How does writing to a file work?

❮ File Methods

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Example

Open the file with "a" for appending, then add some text to the file:

f = open("demofile2.txt", "a")
f.write("See you soon!")
f.close()

#open and read the file after the appending:
f = open("demofile2.txt", "r")
print(f.read())

Run Example »

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Definition and Usage

The write() method writes a specified text to the file.

Where the specified text will be inserted depends on the file mode and stream position.

"a":  The text will be inserted at the current file stream position, default at the end of the file.

"w": The file will be emptied before the text will be inserted at the current file stream position, default 0.

Text files are the normal .txt files. You can easily create text files using any simple text editors such as Notepad.

When you open those files, you'll see all the contents within the file as plain text. You can easily edit or delete the contents.

They take minimum effort to maintain, are easily readable, and provide the least security and takes bigger storage space.

2. Binary files

Binary files are mostly the .bin files in your computer.

Instead of storing data in plain text, they store it in the binary form (0's and 1's).

They can hold a higher amount of data, are not readable easily, and provides better security than text files.

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File Operations

In C, you can perform four major operations on files, either text or binary:

1. Creating a new file
2. Opening an existing file
3. Closing a file
4. Reading from and writing information to a file

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Working with files

When working with files, you need to declare a pointer of type file. This declaration is needed for communication between the file and the program.

FILE *fptr;

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Opening a file - for creation and edit

Opening a file is performed using the

ptr = fopen("fileopen","mode");

2 function defined in the

ptr = fopen("fileopen","mode");

3 header file.

The syntax for opening a file in standard I/O is:

ptr = fopen("fileopen","mode");

For example,

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");

• Let's suppose the file

  ptr = fopen("fileopen","mode");
  

  4 doesn't exist in the location

  ptr = fopen("fileopen","mode");
  

  5. The first function creates a new file named

  ptr = fopen("fileopen","mode");
  

  4 and opens it for writing as per the mode 'w'.
  The writing mode allows you to create and edit (overwrite) the contents of the file.
• Now let's suppose the second binary file

  ptr = fopen("fileopen","mode");
  

  7 exists in the location

  ptr = fopen("fileopen","mode");
  

  5. The second function opens the existing file for reading in binary mode 'rb'.
  The reading mode only allows you to read the file, you cannot write into the file.

Opening Modes in Standard I/OModeMeaning of ModeDuring Inexistence of file

ptr = fopen("fileopen","mode");

9Open for reading.If the file does not exist,

ptr = fopen("fileopen","mode");

2 returns NULL.

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");

1Open for reading in binary mode.If the file does not exist,

ptr = fopen("fileopen","mode");

2 returns NULL.

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");

3Open for writing.If the file exists, its contents are overwritten.
If the file does not exist, it will be created.

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");

4Open for writing in binary mode.If the file exists, its contents are overwritten.
If the file does not exist, it will be created.

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");

5Open for append.
Data is added to the end of the file.If the file does not exist, it will be created.

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");

6Open for append in binary mode.
Data is added to the end of the file.If the file does not exist, it will be created.

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");

7Open for both reading and writing.If the file does not exist,

ptr = fopen("fileopen","mode");

2 returns NULL.

fopen("E:\\cprogram\\newprogram.txt","w");

fopen("E:\\cprogram\\oldprogram.bin","rb");

9Open for both reading and writing in binary mode.If the file does not exist,

ptr = fopen("fileopen","mode");

2 returns NULL.

fclose(fptr);

1Open for both reading and writing.If the file exists, its contents are overwritten.
If the file does not exist, it will be created.

fclose(fptr);

2Open for both reading and writing in binary mode.If the file exists, its contents are overwritten.
If the file does not exist, it will be created.

fclose(fptr);

3Open for both reading and appending.If the file does not exist, it will be created.

fclose(fptr);

4Open for both reading and appending in binary mode.If the file does not exist, it will be created.

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Closing a File

The file (both text and binary) should be closed after reading/writing.

Closing a file is performed using the

fclose(fptr);

5 function.

fclose(fptr);

Here,

fclose(fptr);

6 is a file pointer associated with the file to be closed.

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Reading and writing to a text file

For reading and writing to a text file, we use the functions

fclose(fptr);

7 and

fclose(fptr);

8

They are just the file versions of

fclose(fptr);

9 and

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

0. The only difference is that

fclose(fptr);

7 and

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

2 expects a pointer to the structure FILE.

