**Fun with the TI-81: Part I**

Before there there was the TI-82, TI-83 and its family, and the TI-84 Plus and its family, there was the Texas Instrument’s original calculator from the 1990, the TI-81!

**Translating TI-81 to TI-84 Plus (and Back) **

Most of the commands can be copied directly. Some caveats to keep in mind:

**Disp**

The TI-81’s Disp (Display command) can only display either a variable’s value or a string. The command can only display one item per line.

The TI-84 Plus’ Disp command can list any combination of strings and variable values, separated by a comma. Each argument will be placed on one line.

**Input**

The TI-81’s Input command can ask for one variable. There is no prompt string option. A prompt string will require an extra Disp command. Also, there is no colon character.

Disp “VAR=”

Input A

The TI-84 Plus’ Input command can have an optional prompt string.

Input “VAR=”, A

**Lists **

The TI-81 has two lists that are used for statistical calculations, {x} and {y}. To recall an element of either {x} or {y}, press [ 2nd ] [ 0 ] or [ 2nd ] [ 1 ], respectively. The dimensions of the stat lists can be found by pressing [VARS], and selecting Dim{x} under the DIM menu. Stat lists can’t be resized by storing a value to it.

All of the lists for the TI-84 Plus start with a lower case bold “**L**“. Lists 1-6 can be pressed by [ 2nd ] [ 1 ] through [ 6 ]. There are many lists commands and functions for the TI-84 Plus.

**Linear Regression Options**

Running linear regression is the LinReg command on the TI-81. The equation will always be *a + bx*.

Running linear regression for the TI-84 Plus will need you to designate the x-list and y-list. There are also various options: *a + bx*, *ax + b*, or *Med-Med*.

**The If Command and Loops**

The TI-81 only has a singular If command, no Then or Else. The syntax is:

If condition

do if condition is true

skip to here if condition is false

Loops will require the extensive use of Lbl (label), Goto, DS<(, and IS>(. Lbl and Goto are self-explanatory.

DS<(var, target). The value of var is decreased by 1. The next command is skipped when value < target.

IS>(var, target). The value of var is increased by 1. The next command is skipped when value > target.

In addition to If (which can still do the two-line structure), Lbl, Goto, DS<(, and IS>(, the TI-84 Plus has Then, Else, For, While, and Repeat.

**The STO> Button**

The TI-81 turns on the ALPHA keyboard when pressing [STO>].

The TI-84 Plus doesn’t.

On to the programming…

**TI-81 Decimal to Binary Conversion: BINTODEC**

(75 bytes)

Input the binary integer at the prompt. Use only 1s and 0s.

Variables:

B = binary representation

D = decimal representation

N, M: used

Program:

Disp “BIN>DEC”

Input B

0 → D

0 → N

B → M

Lbl 0

2^N * 10 * FPart(M/10) + D → D

IPart(M/10) → M

IS>(N, IPart(log B) + 1)

Goto 0

Disp D

Example:

Input: B: 1001010

Result: D: 74

**TI-81 Binary to Decimal Conversion: DECTOBIN**

(99 bytes)

Input the decimal integer at the prompt. The integer needs to be in between 1 and 1024. Only positive integers are allowed.

Variables:

B = binary representation

D = decimal representation

N, M: used

Program:

Disp “DEC>BIN”

Disp “1≤D≤1024”

Input D

0 → B

D → M

IPart( log D / log 2 ) → N

Lbl 2

If 2^N ≤ M

B + 1 → B

If 2^N ≤ M

M – 2^N → M

If N ≠ 0

10 * B → B

DS<(N, 0)

Goto 2

Disp B

Example:

Input: D: 516

Result: B: 1000000100

**TI-81 Roots of a Quadratic Equation: QUADEQN**

(121 bytes)

This program solves the equation A*X^2 + B*X + C = 0, which allows for real or complex roots.

Variables:

A, B, C: coefficients

X, Y: roots

If the discriminant is zero or positive, the roots are real, and are stored in X and Y.

If the discriminant is negative, we have complex roots in the form of X ± Yi, X is the real part, Y is the imaginary part.

Program:

Disp “AX²+BX+C=0”

Input A

Input B

Input C

-B / (2A) → X

(B² – 4AC) / (4A²) → Y

If Y

Goto 0

√Y → Y

X + Y → Y

2X – Y → X

Disp “ROOTS”

Goto 2

Lbl 0

√(abs Y) → Y

Disp “X+YI, X-YI”

Lbl 2

Disp X

Disp Y

Examples:

x^2 + 4x + 5 = 0, Roots: 2 ± i

Input: A: 1, B: 4, C: 5

Results: “X+YI, X-YI”, X: -2, Y: 1

x^2 + 5x + 4 = 0, Roots: -4, -1

Input: A: 1, B: 5, C: 1

Results: “ROOTS”, X: -4, Y: -1

Tomorrow will be Part 2. Until then,

Eddie

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