Calculating Average from array and seperate structure - c

Question

Trying to make a program that prints data entered into a file.
Everything is working all and good apart from the Calculating Average for marks entered.
I can't seem to figure out how to do it, even though it should be simple I just cant get my head around it.
The error I am currently getting is:
"temp->mark = temp->mark + studentArray[j];" (Invlalid operands to
binary + (have 'float' and 'char *').
Much appreciated if someone could help me. I have tried the following
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
struct student{
char name[30];
int id;
float mark;
};
int count = 0;
void student_update(char *stuadd);
void display(char *stuadd);
void main(int argc, char *studentArray[100])
{
int choice;
while(1)
{
printf("Welcome to Student Archives\n\n");
printf("1. Display Students' Details\n");
printf("2. Calculate average of all students’ marks \n");
printf("3. Add new student to the record \n");
printf("4. Quit Program\n");
scanf("%d",&choice);
switch(choice)
{
case 1:display(studentArray[100]);
break;
case 2:
break;
case 3:
student_update(studentArray[100]);
break;
case 4: printf("Program Terminated.\n");
exit(0);
default: printf("Wrong Choice. Enter again\n");
break;
}
}
}
void display(char *stuadd)
{
FILE *fptr;
char ch;
int rec = count;
fptr = fopen("stuadd.txt", "r");
struct student *temp = (struct student *)malloc(sizeof(struct student));
if (fptr == NULL)
printf("File does not exist.");
else
{
while (rec)
{
fread(temp->name, 50, 1, fptr);
printf(" %s\n", temp->name);
fread(&temp->id, sizeof(int), 1, fptr);
printf("%d", temp->id);
fread(&temp->mark, sizeof(int), 1, fptr);
printf("%.2f", temp->mark);
rec--;
}
}
fclose(fptr);
free(temp);
free(temp->name);
}
void calculateAverage(char *studentArray[100])
{
struct student *temp = (struct student *)malloc(sizeof(struct student));
int j;
float avg;
temp->mark = avg = 0;
for(j = 0; j < 100; j++)
{
temp->mark = temp->mark + studentArray[j];
}
avg = (float)temp->mark / j;
printf("Average of students' total marks are: %.2f",avg);
}
void student_update(char *stuadd)
{
FILE *fptr;
fptr = fopen("stuadd.txt", "a+");
struct student *temp = (struct student *)malloc(sizeof(struct student));
if (fptr == NULL)
printf("\nError.");
else
{
printf("\nEnter the students' name\n");
scanf(" %[^\n]s", temp->name);
printf("Enter the students' ID\n");
scanf("%d", &temp->id);
printf("Enter the students' mark\n");
scanf("%f", &temp->mark);
fprintf(fptr, "%s %d %.2f", temp->name, temp->id, temp->mark);
count++;
}
fclose(fptr);
free(temp);
free(temp->name);
}

Answer 1

The posted code does not compile!
under ubuntu linux, using:
gcc -ggdb -Wall -Wextra -Wconversion -pedantic -std=gnu11 -c "untitled.c" (in directory: /home/richard/Documents/forum)
the compiler outputs the following:
untitled.c:16:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
void main(int argc, char *studentArray[100])
^~~~
untitled.c: In function ‘main’:
untitled.c:16:15: warning: unused parameter ‘argc’ [-Wunused-parameter]
void main(int argc, char *studentArray[100])
^~~~
untitled.c: In function ‘display’:
untitled.c:48:10: warning: unused variable ‘ch’ [-Wunused-variable]
char ch;
^~
untitled.c:45:20: warning: unused parameter ‘stuadd’ [-Wunused-parameter]
void display(char *stuadd)
^~~~~~
untitled.c: In function ‘calculateAverage’:
untitled.c:83:33: error: invalid operands to binary + (have ‘float’ and ‘char *’)
temp->mark = temp->mark + studentArray[j];
~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~
untitled.c:86:29: warning: conversion to ‘float’ from ‘int’ may alter its value [-Wconversion]
avg = (float)temp->mark / j;
^
untitled.c: In function ‘student_update’:
untitled.c:91:27: warning: unused parameter ‘stuadd’ [-Wunused-parameter]
void student_update(char *stuadd)
^~~~~~
Compilation failed.
There are some other problems, like:
free(temp);
free(temp->name);
That is accessing a pointer into allocated memory after the allocated memory has been passed to free() The result is indefined behavior. Suggest:
free(temp->name);
free(temp);
Regarding the following statements
FILE *fptr;
fptr = fopen("stuadd.txt", "a+");
struct student *temp = (struct student *)malloc(sizeof(struct student));
if (fptr == NULL)
printf("\nError.");
Always check for an error indication immediately after the call to the C library function.
output error messages to stderr, not stdout
When the error indication is from a C library function, then immediately call perror(); to output both your error message AND the text reason the system thinks the error occurred, all to stderr
when calling any of the heap allocation functions: malloc calloc realloc, 1) the returned type is void* which can be assigned to any pointer. Casting just clutters the code, making it more difficult to understand, debug, etc. 2) always check (!=NULL) the returned value to assure the operation was successful. Suggest:
FILE *fptr;
fptr = fopen("stuadd.txt", "a+");
if ( !fptr )
{
perror("fopen failed");
exit( EXIT_FAILURE );
}
// implied else, fopen successful
struct student *temp = malloc(sizeof(struct student));
if( !temp )
{
perror( "malloc failed" );
exit( EXIT_FAILURE );
}
// implied else, malloc successful
regarding:
scanf(" %[^\n]s", temp->name);
That call to scanf() is nonsense. Since the input format specifier %[^\n] will stop input from stdin when it encounters a newline sequence, there is no possible way for the next char in stdin to be a s When calling any of the scanf() family of functions, always check the returned value to assure the operation was successful. when using the input format specifiers %s and/or %[...] always include a MAX CHARACTERS modifier that is one less than the length of the input buffer to avoid any possibility of buffer overflow (and the resulting undefined behavior). Suggest removing the trailing s in the format string and check the return value and limiting the total number of characters that can be input, as in:
if( scanf(" %29[^\n]", temp->name) != 1 )
{
fprintf( stderr, "scanf failed to input the student name\n" );
exit( EXIT_FAILURE );
}
regarding:
for(j = 0; j < 100; j++)
{
temp->mark = temp->mark + studentArray[j];
}
there is no array studentArray[] so this will never produce the desired results.
regarding the error message:
avg = (float)temp->mark / j;
untitled.c:83:33: error: invalid operands to binary + (have ‘float’ and ‘char *’)
temp->mark = temp->mark + studentArray[j];
of course a 'float` value cannot be added to a pointer to a char array. What were you actually trying to accomplish?
All the above is just the 'tip of the iceburg' of problems in the posted code. Suggest using a debugger and stepping through your code to determine the many problems