Thread: Displaying an input backwards

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

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.

"//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 )

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;
}

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...

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.