ADD elements of an array with a given condition

Question

There is an array of n students( stu[n]).If gender is boy then my code adds

• for boy b, 2nd,4th,6th,........even position elements of array and
• for girl g, 1st,3rd,5th....odd position elements of array.

1> Gender of boys denoted by b. 2> Gender of girls denoted by g.

Input info>>

1. The 1st line contains n, denoting the number of students in the class, hence the number of elements in the array.
2. Each of the subsequent lines contains the marks of n students .
3. The last line contains gender b/g;

Output info>>

The output should contain the sum of all the alternate elements of marks as explained above.

#include <stdio.h>

int main() {
int n,i;
scanf("%d",&n);//n denotes number of students.
int stu[n],sum=0;


for(i=1;i<=n;++i)
scanf("%d",&stu[i]);//for accepting input in array.

char gen;
scanf("%s",&gen);//for accepting input gender b/g.


 for(i=1;i<=n;i++){
 if(gen=='g' && i%2!=0){ //girl g adds alternate odd position  elements.
sum=sum+stu[i];
printf("%d",sum);
}

else if(gen=='b' && i%2==0){ //boy b adds alternate even position elements.

sum=sum+stu[i];
printf("%d",sum);
}

}
//code
return 0;
}

Sample Input

3 3 2 5 b

Sample Output 8

explanation>> marks=[3,2,5] and gender = b so it will add 3+5(even position 0,2 alternate elements). If gender in place of b is g then it will produce output = 2.

My code is shows output of 0 in all test cases.

Answer 1

You have the major problem in

int n,i;
int stu[n],sum=0;

here, n being a uninitialized local scoped variable with automatic storage, the initial value is indeterminate.

Now, since the address of the variable was never taken and it has a type that can have trap representation, attempt to use the value (int stu[n]) will invoke undefined behavior.

You need to scan in the value into n first, then use that to define the VLA stu. Something like

int n,i;
scanf("%d",&n);//n denotes number of students.
 // **Note: please check for errors in input with scanf return value.**
int stu[n],sum=0;   // here, n has the scanned value.

That said,

char gen;
scanf("%s",&gen);

is also horribly wrong, you want to scan in a char, not a string, and with the address of a plain char variable, %s conversion specification would be UB, again. You should use %c and discard any whitespaces which is present in buffer altogether.

Answer 2

You're making things more complicated than they need to be. Here is how you can possibly do:

#include <stdio.h>

int main(void)
{
    int mark;
    int b = 0;
    int g = 0;
    char students_marks[5];

    for (int i=0; i<5; i++) {
        scanf("%d", &mark);
        students_marks[i] = mark;
    }
    for (int i=0; i<5; i++) {
        if (i%2 == 0) b += students_marks[i];
        if (i%2 == 1) g += students_marks[i];
    }
    printf("Boys: %d\nGirls: %d\n", b, g);
    return 0;
}

Answer 3

You should probably not use an array, and just ignore the first data point. It is (probably) easier to use a linked list. Or maybe just use two lists, alternating the inputs between them. And I would definitely not use scanf. If you are new to C, do NOT waste your time learning the foibles of the scanf format string language. The only time scanf is ever useful is in introductory courses where instructors incorrectly believe that you will be able to get input more quickly than if you spend time learning other methods. But in fact you will end up burning more time learning when to use spaces in the format string that you saved by not learning fread. After your introduction to C, you will (almost) never use scanf. Also, it seems like a horrible design to put the discriminant at the end of the input. The values to be summed (the gender) should be given as a command line argument. That said, you could just do:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

FILE *
xfopen(const char *path, const char *mode)
{
        FILE *rv;
        if( (rv = fopen(path, mode)) == NULL ) {
                perror(path);
                exit(EXIT_FAILURE);
        }
        return rv;
}

int
main(int argc, char **argv)
{
        int i;
        int size;  /* Should probably be size_t.  Skipping that for now. */
        FILE *in = argc > 1 ? xfopen(argv[1], "r") : stdin;
        int sum = 0;
        if( fscanf(in, "%d", &size) != 1 || size <= 0 ) {
                fprintf( stderr, "invalid input\n" );
                exit(EXIT_FAILURE);
        }
        int grades[size];
        for( i = 0; i < size; i++ ) {
                if(fscanf(in, "%d", grades + i) != 1) {
                        fprintf( stderr, "invalid input on line %d\n", i );
                        exit(EXIT_FAILURE);
                }
        }
        char gender;
        if(fscanf(in, " %c", &gender) != 1) {
                fprintf( stderr, "invalid input on line %d\n", i );
                exit(EXIT_FAILURE);
        }
        if(strchr("bBgG", gender) == NULL) {
                fprintf( stderr, "invalid gender: %c\n", gender);
                exit(EXIT_FAILURE);
        }
        for( i = tolower(gender) == 'b'; i < size; i += 2 ) {
                sum += grades[i];
        }
        printf("sum: %d\n", sum);
}

Answer 4

Hmm… i changed your code a Little bit and hope this runs as described.

int main() {

       int n, index, sum=0;
       int* stu;
       scanf("%d", &n); // input number of studens

       if(n>0)
          stu = malloc(n*sizeof(int)); // allocate memory
       else
          return 0;

       for(index=0;index<n;index++)
          scanf("%d", &stu[index]);

       char gen;
       scanf("%c", &gen);

       for(index=0; index<n; index++){
          if(gen=='g' && index%2!=0) {
             sum += stu[index];
             printf("%d", sum);
          } else if(gen=='b' && index%2==0) {
             sum += stu[index];
             printf("%d", sum);
          }
       }

       return 0;
    }