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I am a novice programmer trying to understand arrays in C. Specifically I want to take the numeric value of a variable and feed it into an array. I tried to assign the value to the array, but failed with error messages. Can someone explain, simplistically, how to push a value into an array and then be able to just access the last digit?

My last attempt:

#include <stdio.h>
unsigned int TMR0 = 158;


int main(void)
{
unsigned int V = TMR0; 
unsigned int Random[2] = {V};
printf("%d \n" , *(Random+2));
return 0;

Thanks.

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    When you say "access the last digit", are you saying you want "158", or do you mean you want "8"? I fear the latter is the case, in which case you need to rewind and learn about integers before you learn about arrays. Commented Feb 10, 2016 at 4:36
  • 2
    *(Random+2) is the same as Random[2] and that invokes UB as you access invalid memory locations beyond the array. Commented Feb 10, 2016 at 4:39
  • You would benefit greatly from a good teaching resource. Buy a PDF or hard copy of C Primer Plus by Steven Prata. Read it cover-to-cover, and do all the examples and exercises at the end of each chapter. Most college students can do this over a long weekend. Commented Feb 10, 2016 at 4:52
  • Could you make the question clearer ? It saves time for others. Commented Feb 10, 2016 at 4:54
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    @paddy - Yes, I am looking to access the last digit '8'. I am new to arrays with only very basic C understanding. Thanks Commented Feb 10, 2016 at 5:11

2 Answers 2

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unsigned int Random[2];

Array will be declared with two positions. 

Random[0] Random[1] // two accessible positions in that array.

When you are assigning the value to the array,

unsigned int Random[2] = { V} ;

Value will be stored in the first position of array. Random[0].

*(Random+2) will access the position Random[2]. Which is not accessible position for this array. It will lead to undefined behaviour.

Another thing is if you need to assign the values to both the position you have to do like this.

unsigned int Random[2] = {V,V} ;

To access the last element in your array,

*(Random+1) or Random[1]

May this link will help.

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5 Comments

I see some of my error, the array needs to be declared as unsigned int Random [3] - to allow for three digits to be stored 1,5 and 8 from variable V. I am looking to access the last digit whose value is 8 (that would be Random[2] since the count begins at [0]. To be clear, V is a variable - can I simply put it in between {} and it will be read into the array?
@chopnhack When you use unsigned int Random[3] = {V};, 158 gets stored in Random[0], not each digit in each array index. For that, you'll have to split the number into digits (using % 10 or some other method) and assign each digit to each slot of the array.
@Cool Guy - ouch, thanks! That was a big misunderstanding for me, I thought each cell in the array was to store only one number (0-9), like a bit!
@chopnhack, Strongly suggest have your instructor/TA explain how data is stored in memory (which should have been explained back in the class that discussed how computers work) also have them explain the hexadecimal number system and how to convert between different number bases, the concept of big and little Endian numeric representation, etc. Just to note: 158 will (with 4 byte/32 bit integers) look like: 0x0000009E or 0x9E000000 depending on the Endian-ness of the underlying hardware architecture.
@user3629249 - no instructor, hacking and self teaching along the way. Thanks!
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You don't actually need an array to access digits of your number -- you just need math.

It's important to realise that "digit" implies a particular numeric base. In the computer, numbers are represented in binary. For convenience, they can be represented in our code using common bases: decimal, hexadecimal, and octal are the ones we generally use in languages like C.

So, to get the last digit in base 10, you would take the value modulo 10:

int val = 158;
int last_digit = val % 10;
printf( "%d\n", last_digit );

If you need to find digits other than the last, you can first perform integer division and then modulo:

int second_to_last_digit = (val / 10U) % 10;

Alternatively you can convert the number into a string, and then look at each character in that string. But I'm not going to go into that here, since it can be confusing to provide too much information to new programmers.

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10 Comments

thanks Paddy! strcopy, etc. was what I thought I was ending up doing by creating the array, I kept messing with the type of data because of errors I received in compiler. I had avoided modulo and division because they call the math library - this is for a small pic mcu with limited word space memory. Can you elaborate on the conversion of the string?
Are you sure? Integer modulo and division shouldn't invoke math processor. Conversion to string will involve much more overhead.
I checked using modulo, it chews up about 63 word spaces, roughly a quarter of my total memory! I think you can see why I would prefer a less taxing approach :-) I have no clue yet how much the string operators will cost.
Do you need all the digits, or just the last?
Hi paddy, yes I only need the last digit.
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