Displaying an input backwards

I am reading the book C Primer Plus, the 5th edition, in order and I'm at the end of chapter 6 where they have some review questions. I'm having trouble with this question:

Quote:

---

Write a program that reads in a line of input and then prints the line in reverse order. You can store the input in an array of char; assume that the line is no longer than 255 characters. (Recall that you can use scanf() with the %c specifier to read a character at a time from input and that the newline character (\n) is generated when you press the Enter key.)

---

For the "line of input," I'm assuming it takes both letters and numbers.

Here is what I have so far:

Code:

---

#include <stdio.h>
#include <string.h>
int main(void)
{
char line;
int letters;
char display[letters];

printf("enter a line:\n");
scanf("%s", line);

letters = strlen(line);

while (letters > 0)
{
printf("%s", display[letters]);
--letters;
}

return 0;
}

---

When compiling, I got the warning: "first.c:10: warning: passing arg 1 of `strlen' makes pointer from integer without a cast" and when I try to run the program, I get the error: "Segmentation fault (core dumped)."

Obviously I did it wrong (I'm not following the directions in the parentheses of the question), but anyway, after I got this far, I am stuck on what I should do and I have no clue as to how to go about doing this. Any pointers in the right direction would be appreciated; thanks.

line is defined as a character - a one byte character.

char line;
int letters;
char display[letters]; // PROBLEM 1,

printf("enter a line:\n");
scanf("%s", line);

letters = strlen(line); //PROBLEM 2, strlen takes a pointer, not one char. strlen of one char is one.


Its not over yet, but that will get you a little closer. Post back what you see next.

Code:

---

char display[letters];

---

Is this a standard declaration yet? You want something like char display[255];
This will make an array with 255 char elements.

Code:

---

while (letters > 0)
{
printf("%s", display[letters]);
--letters;
}

---

This is the right basic idea, but you have a lot of problems with syntax. display[letters] is a single char. %s is used to print a null terminated string. Also, notice that this loop will not print display[0], something you probably want to do.

As you can probably tell, I'm pretty new at this (about 24 hours into learning C) so I'm trying very hard to understand from what I already know of the language.

I was trying to make scanf take the line from the input as a string so each character in the string is placed in a block of an array size of however many characters were entered in the input + 1 (for the null block), then make strlen count the number of blocks (characters) (not including the null block), then assign it to the variable "letters." That was the first step and I knew it was wrong but I didn't know what to do but I tried to move on to the next step anyway, which was take the highest numbered block (the last character of the letters array), print that first, then go down 1 block, print that, etc with the 'display[letters]' thing.

I'm still trying to make out what's wrong based on the advice provided so far k

Code:

---

char line;
int letters;
char display[letters]; 

//you can't declare an array like this. you either hard code it ( display[255] ), or you research pointers and learn how to use malloc.

printf("enter a line:\n");
scanf("%s", line);  // scanf %s will read until you hit enter, but line is a variable that will only store one char, so you need to change this to be scanf("%s", display) because display will hold more than one character.

letters = strlen(line);  // the length of one character is one, you need to say strlen(display) if you want the string

while (letters > 0)  // you want to print all characters, so while letters >= 0
{
printf("%s", display[letters]);
--letters;
}

return 0;
}

---

that'll give you a little more direction. however, you might want to pick your variable names better
Example:
you declared the variable line as one character and display as multiple characters. I would have done something like:

char OneChar
char EntireLine[255];
int NumberOfChar;

NumberOfChar = strlen(EntireLine)

"//you can't declare an array like this. you either hard code it ( display[255] ), or you research pointers and learn how to use malloc."
1st comment: Ah, no wonder. malloc is in the very last chapter of this book, and I haven't learned much about pointers yet except the incredibly basic basics.

"// scanf %s will read until you hit enter, but line is a variable that will only store one char, so you need to change this to be scanf("%s", display) because display will hold more than one character."
2nd comment: This is much simpler to understand if I set display[255] instead of trying to create an array for the exact number of characters that's input

"// the length of one character is one, you need to say strlen(display) if you want the string"
3rd comment: Same as respond for 2nd comment

"// you want to print all characters, so while letters >= 0"
4th comment: I saw that and realized my mistake and fixed it, thanks

If you only have 24 hours or so of C experience, you probably want to start even more basic. String handling is not something you should delve into it without some good foundations in other aspects of the language.

