Thread: Padding string with string

Have you considered using strcat() or (if you remember the need to add a character with value zero as terminator) memcpy() in a loop?

Here is the strcat function from K&R, for reference. Or you can use strcat from the string.h header. It's problem is that it assumes the copied to string (s in the code below) is big enough to contain itself and the thing it's copying. You might be able to fix that with strlen though. Here:

Code:

void strcat(char s[], char t[])   
{       

    int i, j;
    i = j = 0;      

    while (s[i] != '\0')           
        i++;       

    while ((s[i++] = t[j++]) != '\0');  
}

Edit: Also if you want the strings to truly look concatenated, don't forget about newlines. It's not practical to replace the newlines with '\0' on the string being copied into, you might try spaces though. Good luck.

I tried strcat but it doesn't work. It had the same output as 3.

Can someone explain why this does not work?

Code:

j=0;
for(i=0;i<strlen(a);i++){
        bNew[i]=b[j];
        j++;
        if(j==strlen(decode)-1) j=0;    
}

Values of i and j during for loop show that it should work.

Code:

i= 0    j= 0
i= 1    j= 1
i= 2    j= 2
i= 3    j= 3
i= 4    j= 4
i= 5    j= 5
i= 6    j= 6
i= 7    j= 7
i= 8    j= 8
i= 9    j= 9
i= 10   j= 10
i= 11   j= 11
i= 12   j= 12
i= 13   j= 13
i= 14   j= 14
i= 15   j= 15
i= 16   j= 16
i= 17   j= 0
i= 18   j= 1
i= 19   j= 2
i= 20   j= 3
i= 21   j= 4
i= 22   j= 5
i= 23   j= 6
i= 24   j= 7
i= 25   j= 8
i= 26   j= 9
i= 27   j= 10
i= 28   j= 11
i= 29   j= 12
i= 30   j= 13
i= 31   j= 14
i= 32   j= 15
i= 33   j= 16
i= 34   j= 0
i= 35   j= 1
i= 36   j= 2
i= 37   j= 3

It doesn't look like you are copying string a.

Edit: Nevermind, just looked at the desired output again. I thought you were trying to copy both strings into bnew[].

Out of curiosity, what was the maximum value of I?

In my actual program I have:

Code:

char encode[256]="\0";
char decode[256]="\0";
char encrypt[256]="\0";
char encryptPad[256]="\0";

So the max value of i could be less than or equal to 255.


The output I posted was for a random phrase. Heres output of i,j for provided test data.

Code:

i= 0    j= 0
i= 1    j= 1
i= 2    j= 2
i= 3    j= 0
i= 4    j= 1
i= 5    j= 2
i= 6    j= 0
i= 7    j= 1
i= 8    j= 2
i= 9    j= 0
i= 10   j= 1
i= 11   j= 2
i= 12   j= 0
i= 13   j= 1
i= 14   j= 2
i= 15   j= 0
i= 16   j= 1
i= 17   j= 2
i= 18   j= 0
i= 19   j= 1
i= 20   j= 2
i= 21   j= 0

Originally Posted by Alpo

Thanks for replying, I wasn't sure if <strlen of an array of 255 would be 255, or 254.

It's neither. strlen() computes the number of non-zero characters that precedes the first character with value zero. The zero is referred to as a sentinel (a marker indicating something - in this case "end of string").

No, strlen() does not count the zero character (but it always gets that far, since it counts until it finds a zero character).

If the array has no elements with value zero, then strlen() exhibits undefined behaviour (anything can happen). In practice, it'll often keep searching through a random area of memory (unrelated to your array, but happens to be located in memory after the array) until it happens to find a character with value zero. That means it can give a much larger value than the length of your array. If the host system detects your program reading memory it shouldn't, then your program will be terminated with prejudice. In worst case, if there is no character with value zero in memory and the host system doesn't detect your program running rampant, eventually a size_t will overflow and be reset to zero (since size_t is an unsigned integral type), so counting will resume - that means an infinite loop.

I is 253 with the array all full (254 counting 0). I've gotten lucky to have my program crash whenever I've gone off the deep end of an array before. I read it can screw the memory of nearby programs if your computer doesn't detect it, and you are writing to it.

Code:

 main()
 #include <stdio.h>
 #include<string.h>

 #define ARRSIZE 255

 main()
 {
     int i;
     char arr[ARRSIZE];
     memset(arr,'A',ARRSIZE-1);

     for(i=0; i < strlen(arr); ++i){
        printf("%d\n", i);
     }
     printf("%s", arr);
 }

Your compiler happens to have initialised arr with zero bytes, so your strlen() call (and the printf() call) work. That is not guaranteed. You just got lucky .... with your compiler.

I thought memset initialized the array? Do I need to do it differently?

I got how to use it from here: C 'memset' Examples | Lainoox . There was another site that looked better, but it was giving a certificate warning.

Originally Posted by Alpo

I thought memset initialized the array? Do I need to do it differently?

memset() only changes what it is told to change. In your example, the array was of length ARRSIZE, and memset() was told to set the values of ARRSIZE-1 characters. The last character remains uninitialised.