Error in for-loop due to non-constant expression initialization

Question

I am trying to implement insertion sort in GLSL, but there is an error in the for-loop when compiling the shader:

three.min.js:592 THREE.WebGLShader: gl.getShaderInfoLog() ERROR: 0:254: 'j' : Loop index cannot be initialized with non-constant expression

for (int i = 0; i < n - 1; i++)
{
    for (int j = i+1; j > 0; j--)
    {
        if (angle[j - 1] > angle[j])
        {
            float temp = angle[j - 1];
            angle[j - 1] = angle[j];
            angle[j] = temp;
        }
    }
}

Any idea how to solve this problem?

Answer 1

Use bubble sort instead of insertion sort which does not include a non-constant expression in the for-loop. Code is written below.

float temp = 0.0;

for (int write = 0; write < n; write++)
{
    for (int sort = 0; sort < n - 1; sort++)
    {
        if (angle[sort] > angle[sort + 1])
        {
            temp = angle[sort + 1];
            angle[sort + 1] = angle[sort];
            angle[sort] = temp;
        }
    }
}