33 lines
1.1 KiB
Matlab
33 lines
1.1 KiB
Matlab
function [gamma] = gamma_armijo(f, grad_f, dk, xk)
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% Calculates the best step based on amijo method
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%
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% f(xk+ γk*dk) ≤ f(xk) + σ * γk * dk^T * ∇f(xk)
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% γk = β*γk_0
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%
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% f: Objective function
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% grad_fun: Gradient function of f
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% dk: Current value of selected direction -∇f or -inv{H}*∇f or -inv{H + lI}*∇f
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% xk: Current point (x,y)
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% beta: beta factor in [0.1, 0.5]
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% signam: sigma factor in (0, 0.1]
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global amijo_beta
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global amijo_sigma
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gf = grad_f(xk(1), xk(2));
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gamma = 1; % Start with a step size of 1
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% Perform Armijo line search
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while f(xk(1) + gamma * dk(1), xk(2) + gamma * dk(2)) > ...
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f(xk(1), xk(2)) + amijo_sigma * gamma * dk' * gf
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%while f(xk(1) + gamma * dk(1), xk(2) + gamma * dk(2)) > ...
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% f(xk(1), xk(2)) + amijo_sigma * gamma * norm(dk)^2
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gamma = amijo_beta * gamma; % Reduce step size
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if gamma < 1e-12 % Safeguard to prevent infinite reduction
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warning('Armijo step size became too small.');
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break;
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end
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end
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end |