It's basically a switch statement, but without the benefit of descriptive case identifiers. So it's basically, switch (I-20) { case 21: case 22: ...

It's basically a switch statement, but without the benefit of descriptive case identifiers. So it's basically, switch (I-20) { case 21: case 22: ... } but you have to imagine "21" as the string identifier, not the integer value. `I-20` is indexing the list of labels in the GOTO statement. The earlier column beginning with GOTO (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15) I is easier to read switch (I) { case 1: ... case 2: ... } because the integer value of I, the string labels, and their index in the list are all the same. I've never written Fortran. I did learn to program using TI-85 BASIC, though, and am well versed in C. The GOTO in the Fortran code is possibly the easiest part for me to understand.