flights[i].flight_time + flights[i].flight_layover;
    }

    return total_mins;
}

which is then called as

travel_time = distance(user_start,user_end, flights);

edited Nov 10, 2011 at 21:32

answered Nov 10, 2011 at 21:01

K-ballo

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2 Comments

AusCBloke Over a year ago

Should be flights[i].flight_time not flights.flight_time[i].

K-ballo Over a year ago

@AusCBloke: Fixed. Thanks for letting me know, I didn't noticed.

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