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There is probably a fairly simple explenation to this, but how do you a variable to maintain a value it gained in a for loop, specifically a character array? I alter the "value" of the array in the loop and I want that value to carry on thorugh out the rest of the program program. Any help is appreciated.
Can you provide a pseudocode example? I have no idea what you're asking. :confused:
This might be a question of scope? Just declare the value outside the loop?
ok, yeah i should have inclluded an example:
when I do something like this, a contains no data as if it were never inisialized.Code:char a[1000];
int x = o;
for(x = 0; X<100; x++){
a = do some stuff/calculations;
}
printf("%s\n", a);
//and then do other stuff with a
//a contains no data outside of the loop, why?
If you are only assigning the value once then there should be no problem. If you want to assign multiple values and reference them at a later time then you need to use a character array of pointers and say the values to that.
I'm not sure what you're actually experiencing. If you give a value inside the loop it stays outside the loop, unless the value you're setting is declared inside the loop because of scope.
Code:itsme@itsme:~/C$ cat alpha2.c
#include <stdio.h>
int main(void)
{
char a[100];
int i;
for(i = 0;i < 99;++i)
a[i] = (i % 26) + 'a';
a[i] = '\0';
puts(a);
printf("a[2] = %c\n", a[2]);
return 0;
}
Code:itsme@itsme:~/C$ ./alpha2
abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstu
a[2] = c
itsme@itsme:~/C$
Alright, how to explain this. I declaire the arrays value outside of the loop and then alter that value inside the loop. What I get inside the loop is the value I want to continue throughout the rest of the program. I think I found a work around but further knowledge would be helpful.
Is it anything like the code I posted? Why do I bother showing example programs?
Does a contain any data inside the loop?Quote:
//a contains no data outside of the loop, why?
I cant get your code to compile, it says there is an unnessisary @.
>>Does a contain any data inside the loop?
Yes. Prior to the loop I assign input to the array. I then alter that input in the array. If I print out the array in the loop it works and prints out the info. If I try to print outside the loop (after it is terminated, it outputs nothing.
Don't include "itsme@itsme:~/C$ cat alpha2.c" in the source code, silly. That's a *NIX command prompt. "cat alpha2.c" just spits the file out to the screen for me to copy and paste.Quote:
Originally Posted by lilrayray
Then may-be you could post some code. The pseudocode you posted doesn't show that.Quote:
Yes. Prior to the loop I assign input to the array. I then alter that input in the array. If I print out the array in the loop it works and prints out the info. If I try to print outside the loop (after it is terminated, it outputs nothing.
here:
Code:#include <stdio.h>
#include <stdlib.h>
void encrypt();
int main(int argc, char *argv[])
{
encrypt();
getchar();
return 0;
}
void encrypt()
{
//
//declaring variables and file initilization
//
FILE *fp;
char file[150000];
char encrypt[150000];
char decrypt[150000];
int count = 0;
int a = 0;
int x = 0;
char key[16] = "aeiluozcgraolifi";
//
//file is opened here and checked for errors
//
fp = fopen("c:\\test.txt", "r");
if(fp == NULL)
{
puts("No such file or directory.");
}
else{
fread(file, 1000, 150, fp);
}
//
//checks for total number of characters in document
//
for(a = 0; file[a] != '\0'; a++)
{
++count;
}
for(x = 0; x < count; x++){
encrypt[x] = file[x] ^ key[x];
}
printf("%s", encrypt);
}
I'm not even sure where to begin on this, but I'll give it a shot:
Where did you come up with 1000 and 150? Doing 1 and sizeof(file) would make a lot more sense. You're also using way more stack space than you need to. You could read the file in in chunks until you reach EOF. encrypt and decrypt could be dynamically allocated so it only takes up as much space as it really needs to.Code:fread(file, 1000, 150, fp);
So...if the file can't be opened you print an error message and don't try to read the file (which is good), but then you go on to to process file as though something had been read into it (which is bad). For this program, it would probably be best to just print the error message and exit the program if the file can't be opened.Code:if(fp == NULL)
{
puts("No such file or directory.");
}
else{
fread(file, 1000, 150, fp);
}
If you use my above suggestion for fread(), fread() will return the number of characters in the file! If you use the tools at hand correctly, things tend to be a lot easier.Code://checks for total number of characters in document
Are you sure encrypt[x] is being set to a printable character? If not, you're going to have an awfully hard time printing it that way.Code:for(x = 0; x < count; x++){
encrypt[x] = file[x] ^ key[x];
}
printf("%s", encrypt);
Hmm, the program compiles when I fix this error:
Other than that there may be better ways to get the size of the file than getting 150000 characters and then counting how many there actually is.Code:char key[16] = "aeiluozcgraolifi";
//27 initializer-string for array of chars is too long
And then
The length of key was only 16. On my particular run the program didn't crash, but since there apparently were 0-bites beyond key, only the beginning of the text got encrypted.Code:for(x = 0; x < count; x++){
encrypt[x] = file[x] ^ key[x];
}
I would use x%KEYSIZE to get the index for key[] in the loop.
