Creating a top-hat distributed random number generator

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I have this Fortran code which generates a flat distribution as it produces a single random number centered on 0. The function GRNDM (Geant 4 random number generator) produces equally distributed random numbers between the values of 0 and 1. RDUMMY is the name of the vector filled with the random number and the argument “1” states the length of the vector: i.e. GRNDM here will produce a single random number between 0 and 1. The second line then produces random numbers in the interval [μ−σ2,μ+σ2].

I was wondering if there was a way of changing it to produce random numbers with a top hat distribution?