A for loop in C using an array

Question

I've created an array named a that can hold 100 double values,

double a[100];

I set the first element of the array a to NUM, which is a symbolic constant defined early in my code.

a[0] = NUM

I'm curious as to how I would write a for loop that sets each remaining value of a to the value of the preceding element plus 0.1. For example, the second element in the array is the first plus 0.1. I've tried doing

for(i=1; i<=99; i=+0.1)

But I think something is wrong with my initialization of i

You need to learn how for loops work.

HelloWorld123456789
– HelloWorld123456789

2014-05-08 18:05:30 +00:00
Commented May 8, 2014 at 18:05 — HelloWorld123456789
– HelloWorld123456789, Commented May 8, 2014 at 18:05

Mephy · Accepted Answer · 2014-05-08 18:03:32Z

2

Use i to index the array, not to store the value you should put on the array. Remember you can use expressions to access the array, like a[i - 1]

for (i = 1; i < 100; i++)
    a[i] = a[i - 1] + 0.1;

answered May 8, 2014 at 18:03

Mephy

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Comments

user3616528 · Accepted Answer · 2014-05-08 18:08:16Z

1

int i;    
for(i = 0; i < 100; i++)
       a[i] = NUM + 0.1 * i;

dont forget to tell the type int !

answered May 8, 2014 at 18:08

user3616528

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1 Comment

dckuehn Over a year ago

You are not allowed to declare the int in the loop. It must be declared before.

dckuehn · Accepted Answer · 2014-05-08 18:02:57Z

0

int i = 0;

for(i = 0; i < 100; i++){
    if (i == 0)
        a[i] = NUM;
    else
        a[i] = a[i - 1] + .1;
}

Your array definition includes the step. So your array would run about 1000 times, at 1, 1.1, 1.2, but a[1.1] isn't a valid index of your array. Use i to index the array, and then retrieve the previous value to set the next.

answered May 8, 2014 at 18:02

dckuehn

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2 Comments

Marc B Over a year ago

a[0] = NUM; for (i = 1...) and save yourself the repeated/pointless if() test.

dckuehn Over a year ago

That would be more efficient, but I chose to be more explicit/verbose for the example.

Alessandro Suglia · Accepted Answer · 2014-05-08 18:13:27Z

0

From your question I can understand that this is one of your first program in C/C++, so I think that you need to start from basic things and learn how to do it properly, before doing it elegantly.

http://ideone.com/RGZgXL

 for(i = 0; i < ARRAY_SIZE; i++) {
     if(i == 0) { // if we are on the first element, set it to NUM
         array[i] = NUM;
     } else { // otherwise make the sum
         array[i] = array[i-1] + STEP;
     }
 }

In the link you'll find the code and some comments that I hope will help you in understanding it.

Cheers

answered May 8, 2014 at 18:13

Alessandro Suglia

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2 Comments

Iguananaut Over a year ago

They should set array[0] first and then loop starting from i = 1; otherwise you end up with a pointless branch in your loop.

Alessandro Suglia Over a year ago

Yes I know it. But in this way the code i more clear because respect how he has described the problem. After that he will be able to fix it :)

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