a second function key 28 for changing the function of each key to that indicated above the key a menu key 29 for selecting a menu panel of each function a quit key 30 for quitting the selected function or mode and the like the second function key 28 is pressed before a stroke of a desired key for example after the stroke of the second function key 28 the calc mode key works as a solver mode key for solving an expression and the stat key works as a stat graph mode key for displaying a statistical graph fig2 is a system block diagram of the function calculator this system comprises the display section 1 and the input section 2 as described above a central processing unit cpu 3 for controlling the function calculator a read only memory rom 4 and a random access memory ram 5 connected to the cpu 3 the display section 1 has not only the lcd 11 but also an lcd driver 12 for controlling the driving of the lcd 11 the lcd driver 12 is controlled by the cpu 3 the input section 2 detects the stroke of each key that is the input through each key the data having been written in the rom 4 are for example expressions for calculating the process capability index i e a one sided specification cpk and a two sided specification cp as described below.