Thread: Detab - K&R Exercise

write a program 'detab' that replaces tabs in the input with
the proper number of blanks to space to the next tab stop.
Assume a fixed set of tab stops, say every n columns. Should
n be a variable or a symbolic parameter?

That is the exercise. Here is my attempt. I'm having some issues not printing the correct number of spaces. Can some take a look and see if they spot the issue? Thank you guys!

Code:

#include <stdio.h>

int stringtointeger(char **str);  /* converts the argument to integer if capable */

int main(int argc,char *argv[]){
    if(argc != 2)
        printf("Usage: %s n [tabstop every n columns]\n",argv[0]);
    else{
        int tabstop,k,c;
        if((tabstop = stringtointeger(argv))){
            if(tabstop > 10)
                printf("Tab stop must be less than 10\n");
            else{
                k = ++tabstop;
                while((c = getchar()) != EOF){
                    if(c == '\t'){
                        for(k;k > 0;--k)
                            printf(" ");
                    }
                    else
                        printf("%c",c);
                    if(k > 0)
                        --k;
                    else
                        k = tabstop;
                }
            }
        }
        else
            printf("Input Error. Make sure you are using an integer for argument\n");
    }
    return 0;
}
int stringtointeger(char **str){    /* returns int if argument is int, else returns 0 */
    int i= 0,c,t = 0;
    
    if((c = str[1][i]) < '0' || c > '9')
        return 0;
    else{
        for(i = 0;((c = str[1][i]) != '\0' && c >= '0' && c <= '9');++i)
            t = (t * 10) + (c - '0');
        if(c != '\0')
            return 0;
        return t;
    }
}

That is fine, brackets are not necessary since there is only one statment in that if. The closing bracket you quoted belongs to the else that starts on line 8.

You're right, it disappears that the end of that else statement (the one that starts on line 8), but it's not a problem since the OP doesn't try to use the variable after that point.

That would make sense, I had copied it to a online compiler to test it out. But now that you mentioned that it makes sense.

Originally Posted by Vespasian

I ran this programme, still not sure what the problem is... It replaces tabs with spaces whith equivalent number of spaces specified by command line argument. is what I see... Is it not supposed to do this?

Code:

$ ./tab 3
                line 1 (2 tabs)
        line 1 (2 tabs)
                line 2 (also 2 tabs)
       line 2 (also 2 tabs)
                line 3 (also 2 tabs)
       line 3 (also 2 tabs)
$ ./tab 3
a       line 1 (tab in the middle)
a   line 1 (tab in the middle)
a       line 2 (also 1 tab in the middle)
a line 2 (also 1 tab in the middle)
a       line 3 (also 1 tab in the middle)
a    line 3 (also 1 tab in the middle)

It does not line them up consistently (note line 1 is over-indented by 1 space in the first example). This is due to the increment issue i mentioned, and the subsequent manipulation of k, which is a bit awkward. Also, it's just way out of wack when you have tabs in the middle of a line, which is valid input, but processed incorrectly. It's not a straight "each tab char is n spaces", tab takes you to the next column that is a multiple of n.

thanks for the help guys. heres my new code.

Code:

/* detab */
/* john wroblewski */
/* jwroblewski44[at]yahoo[dot]com */
    /*write a program 'detab' that replaces tabs in the input with
      the proper number of blanks to space to the next tab stop.
    Assume a fixed set of tab stops, say every n columns. Should
    n be a variable or a symbolic parameter? */
#include <stdio.h>

int stringtointeger(char **str);  /* converts the argument to integer if capable */

int main(int argc,char *argv[]){
    if(argc != 2)
        printf("Usage: %s n [tabstop every n columns]\n",argv[0]);  //if only one argument
    else{
        int tabstop,c;
        if((tabstop = stringtointeger(argv))){
            if(tabstop > 10)
                printf("Tab stop must be less than 10\n");
            else{
                int k = tabstop;
                while((c = getchar()) != EOF){
                    if(c != '\t'){
                        printf("%c",c);
                        if(k > 0)
                            --k;
                        else if(k == 0)
                            k = tabstop;
                        if(c == '\n')
                            k = tabstop;
                    }
                    else{
                        while(k > 0){
                            printf(" ");
                            --k;
                        }
                        k = tabstop;
                    }
                }
            }
        }
    }
    return 0;
}
int stringtointeger(char **str){    /* returns int if argument is int, else returns 0 */
    int i= 0,c,t = 0;
    
    if((c = str[1][i]) < '0' || c > '9')
        return 0;
    else{
        for(i = 0;((c = str[1][i]) != '\0' && c >= '0' && c <= '9');++i)
            t = (t * 10) + (c - '0');
        if(c != '\0')
            return 0;
        return t;
    }
}