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CHAPTER 4 - The Pascal loops and control structures Every program we have examined to this point has been a simple one pass through with no statements being repeated. As in all other languages, Pascal has extensive capabilities to do looping and conditional branching. We will look at these now. THE FOR LOOP We will start with what may be the easiest structure to understand, the FOR loop. This is used to repeat a single Pascal statement any number of times we desire. Load LOOPDEMO and we will discuss the loops presented there. The first example is the simplest and is simply a repeat of a WRITELN 7 times. We have three new reserved words, FOR, TO, and DO which are used as shown. Any simple variable of type INTEGER, BYTE, or CHAR can be used for the loop index and it must be defined in a VAR statement. Following the DO reserved word is any single Pascal statement that will be repeated the specified number of times. Note that the loop is an incrementing loop but substitution of DOWNTO for TO will make it a decrementing loop as is illustrated in the last example in this program. A COMPOUND PASCAL STATEMENT The second example contains our first compound Pascal statement. It was mentioned in Chapter 1 that the BEGIN END pair of reserved words could be used to mark the limits of a compound statement. In this case, the single statement starting with the BEGIN and extending through and including the END statement is the single Pascal statement that will be executed 10 times. A second variable "total" has been introduced to simply add another operation to the loop. Any valid Pascal operation can be performed within the BEGIN END pair, including another loop, thus resulting in nested loops to whatever depth you desire. The third example shows how the CHAR variable could be used in a FOR loop. Pascal requires that the loop variable, the starting point, and the ending point all be of the same type or it will generate an error message. The fourth example is a decrementing loop as mentioned earlier. THE IF STATEMENT Now we will look at the conditional branching capability, or at least one of them, the IF statement. Load IFDEMO for an onscreen look at the IF THEN pair of reserved Page 15 CHAPTER 4 - The Pascal loops and control structures words. Any condition that can be reduced to a boolean answer is put between the IF THEN pair of words. If the resulting expression resolves to TRUE, then the following single Pascal statement is executed, and if it resolves to FALSE, then the following single statement is skipped over. Of course, you can probably guess that the single statement can be replaced with a compound statement bracketed with a BEGIN END pair and you are correct. Study example 1 and you will see that the line will always be printed in this particular fragment. It is very difficult to come up with a good example without combining some of the control structures which we will do in the next file. The second example is similar to the first but with the single statement replaced with a compound statement and should be easy to understand. The third example contains a new reserved word, ELSE. When the IF condition is FALSE, the single statement is skipped and if a semicolon is encountered, the IF clause is totally complete. If instead of a semicolon, the reserved word ELSE is encountered, then the single Pascal statement following ELSE is executed. One and only one of the single statements will be executed every time the IF statement is encountered in the program. Examination of the third example should clear this up in your mind. THE IF-THEN-ELSE "block" Put on your thinking cap because the next principle is difficult to grasp at first but will suddenly clear up and be one of the most useful facts of Pascal programming. Since the entire IF THEN ELSE "block" of code is itself a single Pascal statement, by definition, it can be used anywhere that an executable statement is legal without BEGIN END separators. This is shown in the fourth example of the IFDEMO Pascal example program. The IF THEN ELSE construct is one of the most used, most useful, and therefore most important aspects of PASCAL. For this reason you should become very familiar with it. Try changing some of the conditions in the example program to see if you can get it to print when you expect it to just for your own practice and enjoyment. When you are ready, we will go on to a program with loops and conditional statements. LOOPS AND IFS TOGETHER Load LOOPIF and observe it for a few minutes. It contains most of what you have studied so far and should be Page 16 CHAPTER 4 - The Pascal loops and control structures understandable to you at this point. It contains a loop with two IF statements within it, and another loop within one of the IF statements. An easily made error should be pointed out at this time. If an extraneous semicolon were put at the end of the second IF statement, the code following the statement would always be executed because the "null" statement (the nothing statement between the THEN and the semicolon) would be the conditional statement. The compiler would not generate an error and you would get no warning. Add a semicolon after the IF statement to see the error. FINALLY, A MEANINGFUL PROGRAM Load TEMPCONV and study its structure. Run it and you will have a list of Centigrade to Fahrenheit temperature conversions with a few added notes. Load, examine, and run DUMBCONV for a good example of poor variable naming. The structure of the program is identical to the last program and when you run it, you will see that it is identical in output, but it is difficult to understand what it does by studying the listing. These programs should both be easily understood by you by now, so we will go on to our next Pascal control structure. THE REPEAT UNTIL LOOP The next two Pascal structures are very similar because they are both indefinite loops (indefinite because they are not executed a fixed number of times). One of the loops is evaluated at the top and the other at the bottom. It will probably be easier to start with the REPEAT UNTIL structure which is the loop that is evaluated at the bottom. Retrieve the file REPEATLP to see a repeat loop. Two more reserved words are defined here, namely REPEAT and UNTIL. This rather simple construct simply repeats all statements between the two reserved words until the boolean expression following the UNTIL is found to be TRUE. This is the only expression I know of that operates on a range of statements rather than a single statement and BEGIN END delimiters are not required. A word of caution is in order here. Since the loop is executed until some condition becomes TRUE, it is possible that the condition will never be TRUE and the loop will never terminate. THE WHILE LOOP The file WHILELP contains an example of another new construct, the WHILE loop. This uses the WHILE DO reserved Page 17 CHAPTER 4 - The Pascal loops and control structures words and will execute one Pascal statement (or one compound statement bounded with BEGIN END) continuously until the boolean expression between the two words becomes FALSE. This loop is also indeterminate and could, like the REPEAT UNTIL, never terminate. You should therefore exercise care in using it. THE CASE STATEMENT The final control structure introduces yet another reserved word, CASE. The CASE construct actually should be included with the IF statement since it is a conditional execution statement, but I chose to save it for last because it is rather unusual and will probably be used less than the others we have discussed in this chapter. The CASE statement is used to select one of many possible simple Pascal statements to execute based on the value of a simple variable. Load the file CASEDEMO and observe the program for an example of a case statement. The variable between the CASE and OF reserved words is the variable used to make the selection. Following that, the various selections are given in the form of a possible value or range, a colon, a single Pascal statement, and a semicolon. Following the list of selections, an ELSE can be added to cover the possibility that none of the selections were executed. Finally, an END; statement is used to terminate the case construct. Note that this is one of the few places in Pascal that an END is used without a corresponding BEGIN. The example file uses "count" for a variable and prints the numbers one through five in text form, and declares that numbers outside this range are not in the allowable list. The program should be self explanatory beyond that point. This is admittedly a very brief explanation of the CASE statement but you will have no trouble using it when you have need for it. You can study it in detail at that time. This brings us to the end of chapter 4 and you now have enough information to write essentially any program desired in Pascal. There would be a few difficulties if you attempted to try to program without the further topics coming up in the next chapters. The additional topics will greatly add to the flexibility of Pascal and will greatly ease programming in it. Page 18 CHAPTER 4 - The Pascal loops and control structures PROGRAMMING EXERCISES 1. Write a program that lists the numbers from 1 to 12 and writes a special message beside the number representing your month of birth. 2. Write a program that lists all of the numbers from 1 to 12 except for the numbers 2 and 9. Page 19