How do I modify thing program??

**gouthamgmv** · 10-29-2010

I've managed to write this following program which accepts values for 5 students and works on it. But, for the life of me, I can't figure out how to modify this for 'n' number of students! I've tried almost everything I know but I can't get it to work without errors! Does anyone have any ideas?? Thanks!

Code:

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

#define NUM_STUDENTS 5 

#define IND_REGNO 0 
#define IND_FAIL 1 
#define IND_RANK 2 
#define IND_TOTAL 3 
#define IND_MARKS 4 

int st[NUM_STUDENTS][9]; 
int sortarr[NUM_STUDENTS]; 

int sortfunc(const void* p1, const void* p2) { 
    int id1=*(int*)p1, id2=*(int*)p2; 

    return st[id2][IND_TOTAL]-st[id1][IND_TOTAL]; 
} 

int main() { 
    printf("\n Enter the register number and marks in each subject");
    int i, j, passers=0, passrate=0, topper=0; 

    for(i=0; i<NUM_STUDENTS; i++) { 
        scanf("%i %i %i %i %i %i", &st[i][IND_REGNO], &st[i][IND_MARKS+0], &st[i][IND_MARKS+1], 
              &st[i][IND_MARKS+2], &st[i][IND_MARKS+3], &st[i][IND_MARKS+4]); 

        for(j=0; j<5; j++) { 
            st[i][IND_TOTAL]+=st[i][IND_MARKS+j]; 
            if(st[i][IND_MARKS+j] < 50) { 
                st[i][IND_FAIL]=1; 
            } 
        } 

        if(st[i][IND_FAIL] == 0) { 
            passers++; 
        } 
    } 

    for(i=0; i<NUM_STUDENTS; i++) { 
        sortarr[i]=i; 
    } 

    qsort(sortarr, NUM_STUDENTS, sizeof(int), sortfunc); 

    for(i=0; i<NUM_STUDENTS; i++) { 
        st[sortarr[i]][IND_RANK]=i+1; 
    } 
    passrate=(passers*100) / NUM_STUDENTS; 
    topper=st[sortarr[0]][IND_REGNO]; 
printf("\nNo \tM1 \tM2 \tM3 \tM4 \tM5 \tTOTAL \tP/F \tRANK \n \n \n");
    for(i=0; i<NUM_STUDENTS; i++) { 
        printf("\n%i \t%i \t%i \t%i \t%i \t%i \t%i \t%s \t%i\n", st[i][IND_REGNO], st[i][IND_MARKS+0], st[i][IND_MARKS+1], 
              st[i][IND_MARKS+2], st[i][IND_MARKS+3], st[i][IND_MARKS+4], st[i][IND_TOTAL], 
              (st[i][IND_FAIL]==0) ? "PASS" : "FAIL", st[i][IND_RANK]); 
    } 

    printf("\nPass Percentage - %i%%\nTopper - %i\n", passrate, topper); 
    getch();
    return 0; 
}

**GReaper** · 10-29-2010

If you want an arbitrary number of students, you will have to use dynamic memory allocation.

**Memloop** · 10-29-2010

You will need to work with dynamic memory if the number of students is unknown at compile time. This basically means replacing all fixed size arrays with dynamically allocated arrays once you know how many students there are.

**askinne2** · 10-29-2010

You could simply ask the user how many students they need and use this number as the size of your array.

**gouthamgmv** · 10-30-2010

If you want an arbitrary number of students, you will have to use dynamic memory allocation.

You will need to work with dynamic memory if the number of students is unknown at compile time. This basically means replacing all fixed size arrays with dynamically allocated arrays once you know how many students there are.

Can someone please explain this concept in detail?? I'm just a beginner and am not sure, thanks!

You could simply ask the user how many students they need and use this number as the size of your array.

I can't do that as I've already defined a function that uses that array above main(). Like I said, I'm a beginner! If I'm wrong, please correct me, thanks!

**rogster001** · 10-30-2010

i cant write code for you at moment but you are being advised to use the malloc function, you will find lots of examples google. you declare a type pointer and later size it according to requirements then if you pass this new array to a function you use an argument for the type pointer and an int for the size then just reference as usual in the function with square brackets

**Adak** · 10-30-2010

A bit of reality check:

1) Your struct is quite small. Any decent PC will easily handle a static array of a thousand student structs.

Dynamic sized arrays are fine, but for a beginner, I'd seriously consider using a large static array, instead. I did a program like this last year, and just sized the array statically, for a large number of club members. The club has never exceeded that number of members, and wasting a bit of RAM, just doesn't matter, when the PC has GB of it.

All the functions are passed the actual number of club members, so there is no problem with the empty elements of the array.

Realistically, how many students do you expect will ever be entered into this program?

2) Yes, you can change parameters to a straight pointer style, if you want, in your function declaration. Not a problem.

This shows a way to allocate a 2D array, in another function:

Reading CSV file

**rogster001** · 10-30-2010

instead of your define num students this will be a local int variable which can obviously be assigned a value by the program or by user input, this value you then pass to malloc function along with type pointer variable like char pointer and hey presto you dun got a dynamic memory allocation to use

**rogster001** · 10-30-2010

i agree with adak about the static array, but only because with the concept of a club or school say there is a max amount of members, but then i dont think its best practice to always reference the entire population whenever you need to get information about it, indexing counts aside,

**BillyTKid** · 10-30-2010

You should your st array directly, not with an extra sortarr.

**gouthamgmv** · 10-30-2010

Originally Posted by rogster001

instead of your define num students this will be a local int variable which can obviously be assigned a value by the program or by user input, this value you then pass to malloc function along with type pointer variable like char pointer and hey presto you dun got a dynamic memory allocation to use

Sorry, but I think I'm still doing it wrongly! I tried that and I'm getting loads of errors! :-/

Can you please provide an example?? Thanks!

**Adak** · 10-30-2010

Remove your #define NUM_STUDENTS -- your num_students will now need to be local, and the user will tell you what that number will be, see?

Also, remove whole line where you declare the array of students, since it will be done inside a local function, if you want to use dynamic memory allocation, for it.

Here's a simple example, I strong suggest you copy and run:
Reading CSV file

It doesn't show anything about a CSV file - just allocating a dynamic 2D array.

**gouthamgmv** · 10-30-2010

I seriously don't get this thing, I'm sorry!!

How about the static array thing?? Is that easier?? What would I need to do?? Thanks !

**gouthamgmv** · 10-30-2010

Did the static array thing by myself!! Can't believe I missed such an easy idea! Thanks for the help guys

**~~CommonTater~~** · 10-30-2010

Originally Posted by gouthamgmv

Sorry, but I think I'm still doing it wrongly! I tried that and I'm getting loads of errors! :-/

Can you please provide an example?? Thanks!

Try ...

Code:

#define NUM_STUDENTS 5000

and recompile.

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