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

Last update on August 08 2025 12:52:53 (UTC/GMT +8 hours)

9. Replace Specific Line in File

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:

For more Practice: Solve these Related Problems:

Write a C program to replace all occurrences of a specific word in a file with another word.
Write a C program to replace a particular line in a file based on a search string, not just by line number.
Write a C program to update a file by replacing the content of a specified line with user input and then display the modified file.
Write a C program to replace multiple lines in a file by mapping each old line to a new line using a configuration file.

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C Programming Code Editor:

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