infoabout.txt

<stdmng.c>
1.Doubt no.1 :Why the Hell fgets not taking any input?
Code:
    //student input
    printf("Enter Roll no :");
    scanf("%d", &s.id);

    printf("\nEnter Name : ");
    fgets(s.name, sizeof(s.name), stdin);

Solution :The problem is arrising because of the scanf function.
The scanf() function takes an input lets's say, 
                Enter Roll no : 1 <Enter>
After inputting the value i hit enter to store the vale in the variable s.id.
NOW, Here the complexity begins. the hitting of "Enter" creates a Newline 
character(\n)--------------> '1' , '\n'

Then, the '1' will be stored in the variable but the newline character(\n) 
can not be stored in to variable s.id due to %d format specifier, Hence can 
not be consumed by scanf. and and and NOW, the newline character will be 
stored in the BUFFER.
After this, Next printf will be compiled and them fgets will be compiled anf 
our problem will arrise.
because, the fgets function reads a string till newline character or reaches
Buffer limit, Here we already have a Newlie character due to our honourable
scanf function, then our fgets function reads it and returns a new line 
character instead of actual inputs.

2.What is Buffer?
Ans : A buffer is a temporary storage area in memory where data is held
before it's processed.
    *When you type something and press enter.
    *The Entire Line (Including \n) goes into input buffer.
    *The functions like fscanf() or fgets() read data from this Buffer.

3. WHY getchar()?
Ans : getchar() function is used to read one character at a time.
Here getchar() used due to clear the input Buffer of  the newline character.
to resolve the problem of **DOUBT NO.1**
Code:
    //student input
    printf("Enter Roll no :");
    scanf("%d", &s.id);

    printf("\nEnter Name : ");
    getchar();//Eats up the \n and clear the input buffer.
    fgets(s.name, sizeof(s.name), stdin);


4.Why we dont need getchar() after using fgets? although i pressed enter 
after the input in fgets, the enter shoulf generate a newline character naa?
Ans : we dont have to use getchar() after fgets() because, the fgets() 
function already reads the newline character and the newline character(\n) 
never goes to input buffer and hence next fgets() functions will not face any 
problems. for EXAMPLE,
            printf("Enter name : ");    
            fgets(Array, size, stdin);------> Jishu <ENTER>

            what it reads ------------------> "Jishu\n"

This just creates some unwanted errors like, missing of some characters etc.

<viewallstd function>
1.What is the use of viewallstd function?
Ans : This function is used to see the available data of the students in the file "students.txt",
in the terminal.
so, we have to use the functions like,
                                    1.fgets(): Which will fetch the data from the from the file.
                                    2.printf(): Display the available data in the terminal.
2. CODE :

        char str[sizeof("student.txt")];
        fgets(str, sizeof("student.txt"), fp);
        what's wrong with this?
Ans : The syntax of fgets()is : fgets(string, size of the string, FILE*ptr);

                                string : const char *NAME;: A character pointer. or simply a string.where the fetch data 
                                will be stored.
                                size of the string : size of how long a string should be fetched from the file.
                                FILE*ptr : file pointer to the file.

    NOW, in the code i declared the size of the str character array as,
                                                                    char str[sizeof("student.txt")];
                                                                    i thought it will just take up 
                                                                    the size of the actual content
                                                                    of the file "student.txt".
    But, i was wrong, it will not take the size of the content inside the "student.txt" instead it will take the size of the
    length of the string "student.txt", which is 12 with NULL termianed character, Which will only be able to fetch data 
    upto one line only or 12 character long string.
    and this fault is also done in the fgets, where i have to put the size of how many character it should fetch or read 
    from the file. i put **fget(...,sizeof("student.txt"),...)** same to same fault. it will read only 12 character long 
    string.

3. When the file pointer fp == NULL happens?
Ans : The file  pointer fp == NULL happens when the file could not be opened successfully.
this occurs in the fopen() function, the fopen()function returns the NULL in the following situations :
        1.File does not exist.
        2.permission denied.
        3.invalid path or file name.

