Function to encrypt a string using pointers

Question

I need to write a function that gets a string and a number N, the function will return in the same pointer the encrypted string. The function will encrypt a string by the following rules:

Reverse the string, e.g: if the string is "Amnon" the result would be "nonmA".
After the reverse, each letter needs to be replaced by the value of N, e.g: if N=3 then instead of "nonmA" the result would be "qrqpD".

I had no problem doing the reverse part but I'm struggling with switching each letter. Here is the code I wrote so far:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void StRreverse(char* mystr, int N);

void StRreverse(char* mystr, int N)
{
    int c, length, n;
    char *begin, *end, temp;
    length = strlen(mystr);
    begin = mystr;
    end = mystr;
    for (c = 0; c < length - 1; c++)
    {
        end++;
    }
    for (c = 0; c < length / 2; c++)
    {
        temp = *end;
        *end = *begin;
        *begin = temp;
        begin++;
        end--;
    }
}
void main()
{
    char string[100];

    printf("Enter some text\n");
    gets(string);

    StRreverse(string);
    printf("Reverse of entered string is \"%s\".\n", string);
    system("pause");
}

void main() should be int main(void) and never use gets as it is dangerous because it doesn't prevent buffer . Use fgets instead. — Spikatrix
– Spikatrix, Commented Mar 29, 2015 at 9:55
You need to pass two arguments to StReverse(), but you give it just one argument. — Spikatrix
– Spikatrix, Commented Mar 29, 2015 at 9:56
What would the encrypted string be if the string to be encrypted is "XYZ" and N=3? — Spikatrix
– Spikatrix, Commented Mar 29, 2015 at 10:00

Jongware · Accepted Answer · 2015-03-29 11:14:41Z

2

First, identify 'letters' in a portable way:

#include <ctype.h>

Then you can use isalpha(mystr[c]) to generally identify letters, and specifically islower and isupper. Then, on each letter, add your constant modulo 26. That is, if a becomes b, then y becomes z and z would be a again.

This operation is encoding-dependent as not all encodings have "a" to "z" and "A" to "Z" defined consecutively. Fortunately, it's unlikely you have such a system :) (although it is an interesting exercise to work out a way where this would not be an issue!).

The trick is to 'wrap around' with an offset of either A or a, so capitals and lowercase need a separate line of code:

if (isupper(mystr[c]))
   mystr[c] = 'A' + ((mystr[c]-'A' + N + 26) % 26);
if (islower(mystr[c]))
   mystr[c] = 'a' + ((mystr[c]-'a' + N + 26) % 26);

where N can be as low as -25, or as high as you want. That lower limit (and the addition of 26 in the statements) is because in some implementations of C, taking the modulus of a negative number returns a negative number as well.

To decode this text -- properly called the Caesar Cypher --, you can apply the same formula with -N.

answered Mar 29, 2015 at 11:14

Jongware

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2 Comments

Amnon Hanuhov Over a year ago

I have not yet studied isupper and islower so I assume I am not allowed to do that, is there any other method?

Amnon Hanuhov Over a year ago

I think without using these functions the code should look like this: for (c = 0; c<length; c++) { if ((mystr[c] >= 'A') && (mystr[c] <= 'Z')) { mystr[c] = 'A' + ((mystr[c] - 'A' + N + 26) % 26); } if ((mystr[c] >= 'a') && (mystr[c] <= 'z')) { mystr[c] = 'a' + ((mystr[c] - 'a' + N + 26) % 26); } }

Yasir Majeed · Accepted Answer · 2015-03-29 12:20:55Z

0

The most simple way to do switching is by adding the number to that character. For example

`char str[n] = {'a','b','c','d'};

for(int i=0; i less than n; i++) str[i] = str[i] +3; // we are adding 3 to switch `

I dont remeber the ascii number for small 'a'. But if its 36 the above code will make it 39 which will be 'd'. Hope it works for u

answered Mar 29, 2015 at 12:20

Yasir Majeed

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