C program to caesar cipher file to output, help needed

I need to write a program that asks the user for the text file, prints its contents, then asks for the output file, encrypts the contents of the first file with caeser cipher and writes the encryption to the output file. So far I have this code, it writes the contents of the file, asks for the output file, and offset key, it has no errors or trouble compiling but after I run it the output file is still empty! :(

Code:

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


int encode (int, int);


int encode(int ch, int key) {
    if (islower(ch)) {
        ch = (ch-'a' + key) % 26 + 'a';
        ch += (ch < 'a') ? 26 : 0;
    }
    else if (isupper(ch)) {
        ch = (ch-'A' + key) % 26 + 'A';
        ch += (ch < 'A') ? 26 : 0;
    }
    return ch;
}
int main (void)
{
    FILE *file_in;
    FILE *file_out;
    char ch;
    char text[300];
    int key;


    printf("Enter the name of the file you wish to see including complete pathway and file\n extension:\n");
    gets(text);
    file_in = fopen(text,"r");


    if( file_in == NULL )
    {
      perror("Error while opening the file.\n");
      exit(EXIT_FAILURE);
    }
      printf("\n The contents of the file are : \n");
   
    while( ( ch = fgetc(file_in) ) != EOF )
    {
      printf("%c",ch);
    }


    printf("\n \n Enter the name of the file you wish to send the encryption\n to including complete pathway and file\n extension:\n");
    gets(text);
    file_out = fopen(text,"w");
   
    printf("\n Enter the alphabetic offset key you would like to use:");
    scanf("%d", &key);
   
    while( ( ch = fgetc(file_in) ) != EOF )
    {
        encode(ch, key);
        fprintf(file_out, "%c", ch);
    }
    printf("file has been encoded");
    fclose(file_out);
    fclose(file_in);
   
    return 0;
}

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> encode(ch, key);
This returns a result, which you're ignoring.

Oh, and stop using gets()
http://sourceforge.net/apps/mediawik...php?title=Gets

Code:

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char ch;

...

while( ( ch = fgetc(file_in) ) != EOF )

---

fgetc returns an int value which is very important when comparing against EOF.

is there any chance you could write some code to replace it with? I see the errors, i'm just not sure how to fix them!

Quote:

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Originally Posted by SaraTerry

is there any chance you could write some code to replace it with? I see the errors, i'm just not sure how to fix them!

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The problem you're having is caused by not knowing how ch is being used here. The ch in main() is NOT the same as the ch variable in encode() - that is a COPY of the ch from main().

If you want to use the value from encode (and you do), you need to "catch" it. How to catch it: ch = encocde(ch, key). Or, you could send ch as a pointer to the variable. But otherwise, you just "throw away" the value of ch, from encode().

So you have two way you can do this, and no, we won't write your code for you. Understand that C ALWAYS copies paramaters for a function. If the variable is a pointer to a variable, then and only then, will the variable it points to be changed. Otherwise, the copy of that variable will be the only thing that is changed.

so I should change it to
ch=encode(ch, key);
fprintf(file_out, "%c", ch);
I've only covered pointers in files to move from line to line, so i'm not sure about the other way

Quote:

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Originally Posted by SaraTerry

so I should change it to
ch=encode(ch, key);
fprintf(file_out, "%c", ch);
I've only covered pointers in files to move from line to line, so i'm not sure about the other way

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Yes, and understand that the "ch" you are having returned from encode() is a COPY of the "ch" you have in main(), and of course, has been given a different value, ( by encode() ).

Thank you for your help, and I understand that now but my output file is still blank after I run the program!

Quote:

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Originally Posted by SaraTerry

Thank you for your help, and I understand that now but my output file is still blank after I run the program!

---

Code:

---

while( ( ch = fgetc(file_in) ) != EOF )
{
    printf("%c",ch);
}
...
while( ( ch = fgetc(file_in) ) != EOF )
{

---

After reading the input file in your first while-loop the file position indicator is at the end of the file. Thus when you try to read a character in your second while-loop you'll get EOF.

You have to put the file indicator back to the beginning. Look at rewind().

Bye, Andreas