4.How does fgets() function work?
Ans : Used to read a LINE OF TEXT from a FILE or INPUT STREAM.
it reads atmost n-1 characters, stores them in a character array, 
and appends a NULL terminated character at the end of the array to make it a proper c string.
SyNTAX:
        char *fgets(char *str, int n, FILE *stream);

        str : ponter to a character array where the string will be stored.
        n : Maximum number of characters to read, Including the Null terminated character.
        stream : Read from. (like stdin for keyboard input or file pointer to a file for the file content input.) 

5.When fgets() function returns NULL ?
Ans : fgets() function can only return NUll value in two main reasons :

        1.End Of the File(EOF) is reached.
        2.An error occurs while reading the file.

6. s.name[strcspn(s.name, "\n")] = '\0' What is this?
Ans : ok, i want to store the data of one student in one line but due to the fgets() function, it generates a \n character after every call to create a perfect string.thats why the data is stored in multiple lines.
to solve this problem we have to remove the \n character from the end of the string inserted by the fgets().
to do that we have to take help of a function which can give us the position of the \n character. and then we can just exchange the value of that position with '\0' a nullc character. and the \n will be removed.

                        size_t pos = strcspn(s.name, "\n");//takes the position where the "\n" is available in the str string
                        s.name[pos] = '\0';// "\n" replaced with '\0' character, "\n" is REMOVED.

        s.name[strcspn(s.name, "\n")] = '\0'// this is the one-liner of the above code. this is called helper box.


1. What sscanf() function returns?
Ans : The sscanf() function returns, no. of the read characters/string/integers, 
for example : 
                sscanf(line,"%s %s %s", name, lastnam, dep);
                it will read 3 strings in the same order in the syntax from the 
                line named buffer, and will return 3 if successfull.
                if not successfull it will return 0.
                if the buffer limit is reached it will return -1.

2.How the stdsearch works?
Ans : 
                        1.while(fgets(line, sizeof(line), fp) != NULL)
                fgets() reads 1 line at a time from "student.txt" through fp untill the file content ends.

                        2.int parsed = sscanf(line, "%d %s %s %s %s", &roll, name, lastname, dep, course);
                this line stores the sscanf return value which should be 5.if the line reaad is a valid file.
                          parsed = 5;

3.STORY OF THIS FUNCTION.
ANS : i didnot write the code like this before,
instead of, int parsed = sscanf(line, "%d %s %s %s %s", &roll, name, lastname, dep, course);
        if (parsed == 5)
        i did,
        if(sscanf(line, "%d %s %s %s %s", &roll, name, lastname, dep, course) == 5)
        {
                //code
        }
        i changed it because it was showing some unwanted errors like,
        not showing of studentnot found.
        and i debugged it and found out,
        that it was due to some invisible garbage values which are present in the "student.txt".
        when the code is run before, the sscanf fails due to that unexpected line and due to failing the error was coming.
        then i solved it by storing the value of the sscanf in the parsed and i wrote the  it worked because.


1.CODE : 
while (fgets(line, sizeof(line), fp1) != NULL)
    {
        int parsed = sscanf(line, "%d %s %s %s %s", &s.id, s.name, s.lastname, s.dep, s.course);
        if (parsed == 5)
        {
            if (s.id == n)
            {
                // Enter Updated Details
                printf("\nEnter new Name : ");
                scanf("%s", s.name);

                printf("\nEnter new LastName : ");
                scanf("%s", s.lastname);

                printf("\nEnter new Department : ");
                scanf("%s", s.dep);

                printf("\nEnter new Course : ");
                scanf("%s", s.course);

                fprintf(fp2, "%d %s %s %s %s\n", s.id, s.name, s.lastname, s.dep, s.course);
                m = 1;
            }
            else
            {
                //this is for the entire input of the file system.
                fputs(line, fp2);
            }
        }
    } work step by step.
    Ans : 
    1st iteration : fgets() Checked, if (parsed == 5) Checked,if (s.id == n) Checked
    2nd iteration : fgets() Checked, if (parsed == 5) Checked,if (s.id == n) not Checked, goes to else.
    3rd iteration : fgets() Checked, if (parsed == 5) Checked,if (s.id == n) not Checked, goes to else.
    4th iteration : fgets() Checked, if (parsed == 5) Checked,if (s.id == n) not Checked, goes to else.
    5th iteration : fgets() Checked, if (parsed == 5) Checked,if (s.id == n) not Checked, goes to else.
    and this logic writes the whole fp1 content in fp2 content.