# IF THEN ELSE

*This is taken from the final issue of the Users Reference Guide:*

## IF-THEN-ELSE

### As used in TI BASIC

The IF-THEN-ELSE statement allows you to change the normal sequence of your program execution by using a conditional branch.

The computer evaluates the expression you have included in the statement, such as A>50. If the expression is true, the computer will jump to line-1, which follows the word THEN. If the condition is false, the computer will jump to line-2 following the word ELSE. If ELSE is omitted, the computer continues with the next program line.

#### The allowable relational operators

in TI Basic are:

- Equal to (=)
- Not equal to (<>)
- Less than (<)
- Less than or equal to (<=)
- Greater than ( >)
- Greater than or equal to (>=)

#### Some valid relationship tests:

- A>7
- CHR$(L)="A"
- A$<"YES"
- (A$&C$)>=D$
- (A +B)/2<>AVG

#### Example

100 REM FIND THE LARGEST OF A SET OF NUMBERS 110 INPUT "HOW MANY VALUES?" :N 120 INPUT "VALUE?":A 130 L=A 140 N=N-1 150 IF N<=0 THEN 180 160 INPUT "VALUE?":A 170 IF L>A THEN 140 ELSE 130 180 PRINT L;"IS THE LARGEST" 190 END >RUN HOW MANY VALUES?3 VALUE?456 VALUE?321 VALUE?292 456 IS THE LARGEST ** DONE **

#### Comparisons

A numeric-expression must be compared to another numeric-expression and a string-expression to another string-expression.

Numeric-expressions are compared algebraically.

String-expressions are compared left-to-right, character by character, using the ASCII character codes. A character with a lower ASCII code will he considered less than one with a higher ASCII code. Thus, you can sort strings into numeric or alphabetic order. If one string is longer than the other, the comparison is made for each character in the shorter string. If there is no difference, the computer considers the longer string to be greater.

Example

>NEW 100 INPUT "AS IS ":AS 110 INPUT "B$ IS ":B$ 120 IF A$=B$ THEN 160 130 IF A$<B$ THEN 180 140 PRINT "B$ IS LESS" 150 GOTO 190 160 PRINT "A$=B$" 170 GOTO 190 180 PRINT "B$ IS GREATER" 190 END >RUN A$ IS TEXAS B$ IS TEX B$ IS LESS ** DONE **

>RUN A$ IS TAXES B$ IS TEX B$ IS GREATER ** DONE **

#### Using a direct value (0, not 0)

An alternative format of the IF-THEN-ELSE statement is to use a numeric-expression with no relationship expressed. In the example below, the computer will evaluate the expression A+B. If the result is zero, the expression is treated as false.

A non-zero result is treated as true.

This is the same as: IF expression <> 0 THEN line-1

Example

>NEW 100 INPUT "A IS ":A 110 INPUT "B IS ":B 120 IF A+B THEN 150 130 PRINT "RESULT IS ZERO,EX PRESSION FALSE" 140 GOTO 100 150 PRINT "RESULT IS NON-ZER O, EXPRESSION TRUE" 160 GO TO 100 >RUN A IS 2 B IS 3 RESULT IS NON-ZERO,EXPRESSIO N TRUE A IS 2 B IS -2 RESULT IS ZERO,EXPRESSION FA LSE

Press CLEAR to end loop.

### As used in Extended BASIC

(Taken from the Extended Basic Manual)

The above paragraphs on TI Basic use also apply to TI Extended Basic, however Extended Basic increases the functionlity as shown below:

The IF-THEN-ELSE statement allows you to transfer control to line-number1 **or to perform statement1**
if relational-expression is true or if numeric-expression is not equal to zero. Otherwise control passes to
the next statement, or optionally to line-number2 or **statement2.**

Statement1 and statement2 may each be several statements long, separated by the statement separator symbol. They are only executed if the clause immediately before them is executed.

The IF-THEN-ELSE statement cannot contain DATA, DEF, DIM, FOR, NEXT, OPTION BASE, SUB, or SUBEND.

Examples

IF X>5 THEN GOSUB 300 ELSE X=X+5

operates as follows: If X is greater than 5, then GOSUB 300 is executed. When the subroutine is ended, control returns to the line following this line.

If X is 5 or less, X is set equal to X+5 and control passes to the next line.

IF Q THEN C=C+1::GOTO 500 ELSE L=L/C::GOTO 300

operates as follows: If Q is not zero, then C is set equal to C+1 and control is transferred to line 500. If Q is zero, then L is set equal to L/C and control is transferred to line 300.

IF A>3 THEN 300 ELSE A=0 :: GOTO 10

operates as follows: If A is greater than 3, then control is transferred to line 300. Otherwise, A is reset to zero and control is transferred to line 10.

109 100 IF A$="Y" THEN COUNT=COU NT+1::DISPLAY AT(24,1):"HERE WE GO AGAIN!"::GOTO 300

IF A$="Y" THEN COUNT=COUNT+1:: DISPLAYAT(24,1):"HERE WE GO AGAIN!"::GOTO 300

operates as follows:

If A$ is not equal to "Y", then control passes to the next line. If A$ is equal to "Y", then COUNT is incremented by 1, a message is displayed, and control is transferred to line 300.

100 IF HOURS<=40 THEN PAY=HO URS*WAGE ELSE PAY=HOURS*WAGE +.5*WAGE*(HOURS-40) :: OT=1

IF HOURS<=40 THEN PAY=HOURS*WAGE ELSE PAY=HOURS*WAGE+.5*WAGE*(HOURS-40) :: OT=1

operates as follows:

If HOURS is less than or equal to 40, then PAY is set equal to HOURS*WAGE and control passes to the next line. If HOURS is greater than 40 then PAY is set equal to HOURS*WAGE+.5*WAGE* (HOURS-40), OT is set equal to 1, and control passes to the next line.

IF A=1 THEN IF B=2 THEN C=3 ELSE D=4 ELSE E=5

operates as follows:

If A is not equal to 1, then E is set equal to 5 and control passes to the next line. If A is equal to 1 and B is not

equal to 2, then D is set equal to 4 and control passes to the next line. If A is equal to 1 and B is equal to 2, then C is set equal to 3 and control passes to the next line.

The following program illustrates a use of IF-THEN-ELSE. It accepts up to 1000 numbers and then prints them in order from smallest to largest.

100 CALL CLEAR 110 DIM VALUE(1000) 120 PRINT "ENTER VALUES TO B E SORTED.": "ENTER '9999' TO END ENTRY." 130 FOR COUNT=1 TO 1000 140 INPUT VALUE(COUNT) 150 IF VALUE(COUNT)=9999 THE N 170 160 NEXT COUNT 170 COUNT=COUNT-1 180 PRINT "SORTING." 190 FOR SORT1=1 TO COUNT-1 200 FOR SORT2=SORT1+1 TO COU NT 210 IF VALUE(SORT1)>VALUE(SO RT2)THEN TEMP=VALUE(SORT1):: VALUE(SORT1)=VALUE(SORT2):: VALUE(SORT2)=TEMP 220 NEXT SORT2 230 NEXT SORT1 240 FOR SORTED=1 TO COUNT 250 PRINT VALUE(SORTED) 260 NEXT SORTED