C++ float array initialization

Question

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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:

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

float studentGrades[ MAX_STUDENTS ] = { 9.9 };

Gave the interesting result:

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

score 48 · Accepted Answer · edited Jun 05 '20 at 12:20

48

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

edited Jun 05 '20 at 12:20

Jayen

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answered Oct 08 '12 at 22:29

Ed S.

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Don't forget that you can use an empty initialization list, in which case *all* elements are set to zero. – Mark Ransom Oct 08 '12 at 22:30
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. – Joseph Mansfield Oct 08 '12 at 22:34
1

@sftrabbit: Yep, it does sound that way doesn't it? Thanks. – Ed S. Oct 08 '12 at 22:37

score 5 · Answer 2 · answered Oct 08 '12 at 22:14

5

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

answered Oct 08 '12 at 22:14

Marcelo Cantos

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C++ float array initialization

2 Answers2

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