Methods and apparatus are provided for computing a probability value of a received value in communication or storage systems. A probability value for a received value in a communication system or a memory device is computed by obtaining at least one received value; identifying a segment of a function corresponding to the received value, wherein the function is defined over a plurality of segments, wherein each of the segments has an associated set of parameters; and calculating the probability value using the set of parameters associated with the identified segment. A probability value for a received value in a communication system or a memory device can also be computed by calculating the probability value for the received value using a first distribution, wherein the first distribution is predefined and wherein a mapped version of the first distribution approximates a distribution of the received values and wherein the calculating step is implemented by a processor, a controller, a read channel, a signal processing unit or a decoder.