TI 30Xa Algorithms: Greatest Common Divisor
To find the greatest common divisor between two positive integers U and V:
Let U ≥ V. Let U = A * V + R where A is the quotient of U / V and R is the remainder. If R≠0, then V becomes the new U and R become the new V. The process repeats until R=0. At that point the value of V prior to the last calculation is the greatest common divisor (GCD) of U and V.
Example:
gcd(166, 78)
U = 166, V = 78
Algorithm Loop:
A = int(U / V)
R = U – V * int(U / V)

A 
R 
U 
V 
Start 
n/a 
n/a 
166 
78 
A = int(166 / 78) = 2 R = 166 – 2 * 78 = 10 
2 
10 
78 
10 
A = int(78 / 10) = 7, R = 78 – 7 * 10 = 8 
7 
8 
10 
8 
A = int(10 / 8) = 1 R = 10 – 1 * 8 = 2 
1 
2 
8 
2 
A = int(8 / 2) = 4 R = 8 – 4 * 2 = 0 
4 
0 *STOP* 


Procedure
Store the greater of the two numbers in memory register 1: [ STO ] [ 1 ].
Store the lesser of the two numbers in memory register 2: [ STO ] [ 2 ].
Divide memory register 1 by memory register 2. Store the integer part (no fractional part) in memory register 3: [ RCL ] [ 1 ] [ ÷ ] [ RCL ] [ 2 ] [ = ], (integer part) [ STO ] [ 3 ]
Figure the remainder and store the result in memory 3: [ RCL ] [ 1 ] [  ] [ RCL ] [ 2 ] [ × ] [ RCL ] [ 3 ] [ = ] [ STO ] [ 3 ]
If the remainder is 0, stop. The GCD is stored in memory 2.
If the remainder is nonzero, then store memory 2 into memory 1 then memory 3 into memory 2. You need to do it in this order. [ RCL ] [ 2 ] [ STO ] [ 1 ], [ RCL ] [ 3 ] [ STO ] [ 2 ]. Go back to Step 3 and repeat.
Examples
Example 1: GCD(26, 14)
M1 = 26, M2 = 14

M1 
M2 
M3 
26 [ STO ] [ 1 ], 14 [ STO ] [ 2 ] 
26 
14 

[ RCL ] [ 1 ] [ ÷ ] [ RCL ] [ 2 ] [ = ] Result: 1.857142857 1 [ STO ] [ 3 ] 
26 
14 
1 
[ RCL ] [ 1 ] [  ] [ RCL ] [ 2 ] [ × ] [ RCL ] [ 3 ] [ = ] Result: 12 [ STO ] [ 3 ] R is not zero, so we continue. 
26 
14 
12 
[ RCL ] [ 2 ] [ STO ] [ 1 ], [ RCL ] [ 3 ] [ STO ] [ 2 ] 
14 
12 
12 
[ RCL ] [ 1 ] [ ÷ ] [ RCL ] [ 2 ] [ = ] Result: 1.166666667 1 [ STO ] [ 3 ] 
14 
12 
1 
[ RCL ] [ 1 ] [  ] [ RCL ] [ 2 ] [ × ] [ RCL ] [ 3 ] [ = ] Result: 2 [ STO ] [ 3 ] R is not zero, so we continue. 
14 
12 
2 
[ RCL ] [ 2 ] [ STO ] [ 1 ], [ RCL ] [ 3 ] [ STO ] [ 2 ] 
12 
2 
2 
[ RCL ] [ 1 ] [ ÷ ] [ RCL ] [ 2 ] [ = ] Result: 6 6 [ STO ] [ 3 ] 
12 
2 
6 
[ RCL ] [ 1 ] [  ] [ RCL ] [ 2 ] [ × ] [ RCL ] [ 3 ] [ = ] Result: 0 [ STO ] [ 3 ] R is zero, so we stop. GCD: [ RCL ] [ 2 ]: GCD(26, 14) = 2 
12 
2 
0 
Example 2: GCD(27, 15)
M1 = 27, M2 = 15

M1 
M2 
M3 
27 [ STO ] [ 1 ], 15 [ STO ] [ 2 ] 
27 
15 

[ RCL ] [ 1 ] [ ÷ ] [ RCL ] [ 2 ] [ = ] Result: 1.8 1 [ STO ] [ 3 ] 
27 
15 
1 
[ RCL ] [ 1 ] [  ] [ RCL ] [ 2 ] [ × ] [ RCL ] [ 3 ] [ = ] Result: 12 [ STO ] [ 3 ] R is not zero, so we continue. 
27 
15 
12 
[ RCL ] [ 2 ] [ STO ] [ 1 ], [ RCL ] [ 3 ] [ STO ] [ 2 ] 
15 
12 
12 
[ RCL ] [ 1 ] [ ÷ ] [ RCL ] [ 2 ] [ = ] Result: 1.25 1 [ STO ] [ 3 ] 
15 
12 
1 
[ RCL ] [ 1 ] [  ] [ RCL ] [ 2 ] [ × ] [ RCL ] [ 3 ] [ = ] Result: 3 [ STO ] [ 3 ] R is not zero, so we continue. 
15 
12 
3 
[ RCL ] [ 2 ] [ STO ] [ 1 ], [ RCL ] [ 3 ] [ STO ] [ 2 ] 
12 
3 
3 
[ RCL ] [ 1 ] [ ÷ ] [ RCL ] [ 2 ] [ = ] Result: 4 4 [ STO ] [ 3 ] 
12 
3 
4 
[ RCL ] [ 1 ] [  ] [ RCL ] [ 2 ] [ × ] [ RCL ] [ 3 ] [ = ] Result: 0 [ STO ] [ 3 ] R is zero, so we stop. GCD: [ RCL ] [ 2 ]: GCD(27, 15) = 3 
12 
3 
0 
I hope you find this useful. What I hope to do with the monthly series is to demonstrate various calculations with the TI30Xa.
Note: For June and July 2024, I will be posting on Saturdays only. I plan to resume the SaturdaySunday schedule in August.
Eddie
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