C++ float array initialization

Question

Possible Duplicate:
C and C++ : Partial initialization of automatic structure

While reading Code Complete, I came across an C++ array initialization example:

float studentGrades[ MAX_STUDENTS ] = { 0.0 };

I did not know C++ could initialize the entire array, so I've tested it:

#include <iostream>
using namespace std;

int main() {
    const int MAX_STUDENTS=4;
    float studentGrades[ MAX_STUDENTS ] = { 0.0 };
    for (int i=0; i<MAX_STUDENTS; i++) {
        cout << i << " " << studentGrades[i] << '\n';
    }
    return 0;
}

The program gave the expected results:

0 0
1 0
2 0
3 0

But changing the initialization value from 0.0 to, say, 9.9:

float studentGrades[ MAX_STUDENTS ] = { 9.9 };

Gave the interesting result:

0 9.9
1 0
2 0
3 0

Does the initialization declaration set only the first element in the array?

Solution 1

You only initialize the first N positions to the values in braces and all others are initialized to 0. In this case, N is the number of arguments you passed to the initialization list, i.e.,

float arr1[10] = { };       // all elements are 0
float arr2[10] = { 0 };     // all elements are 0
float arr3[10] = { 1 };     // first element is 1, all others are 0
float arr4[10] = { 1, 2 };  // first element is 1, second is 2, all others are 0

Solution 2

No, it sets all members/elements that haven't been explicitly set to their default-initialisation value, which is zero for numeric types.

Comments

[Mark Ransom]

Don't forget that you can use an empty initialization list, in which case all elements are set to zero.

[Ed S.]

@MarkRansom: Yep yep, I'll add that, thanks.

[Joseph Mansfield]

You make it sound like 0 is a special case, but it's not. It's setting the first element to 0 and then the rest of them to 0.

[Ed S.]

@sftrabbit: Yep, it does sound that way doesn't it? Thanks.