C Programming: caesar cipher for lower case letters only

[Travercraig]

I have this code which uses the caesar cipher encryption method based on a users input, so the user enters the text then they enter the amount of times they want to shift the text.

 

However I was wondering how I could make it so only the lower case letters would be shifted as specified.

 

I was thinking of adding something like this:

if (input[i] >= 97 && input[i] <= 122)

But i'm not sure where to add it in my program which is this:

#include<stdio.h>  #include<string.h>  int main()  {        int rotate, i;        char input[80];        printf(" Enter the text you wish to be encrypted: \n");        gets(input);	        printf(" Enter the amount of times you want to rotate: \n");  	        scanf("%d",&rotate);  		        if (rotate > 26)      rotate = rotate % 26;  //0 <= rotate < 26.        for(i = 0; i < strlen(input); i++)        {            if(input[i] == ' ')           {            continue;        }       else        {             if(input[i] >= 'x')             {              input[i] = input[i] -26;              }             input[i] = input[i] + rotate;            }       }	       printf("Your encrypted text is: %s\n", input);  }

Could anyone advise me on this? I need only lower case letters to be caeser ciphered.


 

[2FA]

One thing I will point out is that you shouldn't use gets. It causes memory leaks and is a security risk. Use fgets instead.

[Travercraig]

One thing I will point out is that you shouldn't use gets. It causes memory leaks and is a security risk. Use fgets instead.

I did try that but it just gives me errors. gets seems to work

[2FA]

I did try that but it just gives me errors. gets seems to work

The formatting with fgets is bit different.

[Travercraig]

The formatting with fgets is bit different.

could you explain? I know gets is like:

gets(arrayname);

[2FA]

fgets(name of array, size of array, stdin);

The stdin indicates that is scanning standard input. You have to  specify that because fgets can also be used to read from files.

 

Read more here: http://www.dummies.com/how-to/content/how-to-use-the-fgets-function-for-text-input-in-c-.html

[MeshFile]

Put the condition before the

input[i] = input[i] + rotate;

[Travercraig]

 

Put the condition before the

input[i] = input[i] + rotate;

Ahh I see, thank you.

[Stuff_]

Using your gets, what happens if you input more than 80 characters? Run it, and check. You'll see something really odd about the output of your program. 

 

This is why you DO NOT use gets.

[Nineshadow]

I did this some time ago .

void cypher(char *str, int offset){    for(int i = 0; i < strlen(str); i++)    {        if (islower(*(str+i)))        {            *(str+i) = ((26 + ((*(str+i) - int('a')) + (offset % 26))) % 26) + int('a');        }        else        {            /*Whatever*/        }    }}

strlen() is from string.h .

islower() is from ctype.h

 

And of course, you use it something like :

   char c[100];   fgets(c,100,stdin);   cypher(c, 13);   printf("%s",c);

[SteveGrabowski0]

If you're doing that for a class your grader might shit all over it for being nonportable, as you can't assume consecutive letters are represented by consecutive integers.