Making a school management system and getting errors - c

Question

I am new to programming and learning the C language and was practising structures and was trying to make basic school management system by using structures, but I am getting
too many errors (by using pointers in structures). So, can you please tell me what are the errors and how can I avoid these types of mistakes. Because, according to my understanding this should have been working.
Here is my code:
#include <stdio.h>
struct student {
int rollno;
char name[20];
float marks;
};
int main() {
again:;
while (1 == 1) {
struct student array[60];
char input;
int roll;
printf("What you would like to do:\n");
printf("Enter 'R' to fill the details again of all the student\n Enter 'U' to edit the
details of a single student\n");
printf("Enter 'S' to see the details of a particular student\n Enter 'A' to see the
details of every student\n");
scanf("%c", &input);
if (input == 'A' || 'a') {
for (int i = 0; i < 60; i++) {
printf("Roll no. : %d\n Name : %s\n Marks : %f\n", array[i].rollno, array[i].name,
array[i].marks);
}
}
else if (input == 'R' || 'r') {
for (int i = 0; i < 60; i++) {
printf("Roll no. os student is : %d\n", (i + 1));
printf("Enter students name :\n");
gets(array[i].name);
printf("Enter students marks :\n");
scanf("%f", &array[i].marks);
}
}
else if (input == 'S' || 's') {
printf("Enter the roll no. of the student of whom you want to see the details :\n");
scanf("%d", &roll);
printf("Roll no. :\n %d\n", roll);
printf("Name : \n");
puts(array[(roll - 1)].name);
printf("Marks : \n %f \n", (*(array + (roll - 1))->marks));
}
else if (input == 'U' || 'u') {
printf("Enter the roll no. of the student of whom you want to see the details :\n");
scanf("%d", &roll);
printf("Enter the name :\n");
gets((*(array + (roll - 1))->name));
printf("Enter the marks :\n");
scanf("%f", &(*(array + (roll + 1))->marks));
} else {
printf("Error Occured!!! \n");
printf("Re-enter your input\n");
goto again;
}
}
return 0;
}

Answer 1

There are many problems in your code:
there is no need for a goto again;, you already have an infinite loop while (1 == 1), which is usually written for (;;) in C. Don't use goto.
the definition struct student array[60]; should be moved outside the scope of the loop body. As posted, the array is discarded after each iteration and its contents become indeterminate.
array should be initialized to avoid undefined behavior when printing the contents before reading it from the user.
string literals cannot span multiple lines in a C source file. You can split them this way for readability:
printf("What you would like to do:\n"
"Enter 'R' to fill the details again of all the student\n"
"Enter 'U' to edit the details of a single student\n"
"Enter 'S' to see the details of a particular student\n"
"Enter 'A' to see the details of every student\n");
you should have a menu option to quit the program
reading a single character with scanf("%c", &input) is tricky: scanf() will read the pending newline from a previous call. You should use scanf(" %c", &input) where the space will cause pending whitespace to be read and discarded.
if (input == 'A' || 'a') does not test for A or a, it compares input to 'A' and if different compares 'a' to zero, hence the test is always true. You should write:
if (input == 'A' || input == 'a')
gets(array[i].name); is a NO NO. Don't use gets(), throw away the book that tells you to use it. For consistency with the other inputs, you can use
scanf("%19[^\n]", array[i].name);
Note that 19 tells scanf() the maximum number of characters to store into array[i].name before the null terminator, preventing undefined behavior for longer user input, and [^\n] causes scanf() to read and store characters up to and not including the newline character. %19s would stop at any white space, preventing the input of multiple words such as James Bond.
scanf("%f", &(*(array + (roll + 1))->marks)); is a very contorted way to write:
scanf("%f", &array[roll + 1].marks);
you should check the return value of scanf() to detect invalid input and flush the pending input on error.
Here is a modified version:
#include <stdio.h>
struct student {
int rollno;
char name[20];
float marks;
};
int main() {
struct student array[60] = { 0 };
int len = sizeof(array) / sizeof(*array);
int c, i, roll;
char input;
for (;;) {
printf("What you would like to do:\n"
"Enter 'R' to fill the details of all the student\n"
"Enter 'U' to edit the details of a single student\n"
"Enter 'S' to see the details of a particular student\n"
"Enter 'A' to see the details of every student\n"
"Enter 'Q' to quit the program\n");
if (scanf(" %c", &input) != 1)
break;
if (input == 'A' || input == 'a') {
for (i = 0; i < len; i++) {
printf("Roll no.: %d\nName: %s\nMarks: %f\n",
array[i].rollno, array[i].name, array[i].marks);
}
} else
if (input == 'R' || input == 'r') {
for (i = 0; i < len; i++) {
printf("Roll no. os student is: %d\n", i + 1);
printf("Enter student's name:\n");
if (scanf("%19[^\n]", array[i].name) != 1)
break;
printf("Enter students marks:\n");
if (scanf("%f", &array[i].marks) != 1)
break;
}
if (i < len) {
printf("Invalid input\n");
}
} else
if (input == 'S' || input == 's') {
printf("Enter the roll no. of the student of whom you want to see the details:\n");
if (scanf("%d", &roll) == 1 && roll >= 1 && roll <= len) {
printf("Roll no.: %d\nName: %s\nMarks: %f\n",
roll, array[roll - 1].name, array[roll - 1].marks);
} else {
printf("Invalid Roll no\n");
}
} else
if (input == 'U' || input == 'u') {
printf("Enter the roll no. of the student of whom you want to see the details:\n");
if (scanf("%d", &roll) == 1 && roll >= 1 && roll <= len) {
printf("Enter the name:\n");
scanf("%19[^\n]", array[roll - 1].name);
printf("Enter the marks:\n");
scanf("%f", &array[roll - 1].marks);
} else {
printf("Invalid Roll no\n");
}
} else
if (input == 'Q' || input == 'q') {
return 0;
} else {
printf("Invalid entry\n");
printf("Re-enter your input\n");
}
/* read and discard the rest of the input line */
while ((c = getchar()) != EOF && c != '\n')
continue;
}
printf("Premature end of file\n");
return 0;
}