I'm reading the book in order, and I've already done most of the questions with the exception of a few like these. I'm pretty sure after reading chapter 6, that the reader should be able to do the review questions of the chapter, which include everything before it, but nothing that wasn't introduced already. I think the questions are asking, based upon the knowledge the reader knows so far because the book is intended for people without any knowledge of C beforehand or even any programming experience at all (which not might be recommended by other people not to begin with C as their first language). But whatever the chapters have gone over, I think the questions are supposed to be meant for that

Anyway, so far this is what I understand:

Code:

---

#include <stdio.h>
#include <string.h>
int main(void)
{
char line; // declares line as a 1 byte character
int letters;

char display[255]; // sets an array of 255 blocks

printf("enter a line:\n");
scanf("&#37;s", display); // sets the character(s) input 'display' as a string

letters = strlen(display); // counts the number of characters in the string 'display' and makes the variable 'letters' that amount as an integer

while (letters >= 0) // while the integer 'letters' has more than 0 characters left, (block 0 is the first character)
{
printf("%c \n", display[letters]); // output starting from the largest block (the last letter) (I had to change %s to %c because I want to display the character, not a whole string)
--letters; // move down 1 block then repeat until after block # 0 then the loop ends
}
return 0;
}

---

Initially I was just making it real complicated because I was trying to make the array the exact size of the word input first instead of just putting 255 like I was supposed to
At first it didn't work because I had %s instead of %c in the printf function to display. Now I understand and it looks like it works perfectly!

Thanks to all (I know this is all simple stuff for everyone else :p )

You should be printing from index (letters-1), not letters. You're currently trying to print the '\0' char first. Also, you're not using the variable line.

you may want to add some indentation
remove unused variable line
replace comments to the C-style /* */ (// coments are not standard)
and add explicit cast of the return value of strlen - because it returns size_t value - not int

Code:

---

#include <stdio.h>
#include <string.h>
int main(void)
{
        int letters;
       
        char display[255]; /* sets an array of 255 blocks */
       
        printf("enter a line:\n");
        scanf("&#37;s", display); /* sets the character(s) input 'display' as a string */
       
        letters = (int)strlen(display); /* counts the number of characters in the string
                                    * 'display' and makes the variable 'letters'
                                    * that amount as an integer
                                    * we need cast to int to be able to run loop till
                                    * letters >= 0
                                    */
       
        while (letters >= 0) /* while the integer 'letters' has more than 0 characters left, (block 0 is the first character)*/
        {
                printf("%c \n", display[letters]); /* output starting from the largest character (the last letter) (I had to change %s to %c because I want to display the character, not a whole string)*/
                --letters; /* move down 1 character then repeat until index 0 (including) then the loop ends */
        }
        return 0;
}

---

So is this okay for the code?

Code:

---

#include <stdio.h>
#include <string.h>
int main(void)
{
          int letters;
          char display[255];

          printf("enter a line:\n");
          scanf("&#37;s", display);

          letters = (int)strlen(display);
          letters -= 1;

          while (letters >= 0)
          {
                    printf("%c", display[letters]);
                    --letters;
          }

          printf("\n");

          return 0;
}

---

Quote:

---

Originally Posted by guitarscn

So is this okay for the code?

---

Yes.

Just make a mental notice for the future (I do not see a need to deep in this detail right now) that using

scanf("%s"

is insecure. It means user can enter more then 255 characters on the same line broking your over data...

Yea, someone earlier was talking about pointers and malloc or something so that probably wouldn't limit it to 255 chars like this one. But thanks, at least the problem is now solved. This isn't that much of a realistic program anyway since it's just practice for the reader of the book to challenge themselves

Much better now. Just a couple of suggestions:

I think a for loop would be preferred over a while in this case.
Now that everything works, why don't you try the problems suggestion of reading the string in 1 char at a time? Although you probably won't ever actually read from stdin in this fashion, a similar technique will probably be used when you get to file i/o.
Learn about better ways to read strings. I like using ^ with scanf (read the link in Elysia's signature), but most users here probably prefer some sort of getline function.

Maybe set those aside for now. Based on your initial attempt, I think you should spend some time on the material you've already learned before trying to go through the book too fast. I wouldn't recommend doing more than 1-2 chapters per day, especially as you get to mre advanced topics.

And in my opinion, C is probably the best first language. Sure, maybe the syntax is more difficult than say python and the standard library isn't as big as java's, but C is the most versatile and powerful language there is, and most importantly it will teach you good programming practices. C doesn't hold your hand so it's your own responsibility to make sure you don't do something stupid.

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.

Quote:

---

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

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)

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.

Quote:

---

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.

Hmmm...this is what I could get

Code:

---

#include <stdio.h>
#include <string.h>
int main(void)
{
        int line = 0; /* start at the first array slot */
        char count[255]; /* max 255 characters */

        printf("enter a line: ");

        do
        {
                scanf("%c", &count[line]);      /* user enters character starting with first array slot */
                ++line;                          /* keep going (++line) until '\n' becomes true or */
        } while (printf("\n") == 0, line < 255); /* exceed 255 characters */

        for (line = line - 1; line >= 0; --line) /* start at the last spot (excluding the '\n') */
                printf("%c", count[line]);      /* output all characters backwards from each array spot (--line) */

        printf("\n");

        return 0;
}

---

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)