macOS My test array problem.

[larswik]

In Short: I am learning array's so I did this test. The goal was to generate a random number from 1-100. Then divided that by 10 so 67 would be 6 as an example. Then with a for loop fill the array with 6 'X''s and then print it delivering 6 X's.

The code works but it prints 1 more X then it should. I tried to fix this by changing the for loop from i=0 to i=1. But when I do that it stops printing the X's when I run the program?

#include <stdio.h>
#include <time.h>
#include <stdlib.h>

int main (int argc, const char * argv[]) {

	int guess, toPrint, i, terminate; // my int's
	char array[11];	//my array

	srand( clock() ); //my random seed
	
	guess = (rand() % 100) + 1; // my random num 1-100
	printf("The random number was %d\n",guess); // test to tell me what num it is
	
	toPrint = (guess / 10); // rounds the rand num ie. 67 would be 6
	printf("The rounded is %d\n",toPrint); //tests to see if the nuber is right
	
	terminate = (toPrint + 1); // adding 1 number above my toPrint for array terminate
	printf("The terminate array num is %d\n",array[terminate]); //Test this number
	array[terminate] = '\0'; //set my 0 in my array to 1 number higher then toPrint.
	
	for(i=0; i<=toPrint; toPrint--){ 
		array[toPrint] = 'X'; // adds an 'X' to each Char array 
		printf("The i counter is %d and toPrint is %d\n",i,toPrint); //tests this
	}
	printf("%s", array); // prints the X'x
	printf("\n");

    return 0;
}

I know the code might look funky and not what you guys would write. But this I understand with what I have learned so far in the book. What am I doing wrong?

Thanks

 

[larswik]

I think I am trying to populate my array backwards and that is my problem. I'm rewriting it a little to fix this and have my for loop use the i counter to increment instead decrementing the toPrint.

-Lars

 

[itickings]

Since it looks like you're actually interested in learning, I'm just giving you some relevant things to think about...

Lesson #1
The two most important thing to remember about arrays is that they begin at 0, and that you must be very careful to not try to use any more positions than is allocated.

If you allocate an array with char array[3], it will have the positions array[0], array[1] and array[2]. The initial values for array[0], array[1] and array[2] can be totally random. You cannot trust positions that you haven't assigned values.

Lesson #2
A standard string is an array of chars, terminated by a zero. It is printed from the character at the first position and onwards, until the zero-terminator is found. If the first position, in this case array[0], contains the terminator, then it doesn't matter what the remaining positions contain.

When constructing a string of this kind, it is usually a splendid idea to construct it from the first character and onwards, and then making sure that the terminating character is the one following directly after the last character that is to be displayed.

Lesson #3
The customary use of for is more like

for (i=0; i<toPrint; i++ )

or

for (i=0; i<=toPrint; i++ )

Consider the difference between < and <= in this case, what implications will picking one over the other have? They are both useful, but in different situations.

Your use of the loop is more like the other common loop, while.

 

[larswik]

I solved it, but would like a little clarification. It seems that char array[0] must contain a value or you can't print the %s for the array. By starting to populate from array[1] it was causing the problem and would not print. Is this because array[0] had no value. But when I wrote this code: array[0] = 'x'; then it worked just fine populating from array [1] and up or rewriting the value of array[0] even after I passed the value of 'x' to it.

So I guess my question is: If a char array has no value assigned to like the beginning of it 'array[0]' but the rest of the array has values assigned to it, is the empty array [0] treated like the array termination, 0?

the new code:

#include <stdio.h>
#include <time.h>
#include <stdlib.h>

	
	int main (int argc, const char * argv[]) {
		
		int guess, toPrint, i; // my int's
		char array[11];	//my array
		
		srand( clock() ); //my random seed
		
		guess = (rand() % 100) + 1; 
		printf("The random number was %d\n",guess); 
		
		toPrint = (guess / 10); 
		printf("The rounded is %d\n",toPrint); 
		
		array[0] = 'x';
		
		for(i=1; i<=toPrint; i++){ 
			array[i] = 'X'; 
			printf("The i counter is %d and toPrint is %d\n",i,toPrint); 
		}
		array[--i] = 0;
		
		printf("%s", array); 
		printf("\n");
		
		return 0;
	}

 

[itickings]

So I guess my question is: If a char array has no value assigned to like the beginning of it 'array[0]' but the rest of the array has values assigned to it, is the empty array [0] treated like the array termination, 0?

Sort of. As I wrote in my previous post, unassigned array values are not empty, and you can't trust them. They contain values, just not values set by you. If array[0] happens to contain a termination, then it won't print anything.

 

[larswik]

Cool - thanks itickings. we were cross posting and that is what I came up with for my for loop too. The question I was not sure of was if array[0] was left blank, with no value assigned to it, is that also treated as the zero-terminator?

-Lars

 

[ianray]

Cool - thanks itickings. we were cross posting and that is what I came up with for my for loop too. The question I was not sure of was if array[0] was left blank, with no value assigned to it, is that also treated as the zero-terminator?

-Lars

If by 'left blank' you mean unassigned, then, the value in an unassigned memory location cannot be pre-determined.

A 'string' is an array of characters which by convention ends with a NUL terminator (ASCII zero, 0, or character literal '\0'). There are other conventions for storing strings (for example Pascal strings) but they are irrelevant here.

Loops operating over strings typically iterate while the condition "*s" is true -- note that the condition is a contraction of "*s != 0" which may also be written "*s != '\0'".

// gcc -W -Wall print.c
#include <stdio.h>

void printString(char const* s)
{
    printf("printf (%s)\n", s);
    printf("loop   (");
    while (*s) {
        putchar(*s++);
    }
    printf(")\n");
}

int main()
{
    {
    char const* pointer = "Hello";
    printString(pointer);
    printString(pointer + 1);
    }

    {
    char array[2];
    array[0] = '!';
    array[1] = 0;
    printString(array);
    printString(array + 1);
    }

    return 0;
}