program returning garbage values

Question

This program is returning garbage values in output of total marks and average marks. even after declaring s.total=0 and s.marks[15]=0 in the beginning of program

//student database
#include<stdio.h>
#include<conio.h>
#include<string.h>
main()
{
    struct student
{
    int name[20];
    int roll[5];
    int total;
    float avg;
    int marks[15];
};
    struct student s;
    int subject_no,i;
    s.total=0;
    s.marks[15]=0;  
    printf("Enter the name of student:\t");
    scanf("%s",s.name);
    printf("enter the roll no:\t");
    scanf("%d",&s.roll);
    printf("Enter the number of subjects:\t");
    scanf("%d",&subject_no);
    for(i=0;i<subject_no;i++)
{
    printf("Enter marks in subject %d:\t",i);
    scanf("%d",&s.marks);
    s.total=s.total+s.marks[i];
}
    printf("total marks are:%d\n",s.total);
    s.avg=(s.total)/(subject_no);
    printf("The average is :%d",s.avg);
    getch();
}

You should probably also tag this with "C". There is a pending edit so i cant add it — Jimbo, Feb 13 '14 at 12:04
Better make your 20 some defined macro e.g. `#define NAME_LEN 20` — Basile Starynkevitch, Feb 13 '14 at 12:05

barak manos · Answer 1 · 2014-02-13T12:24:06.710

2

This is an illegal memory access: s.marks[15]=0.

If you have 15 entries in the array, then legal indexes are between 0 and 14.

So you can do s.marks[0]=0 or s.marks[6]=0, but you should not do s.marks[15]=0.

If you want to set the entire array to zero values, then you can iterate it:

for (i=0; i<sizeof(s.marks)/sizeof(*s.marks); i++)
    s.marks[i] = 0;

Or simply use memset:

memset(s.marks,0,sizeof(s.marks));

edited Feb 13 '14 at 12:24

answered Feb 13 '14 at 12:02

barak manos

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-Everything you wrote was ok.But the problem with the program was something else.Actually i deleted the s.marks[15] line.It wasn't needed and declared the s.marks[i] in the for loop.But the real problem actually was==== i wrote %d in the last printf statement,but actually i declared it float.so i had to write %f.and the program started working. – john_conner Feb 13 '14 at 13:18

Basile Starynkevitch · Answer 2 · 2014-02-13T12:41:47.267

1

You should explicitly clear your s structure with

 memset (&s, 0, sizeof(s));

Of course your filling of s.marks[15] is out of bounds (undefined behavior and/or buffer overflow!!)

Your code never fills or uses s.roll[1] so why make s.roll an array?

At last, your first for loop should be

if (subject_no >= 15) exit(EXIT_FAILURE);
for(i=0;i<subject_no;i++) {
  printf("Enter marks in subject %d:\t",i);
  scanf("%d",&s.marks[i]);
  s.total=s.total+s.marks[i];
}

edited Feb 13 '14 at 12:41

answered Feb 13 '14 at 12:03

Basile Starynkevitch

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`memset (s, 0, sizeof(s))` will not work. should pass `&s` as the first argument. – barak manos Feb 13 '14 at 12:06

score 0 · Answer 3 · answered Feb 13 '14 at 12:15

0

It would be better to store the name in a char array, rather than an int array (although that will work)

Also, you have been inconsistent in you use of scanf with arrays.

answered Feb 13 '14 at 12:15

pecks

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program returning garbage values

3 Answers3