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C Exercises: Decrypt a previously encrypted file

C File Handling : Exercise-14 with Solution

Write a program in C to decrypt a previously encrypted file.

 Assume that, the content of the file test.txt was  :                                                                       
������Ʉ�ӄۗ�������ɒ��ђn

Sample Solution:

C Code:

#include <stdio.h>
#include <stdlib.h>

void main()
{
	char ch, fname[20];
	FILE *fpts, *fptt;
	
	printf("\n\n Decrypt a text file :\n");
	printf("--------------------------\n"); 	
	
	printf(" Input the name of file to decrypt : ");
	scanf("%s",fname);	
	
	fpts=fopen(fname, "w");
	if(fpts==NULL)
	{
		printf(" File does not exists or error in opening..!!");
		exit(7);
	}
	fptt=fopen("temp.txt", "r");
	if(fptt==NULL)
	{
		printf(" File does not exists or error in opening..!!");
		fclose(fpts);
		exit(9);
	}
	while(1)
	{
		ch=fgetc(fptt);
		if(ch==EOF)
		{
			break;
		}
		else
		{
			ch=ch-100;
			fputc(ch, fpts);
		}
	}
	printf(" The file %s decrypted successfully..!!\n\n",fname);
	fclose(fpts);
	fclose(fptt);
}

Sample Output:

 Decrypt a text file :                                                                                        
--------------------------                                                                                    
 Input the name of file to decrypt : test.txt                                                                 
 The file test.txt decrypted successfully..!! 
After decrypt, the content of the file is : 
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Flowchart:

Flowchart: Decrypt a previously encrypted file

C Programming Code Editor:

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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