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Example 1: Write to a text file

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

This program takes a number from the user and stores in the file

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

3.

After you compile and run this program, you can see a text file

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

3 created in C drive of your computer. When you open the file, you can see the integer you entered.

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Example 2: Read from a text file

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

This program reads the integer present in the

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

3 file and prints it onto the screen.

If you successfully created the file from Example 1, running this program will get you the integer you entered.

Other functions like

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

6,

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

7 etc. can be used in a similar way.

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Reading and writing to a binary file

Functions

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

8 and

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

9 are used for reading from and writing to a file on the disk respectively in case of binary files.

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Writing to a binary file

To write into a binary file, you need to use the

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

9 function. The functions take four arguments:

1. address of data to be written in the disk
2. size of data to be written in the disk
3. number of such type of data
4. pointer to the file where you want to write.

fwrite(addressData, sizeData, numbersData, pointerToFile);

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Example 3: Write to a binary file using fwrite()

#include 
#include 

struct threeNum
{
   int n1, n2, n3;
};

int main()
{
   int n;
   struct threeNum num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.bin","wb")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   for(n = 1; n < 5; ++n)
   {
      num.n1 = n;
      num.n2 = 5*n;
      num.n3 = 5*n + 1;
      fwrite(&num, sizeof(struct threeNum), 1, fptr); 
   }
   fclose(fptr); 
  
   return 0;
}

In this program, we create a new file

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

1 in the C drive.

We declare a structure

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

2 with three numbers - n1, n2 and n3, and define it in the main function as num.

Now, inside the for loop, we store the value into the file using

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

9.

The first parameter takes the address of num and the second parameter takes the size of the structure

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

2.

Since we're only inserting one instance of num, the third parameter is

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

5. And, the last parameter

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

6 points to the file we're storing the data.

Finally, we close the file.

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Reading from a binary file

Function

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

8 also take 4 arguments similar to the

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   // use appropriate location if you are using MacOS or Linux
   fptr = fopen("C:\\program.txt","w");

   if(fptr == NULL)
   {
      printf("Error!");   
      exit(1);             
   }

   printf("Enter num: ");
   scanf("%d",&num);

   fprintf(fptr,"%d",num);
   fclose(fptr);

   return 0;
}

9 function as above.

fread(addressData, sizeData, numbersData, pointerToFile);

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Example 4: Read from a binary file using fread()

#include 
#include 

struct threeNum
{
   int n1, n2, n3;
};

int main()
{
   int n;
   struct threeNum num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.bin","rb")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   for(n = 1; n < 5; ++n)
   {
      fread(&num, sizeof(struct threeNum), 1, fptr); 
      printf("n1: %d\tn2: %d\tn3: %d\n", num.n1, num.n2, num.n3);
   }
   fclose(fptr); 
  
   return 0;
}

In this program, you read the same file

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

1 and loop through the records one by one.

In simple terms, you read one

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

2 record of

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

2 size from the file pointed by *fptr into the structure num.

You'll get the same records you inserted in Example 3.

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Getting data using fseek()

If you have many records inside a file and need to access a record at a specific position, you need to loop through all the records before it to get the record.

This will waste a lot of memory and operation time. An easier way to get to the required data can be achieved using

fwrite(addressData, sizeData, numbersData, pointerToFile);

2.

As the name suggests,

fwrite(addressData, sizeData, numbersData, pointerToFile);

2 seeks the cursor to the given record in the file.

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Syntax of fseek()

ptr = fopen("fileopen","mode");

0

The first parameter stream is the pointer to the file. The second parameter is the position of the record to be found, and the third parameter specifies the location where the offset starts.

Different whence in fseek()WhenceMeaning

fwrite(addressData, sizeData, numbersData, pointerToFile);

4Starts the offset from the beginning of the file.

fwrite(addressData, sizeData, numbersData, pointerToFile);

5Starts the offset from the end of the file.

fwrite(addressData, sizeData, numbersData, pointerToFile);

6Starts the offset from the current location of the cursor in the file.

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Example 5: fseek()

ptr = fopen("fileopen","mode");

1

This program will start reading the records from the file

#include 
#include 

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

1 in the reverse order (last to first) and prints it.

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