Alright, Ill try these suggestions, thanks.
So I gather you're experimenting with XOR encryption? (Sometimes it's easier -- and better -- to state the problem, and/or the "big picture", rather than perceived issues with assumed solutions.) Anyways, a number of your latest posts seems to point me toward that.
And it's been kinda slow, and I've never written one that I can remember, so I decided to play along at home. Here is something to compare with after you've finished or as you continue developing your code. I'm sure others will be along soon to nitpick it as well.
Doing something like this...Code:#include <stdio.h>
/*
* argv[1] - name of input file to encrypt/decrypt
* argv[2] - name of output file
* argv[3] - crypt key
*/
int main(int argc, char *const *argv)
{
if ( argc == 4 )
{
FILE *input = fopen(argv[1], "rb");
FILE *output = fopen(argv[2], "wb");
if ( input != NULL && output != NULL )
{
unsigned char buffer[BUFSIZ];
size_t count, i, j = 0;
do {
count = fread(buffer, sizeof *buffer, sizeof buffer, input);
for ( i = 0; i < count; ++i )
{
buffer[i] ^= argv[3][j++];
if ( argv[3][j] == '\0' )
{
j = 0; /* restart at the beginning of the key */
}
}
fwrite(buffer, sizeof *buffer, count, output);
} while ( count == sizeof buffer );
fclose(input);
fclose(output);
}
}
return 0;
}
You should find that main.c matches file.txt.Quote:
H:\Forums>xorcrypt main.c file.bin key
H:\Forums>xorcrypt file.bin file.txt key
KABLAM.Code:for(x = 0; x < count; x++){
encrypt[x] = file[x] ^ key[x];
}
printf("%s", encrypt);
Surely one of the XOR operations will produce zero, by chance. encrypt[x] is non-zero up to a certain x. You will only see the first x characters printed. After that, well, there goes. printf() will die there and then because it has encountered a zero character.
Listen everyone, it gets better...
And what if no zero character is produced? Then printf() will print all 150,000 bytes of encrypt and perhaps more, until it does reach a zero.
Toodles.
~
so I should use puts()?
No it won't:Quote:
Originally Posted by jafet
It will simply print nothing. A string is defined as zero or more characters, terminated by a null character. If all you have is the null, you have what is considered an empty string. It's still a legal string. Thus, any Standard C function which handles strings, will still be able to handle it.Code:#include<stdio.h>
int main( void )
{
char foo[] = { 0 };
printf("%s", foo );
return 0;
}
So? As long as you're not running off the end of your allocated space, who cares how much it prints? It's still legal, assuming you are in fact staying in bounds of your allocated space.Quote:
Originally Posted by jafet
Quzah.
Yep, so printf only prints part of the string... that's prolly why the OP thought the rest of the data had "vanished"...