using scanf in pointers to integer array

Question

This program is supposed to store inputted grades of 3 students and prints the average. However, if I ask for the average of student B, it prints the average of student A. and Student A's average is 0. I can't seem to find where I went wrong.. Please help e.g. Student_A = {7,7,7}, Student_B = {8,8,8}; ave(Student_B) = 7

#include<stdio.h>

int i;
char j;
int student_A[4];
int student_B[4];
int student_C[4];
float grade_input(int student[]);
float ave(int student[]);

main(){
    printf("For Student A:\n");
    grade_input(&student_A[4]);
    printf("For Student B:\n");
    grade_input(&student_B[4]);
    printf("For Student C:\n");
    grade_input(&student_C[4]);
    do{
        printf("Whose average grade do you want to see, a ,b ,c?  ");
        getchar();
        scanf("%c", &j);

        if(j=='a'){
            printf("%.2f\n", ave(student_A));
        }
        if(j=='b'){
            printf("%.2f\n", ave(student_B));
        }
        if(j=='c'){
            printf("%.2f\n", ave(student_C));
        }
    }while(j=='a' || j=='b' || j=='c');
}

float grade_input(int student[]){
    int i;
    for(i=0; i<3; i++){
        printf("Enter grade %d: ", i+1);
        scanf("%d", &student[i]);
    }
}

float ave(int student[]){
    return (student[0] + student[1] + student[2])/3.0;
}

Your problem is an out-of-bounds access by referencing element 4 of a 4 element array; see detailed answer below. The issue is not to do with scanf() at all; scanf is doing just fine, you are giving it a bad pointer to use. — amrith
– amrith, Commented Aug 22, 2013 at 11:07
Concerning a potential input problem that may remain after using the accepted answer: Replace scanf("%c", &j); with scanf(" %c", &j); (An extra space). This will consume any previous \n. — chux
– chux, Commented Aug 22, 2013 at 11:47

amrith · Accepted Answer · 2013-08-22 11:10:07Z

The problem you have is passing &student_X[4] to grade_input(). This is just straight illegal as it is a 4 element array; accessing an element with subscript 4 is not defined. Since they are defined contiguously, you end up effectively sending the pointer to student_B when you call the grade_input() function with &student_A[4] and that's exactly what you are seeing!

You should instead pass a pointer to the array. I've slightly rewritten your code below to illustrate.

#include<stdio.h>

int i;
char j;
int student_A[4];

/* ******* */
/* note that student_A is defined as a 4 element array */

int student_B[4];
int student_C[4];
float grade_input(int student[]);
float ave(int student[]);

main(){
  printf("For Student A:\n");

  /* ******* */
  /* note that student_A is defined as a 4 element array */
  /* a pointer to the array is just student_A, not &student_A[4] */
  grade_input(student_A);
  printf("For Student B:\n");
  grade_input(student_B);
  printf("For Student C:\n");
  grade_input(student_C);
  do{
    printf("Whose average grade do you want to see, a ,b ,c?  ");
    scanf("%c", &j);

    if(j=='a'){
      printf("%.2f\n", ave(student_A));
    }
    else if(j=='b'){
      printf("%.2f\n", ave(student_B));
    }
    else if(j=='c'){
      printf("%.2f\n", ave(student_C));
    }
    else
      printf ( "Enter a, b or c\n" );

  }while(j != 'q');
}

float grade_input(int student[]){
  int i;
  for(i=0; i<3; i++){
    printf("Enter grade %d: ", i+1);
    scanf("%d", &student[i]);
  }
}

float ave(int student[]){
  return (student[0] + student[1] + student[2])/3.0;
}

No Idea For Name · Accepted Answer · 2013-08-22 10:51:27Z

0

there is no need to do getchar(); before scanf("%c", &j); whoever you should clean the buffer with fflush(stdout); or do

char c;
while ((c = getchar()) != '\n');

so you're full code should be like:

#include<stdio.h>

int i;
char j;
int student_A[4];
int student_B[4];
int student_C[4];
float grade_input(int student[]);
float ave(int student[]);

main(){
    printf("For Student A:\n");
    grade_input(&student_A[4]);
    printf("For Student B:\n");
    grade_input(&student_B[4]);
    printf("For Student C:\n");
    grade_input(&student_C[4]);
    do{
        printf("Whose average grade do you want to see, a ,b ,c?  ");
        fflush(stdout); // THE CHANGE
        scanf("%c", &j);

        if(j=='a'){
            printf("%.2f\n", ave(student_A));
        }
        if(j=='b'){
            printf("%.2f\n", ave(student_B));
        }
        if(j=='c'){
            printf("%.2f\n", ave(student_C));
        }
    }while(j=='a' || j=='b' || j=='c');
}

float grade_input(int student[]){
    int i;
    for(i=0; i<3; i++){
        printf("Enter grade %d: ", i+1);
        scanf("%d", &student[i]);
    }
}

float ave(int student[]){
    return (student[0] + student[1] + student[2])/3.0;
}

answered Aug 22, 2013 at 10:51

No Idea For Name

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7 Comments

John Noel Maape Over a year ago

the problem still exists. :/ if I chose to see student B's grade, it prints student A's ave

No Idea For Name Over a year ago

@JohnNoelMaape how do you know it's a's ave?

John Noel Maape Over a year ago

if I unputted 3 7's for A and 3 8's for B, I get 7 when I ask for B's average

No Idea For Name Over a year ago

@JohnNoelMaape i've tested and got the right result... i really donno what's wrong, are you sure it's not the input? do you put "B" or "b" when you ask for B's avg?

John Noel Maape Over a year ago

i enter small characters. so i input "b"

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