For(
TI Basic programming instruction, accessed from Catalog or programming editor.
Usage:
For(A,B,C[,D])...End
Causes all commands from For( to End to be repeated a certain number of times. A is any letter variable, B is any numerical expression, C is numerical expression, and D is numerical expression.
The For loop first loads the value of B into variable A, and then repeats the code until the End tag, adding the value of D (or 1 if D is not specified) to variable A, until A is greater than C (less than C if D is negative), and then jumps to the End tag without executing the commands.
For example, the code below will display "A" two times.
:For(A,1,2)
:Disp "A"
:End

Explanation:
Start - 1 is loaded into A, Disp "A" is executed, 1 is added to A (A=2), Disp "A" is executed again, 1 is added to A (A=3), A>2, so Disp "A" is not executed again and the program moves on to the next set of commands. Note that you can specify a negative value for D (For(A,2,1,-1)), which would have the same effect if replaced with the For( command in the program above, except that A will start as 2 and end as 0. A can also be used within the code inside the For loop if needed, and it is not recommended to alter A within the For loop for novice programmers, even though it is legal.

Note: in argument For(A,B,C[,D]), if D is negative and C>B, -or- if D is positive or not specified and B>C, the For loop is not executed at all, but the value of B is still stored into A. Also, the increment value (D) cannot be Zero, if D=0 your program will stop with an error. If B=C, your For loop will be executed once. The values of B,C, and D cannot be changed within the For loop (they can be set to different values, but they will not affect the conditions of the loop).