why error?

Question

static void Main(string[] args)
{
int[ ] a = new int[10];
for (int k = 5; k < 12; k++) {
a[k] = k*2;
}
for (int x = 0; x < 10; x++) {
Console.WriteLine(a[x]);
}
}

Answer 1

Your array only contains 10 elements, from 0 - 9, your first for loop tried to assign a value to an element that doesn't exist, because the loop was designed to start from 5 - 11.

Hth, cmiiw

Answer 2

@William, your loop can start from any number, but in your first loop case, when k is incremented to 10 that's when error happens, your array only has elements from index 0 - 9, there's no 10th and 11th element.

Hth, cmiiw

Answer 3

This is your array (a) of ten elements.

-----------------------------------------------
Index | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
----------------------------------------------
Value | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
-----------------------------------------------
^ ^ ^ ^ ^ # #

Then you use for loop to assign values to array elements, starting from index 5 until 11.

for(int k = 5;k < 12;k++)

The loop succeeded to assign values to elements from a[5] until a[9] (marked by ^), but the loop didn't stop there, it continued to try assigning values to a[10] and a[11] (marked by #) which failed, because array (a) only had ten elements, with index from 0-9.

P.S. Your most recent changed code will work just fine.

Hth, cmiiw

Answer 4

its a rule of arrays to start at 0, like a list the first number is the first, not the fifth.

Answer 5

thanks. but why it cannot start from 5? why it must start from 0? then how i can create a array with first number is 10?

Answer 6

the index should start from 0, but the value could from any number. I just don't know how make it start from numbers not equal to 0.

Answer 7

i find the answer. maybe it's because of [k].it make k play double roles. 1: index; 2: help to create element value.
so if i want to create an array start from other numbers, maybe i should declare another int besides k.

Answer 8

i find the answer. maybe it's because of [k].it make k play double roles. 1: index; 2: help to create element value.
so if i want to create an array start from other numbers, maybe i should declare another int besides k. or like below.

static void Main(string[] args)
{
int[ ] a = new int[10];
for (int k = 0; k < 10; k++) {
a[k] = k*2+10;
}
for (int x = 0; x < 10; x++) {
Console.WriteLine(a[x]);
}
}