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C Exercises: Replace a specific line in a file with a new text

C File Handling : Exercise-9 with Solution

Write a program in C to replace a specific line with another text in a file.

Assume that the content of the file test.txt is :                                                                       
test line 1                                                                                                   
test line 2                                                                                                   
test line 3                                                                                                   
test line 4                                                                                                   

Sample Solution:

C Code:

#include <stdio.h>
#include <string.h>

#define MAX 256

  int main() 
  {
        FILE *fptr1, *fptr2;
        int lno, linectr = 0;
        char str[MAX],fname[MAX];        
        char newln[MAX], temp[] = "temp.txt";
		
		printf("\n\n Replace a specific line in a text file with a new text :\n");
		printf("-------------------------------------------------------------\n"); 
		printf(" Input the file name to be opened : ");
        fgets(fname, MAX, stdin);
        fname[strlen(fname) - 1] = '\0';
        fptr1 = fopen(fname, "r");
        if (!fptr1) 
        {
                printf("Unable to open the input file!!\n");
                return 0;
        }
        fptr2 = fopen(temp, "w");
        if (!fptr2) 
        {
                printf("Unable to open a temporary file to write!!\n");
                fclose(fptr1);
                return 0;
        }
        /* get the new line from the user */
        printf(" Input the content of the new line : ");
        fgets(newln, MAX, stdin);
        /* get the line number to delete the specific line */
        printf(" Input the line no you want to replace : ");
        scanf("%d", &lno);
        lno++;
         // copy all contents to the temporary file other except specific line
        while (!feof(fptr1)) 
        {
            strcpy(str, "\0");
            fgets(str, MAX, fptr1);
            if (!feof(fptr1)) 
            {
                linectr++;
                if (linectr != lno) 
                    {
                        fprintf(fptr2, "%s", str);
                    } 
                    else 
                    {
                        fprintf(fptr2, "%s", newln);
                    }
                }
        }
        fclose(fptr1);
        fclose(fptr2);
        remove(fname);
        rename(temp, fname);
        printf(" Replacement did successfully..!! \n");
        return 0;
  }
  

Sample Output:

Replace a specific line in a text file with a new text :                                                     
-------------------------------------------------------------                                                 
 Input the file name to be opened : test.txt                                                                  
 Input the content of the new line : 2                                                                        
 Input the line no you want to replace : 2                                                                    
 Replacement did successfully..!!   

Flowchart:

Flowchart: Replace a specific line in a file with a new text

C Programming Code Editor:

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Previous: Write a program in C to delete a specific line from a file.
Next: Write a program in C to append multiple lines at the end of a text file.

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C Programming: Tips of the Day

Difference between signed / unsigned char :

There's no dedicated "character type" in C language. char is an integer type, same (in that regard) as int, short and other integer types. char just happens to be the smallest integer type. So, just like any other integer type, it can be signed or unsigned.

It is true that (as the name suggests) char is mostly intended to be used to represent characters. But characters in C are represented by their integer "codes", so there's nothing unusual in the fact that an integer type char is used to serve that purpose.

The only general difference between char and other integer types is that plain char is not synonymous with signed char, while with other integer types the signed modifier is optional/implied.

Ref : https://bit.ly/3esiVjT