Thread: Displaying an input backwards

The book didn't say anything about ^ or getline yet, so those weren't really an option I guess (yet). I was thinking of reading through the entire book, then going through the entire book again (and again, etc. as necessary). Or do you think it's better to carefully study one chapter at a time as much as possible?

Maybe I'm tired...I can't find a way to do the actual question...is it saying to do scanf("%c", line) then declare char line[255] or something like that? I don't know how this can read a character at a time like we just did..

Yes, I would definitely go slowly and learn everything very well before moving on. You will have a hell of a time with the more advanced topics unless you have a very solid understanding of the basics.

This is the best I could come up with:

Code:

#include <stdio.h>
#include <string.h>
int main(void)
{
        int line;
        char count[255]; /* set a max array of 255 characters */

        printf("enter a line: ");
        for (line = 0; ; ++line)          /* start at array block 1, let the user input a character, */
                scanf("&#37;c", count[line]); /* continue input with next block (++line) until press Enter */

        for (line = 255; line >= 0; --line) /* start at the last character block and go down to the first */
                printf("%c", count[line]); /* print all characters on the whole array going backwards (--line) */

        printf("\n");

        return 0;
}

I can compile it fine, but when I try the program, after I input a line and press Enter, it gives me a "Segmentation fault (core dumped)" error.

Code:

        for (line = 0; printf("\n") != 1; ++line) /* start at array block 1, as long as you don't press enter, */
                scanf("%c", count[line]);         /* let the user input a character on the next block (++line) */

if you want safly read line use

Code:

if(fgets(count, sizeof count, stdin) == NULL)
{
   /* failed to read line - notify and exit */
}
else
{
   /* line read */
   char* p = strchr(line, '\n');
   if(p)
   {
      /* new line char found - remove it */
      *p = '\0';
   }
   /* here process the line as you need */
}

Ah, these chapters have not yet covered 'if' and 'else' stuff yet. Also definitely not covered strchr and not even stdin either, I think. I know what NULL is so that's fine even though the book did not teach to use it yet. Also, I am not familiar with the char* and *p stuff either.

The question is asking for the answer only using the very basics that have been convered in the chapters so far...no doubt anyone here can probably come up with a very sweet and short solution, but I guess it's pretty weird to just use the basic stuff.

Originally Posted by guitarscn

Ah, these chapters have not yet covered 'if' and 'else' stuff yet. Also definitely not covered strchr and not even stdin either, I think. I know what NULL is so that's fine even though the book did not teach to use it yet. Also, I am not familiar with the char* and *p stuff either.

The question is asking for the answer only using the very basics that have been convered in the chapters so far...no doubt anyone here can probably come up with a very sweet and short solution, but I guess it's pretty weird to just use the basic stuff.

In that case - I will stick with the original version with scanf and leave the advanced stuff till you learn the corresponding functions

Your first loop has no control condition. Here, a while loop would be more appropriate.
Here are the general guidelines on choosing a loop (though there may be exceptions):

for loop: The most commonly used. Use the for when you know exactly how many times the loop will run.
for example, when you want to print the string one character at a time, you know how many characters are in the string, so it will loop once for each one

Code:

for (i=length-1; i>=0; --i)
{
     printf ("%c", &string[i]);
}

Use a while loop when you don't know how many times the code will loop. When you are reading in characters, you have no idea what I will type before I hit enter. You just have to keep reading in characters until I hit enter (until you encounter the character '\n')

The do...while loop is bottom tested, so it will run at least one time. The regular-ass while loop is top tested and it may run zero or more times. Don't forget the semicolon in the do...while loop

Code:

do
{ ...
} while (condition);

Damn... I missed a lot! I must type too slowly.

Originally Posted by guitarscn

I am pretty satisfied with the way the original solution came out - but NeonBlack did make a suggestion that I try to follow the instructions in the beginning, which I did not follow to get my current solution. So basically I'm trying to solve this all over again, but following the directions which the question states. (Reading the input string 1 character at a time)

In this case you need to fix your condition of the for loop

Here is what I have so far:

Code:

#include <stdio.h>
#include <string.h>
int main(void)
{
        int line = 0;     /* start at array block 1 */
        char count[255]; /* set a max array of 255 characters */

        printf("enter a line: ");
        while (line < 255)                /* start at array block 1, as long as you don't press enter, */
        {                                 /* and don't go over 255 characters, */
                scanf("&#37;c", count[line]); /* let the user input a character on the next block (++line) */
                ++line;                   /* and store the input character to the corresponding array block number */
        }

        for (line = line - 1; line >= 0; --line) /* start at the last character block inputted and go down to the first */
                printf("%c", count[line]); /* print all characters on the whole array going backwards (--line) */

        printf("\n");

        return 0;
}

Still gives me a "Segmentation fault (core dumped)" error after I press Enter after my input

Closer. I kept trying to give you this hint: When you hit enter, the newline character '\n' will be put into count[line]. So you also have to check that the character you've just read in is not '\n'. What I said about do...while was another hint. You can't check the character until after you've read it in, so I would go with the bottom tested do...while (you also need to read at least one character).
Also, you scanf should be

Code:

scanf ("%c", &count[line]);

Notice the & infront of count[line] (yes, I forgot it in my example). Just remember this right now. You will understand the reason when you learn more about pointers.

well, you made some good attempts.
try

Code:

while (count[line-1] != '\n' && line<255);

You see, if the character just read in was the enter key, or if we read in too many characters, the loop will break.
I don't know if you know this yet, && is the logical AND, so both conditions must be true for the loop to continue.

Wow, that's pretty nifty! (It worked flawlessly btw).

At this point I'm not sure if it's my lack of knowledge or the ability to think outside the box, but I wish I could have thought of that!

It's always after seeing the solution you feel like slapping yourself on the head for not thinking of that, because you knew all the content but just couldn't recall the pieces together!

As you learn to program, you not only learn solutions to problems, you also learn how to think of your own solutions to new problems. Just keep at and in no time you'll look back on this and wonder how you could have been so blind to the obvious solution.
That's all for tonight, I'm going to bed. (I haven't been able to sleep since we changed to daylight-saving time)