Introduction to BASIC   IF～END IF

IF～END IF
When the condition holds, the statements written between an IF line and an END-IF line shall be executed.
Example. The solutions of a quadratic equation.

10 INPUT a,b,c
20 LET D=b^2-4*a*c
30 IF D>=0 THEN
40    PRINT (-b-SQR(D))/(2*a),(-b+SQR(D))/(2*a)
50 END IF
60 END

IF～ELSE～END IF
When the condition holds, the statements written between an IF line and an ELSE line shall be executed and when the condition fails, the statements written between the ELSE line and the END-IF line shall be executed.
Example. A program which solves a quadratic equation.

10 INPUT a,b,c
20 LET D=b^2-4*a*c
30 IF D>=0 THEN
40    PRINT (-b-SQR(D))/(2*a),(-b+SQR(D))/(2*a)
50 ELSE 
60    PRINT "No solutions"
70 END IF
80 END

[Note] inequality signs
The inequality signs ≤，≥，≠ are denoted by <=，>=，<> ,respectively.

IF ～ END IF

IF statements
In case that there is only one statement that shall be excecuted when the condition holds, we can write that statement succeeding THEN and omit END IF.
Note that the presence of END IF depends on that THEN have a statement on the righthand side.
Example. The solutions of a quadratic equation.

10 INPUT a,b,c
20 LET D=b^2-4*a*c
30 IF D>=0 THEN PRINT (-b-SQR(D))/(2*a),(-b+SQR(D))/(2*a)
40 END