How to define the number of elements in an array with user input of type int?

Question

I am a beginner. I'm trying to solve a pair-sum program. The program calls for the number of pairs being added and the actual pairs, then it returns the sum of each pair. I'm using an array to store the sum of each pair, but when I print out every element the last one is always wrong. This is what I have so far:

int c, val1, val2, x, i;

printf("Enter the number of pairs to sum: \n");
scanf("%d", &c);
int sum [c];
printf("Enter the pairs: \n");
for ( x = 0; x < (c - 1); ++x)
{
    scanf("%d", &val1); 
    scanf("%d", &val2);
    sum [x] = val1 + val2;
    val1 = 0;
    val2 = 0;
}

printf("The sum of each pair is: \n");
for (i = 0; i < c; ++i)
{
     printf("%d\t", sum[i]);
}

Incidentally I don't see why you need to care how many numbers there are...you could solve this by outputting the sum of each pair right after the user types it in. — nneonneo
– nneonneo, Commented May 13, 2014 at 22:09
@nneonneo he wants to not display the output until after all input has been entered — M.M
– M.M, Commented May 14, 2014 at 6:00

gsamaras · Accepted Answer · 2014-05-13 22:11:48Z

1

Change this

for ( x = 0; x < (c - 1); ++x)

to this:

for ( x = 0; x < c; ++x)

You are going until (c-1), where you print c times.

So, after the fix, your loop will be like this:

printf("Enter the pairs: \n");
for ( x = 0; x < (c - 1); ++x)
{
    scanf("%d %d\n", &val1, &val2 );
    sum [x] = val1 + val2;
    val1 = 0;        // Do you really need me?
    val2 = 0;        // and me?
}

You do not need to nullify val1 and val2 at the end of every loop.

Think the first time you enter the loop. These variables are uninitialized, but they get initialized by scanf().

Let's say the user typed 0 and 1, so your variables got these values respectively.

Then, you set them to zero.

Then, you go again inside the loop and the variables get assigned the input of the user.

And so on...

Think how this would work, if you would discard this step:

Then, you set them to zero.

It would be the same!

The only difference is that when you exit the loop, these variables will have what the user typed for the last run of the loop and not the zero values, but I think that's not a problem. :)

edited May 13, 2014 at 22:11

answered May 13, 2014 at 22:06

gsamaras

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4 Comments

M.M Over a year ago

There would be a difference if he types some words instead of numbers during the number input stage (all the number reads would fail and so it would use the last value set).

gsamaras Over a year ago

@MattMcNabb I do not think that this is what (s)he tried to do. I would say that (s)he thought he needed that for the input procedure as numbers.

Lucas Ranieri Over a year ago

Turns out something about scanf("%d %d\n... was messing up what was being recorded in each element of the array. (c - 1) was also incorrect, I don't know why I thought that would work. Thanks for the help!

gsamaras Over a year ago

@sushiknives, nice that you worked out it by yourself. This means that you tried, I am going to up-vote your question. ;)

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