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| Chapter 12.1 - Looping |
| Suppose you want to input five numbers & add them together. One way (don't type this in unless you're feeling dutiful) is to write 10 LET TOTAL=0 20 INPUT A 30 LET TOTAL=TOTAL+A 40 INPUT A 50 LET TOTAL=TOTAL+A 60 INPUT A 70 LET TOTAL=TOTAL+A 80 INPUT A 90 LET TOTAL=TOTAL+A 100 INPUT A 110 LET TOTAL=TOTAL+A 120 PRINT TOTAL This method is not good programming practice. It may be just about controllable for five numbers, but you can imagine how tedious a program like this to add ten numbers would be, & a hundred would be just impossible. Much better is to set up a variable to count up to five & then stop the program, like this (which you should type in): 10 LET TOTAL=0 20 LET COUNT=1 30 INPUT A 40 REM COUNT = NUMBER OF TIMES THAT A HAS BEEN INPUT SO FAR 50 LET TOTAL=TOTAL+A 60 LET COUNT=COUNT+1 70 IF COUNT<=5 THEN GOTO 30 80 PRINT TOTAL Notice how easy it would be to change line 70 so that this program adds ten numbers, or even a hundred. This sort of counting is so useful that there are two special statements to make it easier: the FOR statement, & the NEXT statement. They are always used together. Using these, the program you have just typed in does exactly the same as 10 LET TOTAL=0 20 FOR C=1 TO 5 30 INPUT A 40 REM C = NUMBER OF TIMES THAT A HAS BEEN INPUT SO FAR 50 LET TOTAL=TOTAL+A 60 NEXT C 80 PRINT TOTAL (To get this program from the previous one you just have to edit lines 20, 40, 60 & 70. TO is shifted 4.) Note that we have changed COUNT to C. The counting variable - or control variable - of a FOR-NEXT loop must have a single letter for its name. The effect of this program is that C runs through the values 1 (the initial value), 2, 3, 4 & 5 (the limit), & for each one, lines 30, 40 & 50 are executed. Then, when C has finished its five values, line 80 is executed. An extra subtlety to this is that the control variable does not have to go up by 1 each time: you can change this 1 to anything else you like by using a STEP part in the FOR statement. The most general form for a FOR statement is FOR control variable = initial value TO limit STEP step where the control variable is a single letter, & the initial value, limit & step are all numeric expressions. So, if you replace line 20 in the program by 20 FOR C=1 TO 5 STEP 3/2 then C will run through the values 1, 2.5 & 4. Notice that you don't have to restrict yourself to whole numbers, & also that the control value does not have to hit the limit exactly - it carries on looping as long as it is less than or equal to the limit (but see exercise 4). You must be careful if you are running two FOR-NEXT loops together, one inside the other. Try this program, which prints out a complete set of 6-spot dominoes. 10 FOR M=0 TO 6 N-loop M-loop 20 FOR N=0 TO M | | 30 PRINT M;":";N;" "; N-loop | 40 NEXT N | 50 PRINT | 60 NEXT M M-loop You can see that the N-loop is entirely inside the M-loop - they are properly nested. What must be avoided is two FOR-NEXT loops that overlap without either being entirely inside the other, like this: 10 FOR M=0 TO 6 M-loop 20 FOR N=0 TO M | N-loop 30 PRINT M;":";N;" "; | | WRONG 40 NEXT M M-loop | 50 PRINT | 60 NEXT N N-loop The FOR-NEXT loops must either be one inside the other, or be completely separate. Another thing to avoid is jumping into the middle of a FOR-NEXT loop from the outside. The control variable is only set up properly when its FOR statement is executed, & if you miss this out the NEXT statement will confuse the computer. You might get error report 1 or 2 (meaning that a NEXT statement does not contain a recognised control variable) if you're lucky. Summary Statements: FOR, NEXT, TO, STEP |
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