You can download the zip file given in this answer: https://www.mathworks.com/matlabcentral/answers/98293-is-there-a-fixed-step-ordinary-differential-equation-ode-solver-in-matlab-8-0-r2012b#answer_107643 and study the code of ode1.m. It is the implementation of the Euler method provided in very early releases of MATLAB. It is no longer included in MATLAB, but it is still useful to understand the implementation of the Euler method.
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