K&R Programming in C: EX 3.3 - c

Question

EX 3.3: Write a function expand(s1,s2) that expands shorthand notations like a-z in the string s1 into the equivalent complete list abc...xyz in s2 Allow for letters of either case and digits, and be prepared to handle cases like a-b-c and a-z0-9 and -a-z. Arrange that a leading or trailing - is taken literally.
I'm trying to solve exercise 3.3 in K&R and this is what I have:
void expand(char s1[], char s2[]){
int i; // index for first string
int j; // index for 2nd string
for(i = 0, j = 0; s1[i] != '\0'; ++i, ++j){
if(isalnum(s1[i]) && s1[i+1] == '-'){
char c = s1[i];
for(char c = s1[i]; c <= s1[i+2]; ++c, ++j){
s2[j] = c;
}
++i;
} else{
s2[j] = s1[i];
}
}
s2[j] = '\0';
}
It succesfully expands any range, as long as it is not after any other range, ie. it doesn't add anything to s2 after the first range is finished. If I put this statement:
printf("%c\n", c);
in the second for loop, it prints out the right characters, but it doesn't add it to s2.
Sample inputs & outputs:
In: akls aldio a-h 19 aodk
Out: akls aldio abcdefgh
In: 0-6 a-c lol
Out: 0123456
In: a-c-g 1okd 2-4
Out: abc
Can anybody point me in the right direction to fix my mistake?
Thank you.

Answer 1

After your inner for loop, j is one past where it should be, so you skip writing to one position. If that position happens to contain the value 0, it terminates the string and you don't see anything after it.
Also, i is one before where it should be.
Replace this:
for(char c = s1[i]; c <= s1[i+2]; ++c, ++j){
s2[j] = c;
}
++i;
With this:
for(char c = s1[i]; c <= s1[i+2]; ++c, ++j){
s2[j] = c;
}
i+=2;
j--;

Answer 2

As, suggested by dbush, you need to do one j--, but for cases like a-c-g to work you need to change your inner for condition too. Instead of checking c <= s1[i+2], you need to check only till c < s1[i+2]
void expand(char s1[], char s2[]){
int i; // index for first string
int j; // index for 2nd string
for(i = 0, j = 0; s1[i] != '\0'; ++i, ++j){
if(isalnum(s1[i]) && s1[i+1] == '-'){
char c = s1[i];
/* Do it c < instead of c<= */
for(char c = s1[i]; c < s1[i+2]; ++c, ++j){
s2[j] = c;
}
--j; /* Decrement j once */
++i;
} else {
s2[j] = s1[i];
}
}
s2[j] = '\0';
}
P.S: This code solves OP's immediate issues at hand (i.e the inputs that was shown in the original post). But, this code fails for inputs like a-b-c-. Also additional code is needed for inputs like abc-DEF, abc--def, abc-456, etc. However, as seen in comments (of this answer) OP wants to solve those issues himself.

Answer 3

As sps and dbush already made it work, let me add my two cent, too.
You should keep your stuff simple and readable if possible. For example: you load too much into your loops, its not the IOCCC. Increment/decrement only the index you defined in the first part of the for-loop. That would be the variables i and c in your case. The iterations over the vectors s1 and s2 should be done as close as possible to the vectors.
That gives:
void expand(char s1[], char s2[]){
int i;
int j;
char c;
for(i = 0, j = 0; s1[i] != '\0'; i++){
if(isalnum(s1[i]) && s1[i+1] == '-'){
for(c = s1[i]; c < s1[i+2]; c++){
s2[j++] = c;
}
i++;
} else{
s2[j++] = s1[i];
}
}
s2[j] = '\0';
}
Doing it that way gets rid of the otherwise necessary correction j--
Test it:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(int argc, char **argv)
{
char s[200];
char s2[200];
int i;
memset(s,0,200);
memset(s2,0,200);
// put some spaces in between the arguments
for(i=1;i<argc;i++){
// counting ommitted!
strcat(s2,argv[i]);
s2[strlen(s2)] = ' ';
}
printf("In: %s\n",s2);
expand(s2,s);
printf("Out: %s\n",s);
exit(EXIT_SUCCESS);
}
$ gcc -W -Wall -std=c11 expand.c -o expand
./expand 0-9 ASD a-z QWE a-ch-r
In: 0-9 ASD a-z QWE a-c-r
Out: 0123456789 ASD abcdefghijklmnopqrstuvwxyz QWE abchijklmnopqr
Oh, and the ++i et al. in the third part of the for loops: we have 2016 now.