John Miller John Miller - 5 days ago 4
C Question

C language-if statement issue

I've created the following program to make a Caesar and Vigenere cipher. I've gotten the Caesar cipher to work correctly, however I cannot get the Vigenere to work properly.

What I want to happen is have my if statements "catch" all the various numbers with an interval of 5. However whenever I run the program my Viegenere cipher outputs the exact same input. I believe this is because I'm making a mistake with my if statements, but i'm not sure what it is.

The beginning of the Viegenere cipher is commented in the code as //Vigenere Cipher-- keyword is "apple"

#include <stdio.h>


int main(){
int i=0;
//setting the individual slot number for the array-- later used in the while loop
char guy[100];
printf("Enter the plain text:");
fgets(guy,100,stdin); //takes user's input-- such as "abc" and puts it into its respective slot in the array guy[10] r-right?

while (guy[i] != '\0'){ //while loop that runs until it reaches the end of the string


if ((guy[i]) >= 'A' && (guy[i]<= 'Z')){ //moves capital letter values up 1
if (guy[i]=='Z'){
guy[i]=guy[i]-25;
}
else {
guy[i]=guy[i]+1; //makes the current "slot" number go up 1 value. Example: a = 97 + 1 -> b = 98
}
}
if ((guy[i]) >= 'a' && (guy[i]) <= 'z'){// moves lower case letter values up 1
if (guy[i]=='z'){
guy[i]=guy[i]-25;
}
else{
guy[i]=guy[i]+1;
}
}
i++; //moves the array's interval up to the next "slot"

}
printf("Encrypted text is: %s\n",guy);

//Vigenere Cipher-- keyword is "apple"
//a = 1 value shift
//p = 16 value shift
//p = 16 value shift
//l = 17 value shift
//e = 5 value shift

printf("Enter the plain text: ");
fgets(guy,100,stdin);//takes user's input

while (guy[i] != '\0'){ //while loop that runs until it reaches the end of the string

if (i%5==0 || i==0){ //checks to see which character it is in the string, for instance the numbers 0,5,10,15,20 should all be added by 1
guy[i] = guy[i]+1;
}

if ((i-1)%5==0 || i==1){ //all numbers that are second in the key word 'apple', such as 1,6,11,16
guy[i]=guy[i]+16;
}
if ((i-2)%5==0 || i==2){// all numbers that are third to the key word 'apple' , such as 2,7,12,17,22
guy[i]=guy[i]+16;
}
if((i-3)%5==0 || i==3){// all numbers that are fourth to the key word 'apple', such as 3,8,13,18
guy[i]=guy[i]+17;
}
if((i-4)%5==0 || i==4){// all numbers that are fifth in the key word 'apple', such as 4,9,14,19
guy[i]=guy[i]+5;
}
else {
i++;
}
}
printf("Encrypted text is: %s\n",guy);
}

Answer

Before encrypting the data again - you should re-initialize the the value of "i" variable

#include <stdio.h>


    int main(){
    int i = 0;
    //setting the individual slot number for the array-- later used in the  while loop
    char guy[100];
    printf("Enter the plain text:");
    fgets(guy, 100, stdin); //takes user's input-- such as "abc" and puts it into its respective slot in the array guy[10] r-right?

    while (guy[i] != '\0'){ //while loop that runs until it reaches the end of the string


        if ((guy[i]) >= 'A' && (guy[i] <= 'Z')){ //moves capital letter values up 1
            if (guy[i] == 'Z'){
                guy[i] = guy[i] - 25;
            }
            else {
                guy[i] = guy[i] + 1; //makes the current "slot" number go up 1 value. Example: a = 97 + 1 -> b = 98
            }
        }
        if ((guy[i]) >= 'a' && (guy[i]) <= 'z'){// moves lower case letter values up 1
            if (guy[i] == 'z'){
                guy[i] = guy[i] - 25;
            }
            else{
                guy[i] = guy[i] + 1;
            }
        }
        i++; //moves the array's interval up to the next "slot"

    }
    printf("Encrypted text is: %s\n", guy);

    //Vigenere Cipher-- keyword is "apple"
    //a = 1  value shift
    //p = 16 value shift
    //p = 16 value shift
    //l = 17 value shift
    //e = 5  value shift
    printf("The value of i = %d\n", &i);
    i = 0;
    printf("Enter the plain text: ");
    fgets(guy, 100, stdin);//takes user's input

    while (guy[i] != '\0'){ //while loop that runs until it reaches the end of the string

        if (i % 5 == 0 || i == 0){ //checks to see which character it is in the string, for instance the numbers 0,5,10,15,20 should all be added by 1
            guy[i] = guy[i] + 1;
        }

        if ((i - 1) % 5 == 0 || i == 1){ //all numbers that are second in the key word 'apple', such as 1,6,11,16
            guy[i] = guy[i] + 16;
        }
        if ((i - 2) % 5 == 0 || i == 2){// all numbers that are third to the key word 'apple' , such as 2,7,12,17,22
            guy[i] = guy[i] + 16;
        }
        if ((i - 3) % 5 == 0 || i == 3){// all numbers that are fourth to the key word 'apple', such as 3,8,13,18
            guy[i] = guy[i] + 17;
        }
        if ((i - 4) % 5 == 0 || i == 4){// all numbers that are fifth in the key word 'apple', such as 4,9,14,19
            guy[i] = guy[i] + 5;
        }
        else {
            i++;
        }
    }
    printf("Encrypted text is: %s\n", guy);
    getchar();
}

Output:-

Enter the plain text:apple Encrypted text is: bqqmf

The value of i = 1900200 - The value of i before re-initializing.

Enter the plain text: apple Encrypted text is: bÇÇ}

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