checking the end of a c string array [closed] - c

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Im trying to find the end of a word for example school and at the end of a word put a s on to it.
Here is what I have so far:
for (int i = 0; i < 10; i++) {
plural[i] = orig[i];
if (plural[i] == NULL) {
plural[i] = 's';
plural[i + 1] = '\0';
}
}

Answer 1

Your code may function of the string in orig is less than 10 characters long and NULL is defined as 0.
But note that NULL is used to represent the null pointer, not the null byte at the end of a string. NULL can be defined this way:
#define NULL ((void*)0)
In this case, your code would generate a warning upon compilation.
It is considered good style to write '\0' for the null byte at the end of a string. The null character constant has the same value, 0 with type int, but is more explicit for the purpose of representing the null byte.
You should test if orig[i] is non null instead of iterating to 10:
char orig[] = "world";
char plural[10];
int i;
for (i = 0; orig[i] != '\0'; i++) {
plural[i] = orig[i];
}
plural[i] = 's';
plural[i + 1] = '\0';

Answer 2

You can determine the position of null-terminator ('\0') by the following:
int len;
for(len = 0; cstr[len]; ++len);
This is a possible minimal implementation of strlen which is stands for to determine the length of a char array. In example:
#include <stdio.h>
int main() {
char cstr[10] = "Test";
size_t len;
for(len = 0; cstr[len]; ++len);
if(sizeof(cstr) > len + 1) {
cstr[len++] = 's';
cstr[len] = '\0';
}
printf("%s\n", cstr);
}
Note: As David C. Rankin mentioned in comments, you have to protect the array bounds. Knowing that this is an array, you can read its size with the sizeof operator.

Answer 3

The most important part of this exercise is to insure you learn to protect your array bounds. If you declare an array of char [10], then the longest string it can hold is 9-chars + the nul-byte. If you plan to add a character to the end (e.g. 's'), then that means the original string can be no longer than 8-chars.
If you declare plural as:
char plural[10] = ""; /* note the initialization to all `0` */
then the maximum number of characters that can be held in plural in order to use plural as a string is sizeof plural - 1 chars (*preserving space for the nul-byte). So you can set a max for the length of your string with:
char plural[10] = ""; /* note the initialization to all `0` */
int max = sizeof plural - 1;
Then after you find your original string length, you can validate that there is sufficient room for the nul-byte, e.g.
if (len >= max) { /* validate room for 's' available */
fprintf (stderr, "error: adding 's' will exceed array size.\n");
return 1;
}
Putting all the pieces together in a short example, you could do something similar to the following:
#include <stdio.h>
int main (int argc, char **argv) {
char plural[10] = "", *def = "school";
int len = 0,
max = sizeof plural - 1;
if (argc == 1) { /* if no argument given, copy def to plural */
char *p = def;
for (int i = 0; *p && i < max; i++, len++)
plural[i] = *p++;
}
else /* otherwise copy argv[1] to plural */
len = snprintf (plural, max, "%s", argv[1]);
if (len >= max) { /* validate room for 's' available */
fprintf (stderr, "error: adding 's' will exceed array size.\n");
return 1;
}
plural[len] = 's'; /* add 's' - (nul-terminated via initialization) */
printf ("original : %s\nappended : %s\n", argc > 1 ? argv[1] : def, plural);
return 0;
}
Example Use/Output
$ ./bin/plural
original : school
appended : schools
$ ./bin/plural 12345678
original : 12345678
appended : 12345678s
$ ./bin/plural 123456789
error: adding 's' will exceed array size.
note: if you are more comfortable with array indexes than with pointer arithmetic, you can use the following equivalent statement for the length finding and copy:
if (argc == 1) /* if no argument given, copy def to plural */
for (int i = 0; def[i] && i < max; i++, len++)
plural[i] = def[i];
else /* otherwise copy argv[1] to plural */
len = snprintf (plural, max, "%s", argv[1]);
Look things over. There are many, many different ways to approach this. A normal addition would be to include string.h and use strlen and strcpy or memcpy instead of a loop to find your length or copy characters to plural (note: for long strings memcpy will be more efficient, but for 10 char -- it makes no difference) Let me know if you have any questions.