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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. It should be pointed out that the loop control variable can only be incremented or decremented by 1 each time through the loop in Pascal. 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" at the end of line 17 and extending through and including the end statement in line 21 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 for loop, resulting in nested loops to whatever depth you desire. The third example shows how the char type 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. In addition, it must be a variable of type integer, byte, or char. The starting point and ending point can be constants or expressions of arbitrary complexity. Page 20 CHAPTER 4 - The Pascal loops and control structures The fourth example is a decrementing loop as mentioned earlier. It uses the reserved word "downto". THE IF STATEMENT Pascal has two conditional branching capabilities, the "if" and the "case". We will look at one of them now, the if statement. Load IFDEMO for an onscreen look at the "if then" pair of reserved 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 because Three is equal to One + Two. It is very difficult to come up with a good example without combining some of the other control structures but we will do so in the next file. The second example in lines 14 through 19, is similar to the first but has the single statement replaced with a compound statement and should be easy to understand. The third example in lines 21 through 24, 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 two 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. Notice that the Pascal compiler is looking for either a semicolon to end the if, or the reserved word "else" to continue the logic. It is therefore not legal to use a semicolon immediately preceding the reserved word "else". You will get a compiler error if you do so. 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 Page 21 CHAPTER 4 - The Pascal loops and control structures end separators. This is shown in the fourth example of the IFDEMO Pascal example program. Lines 27 through 30 comprise a single Pascal statement, and lines 32 through 35 comprise another. The if statement begun in line 26 therefore has a single statement in each of its branches. 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 for your own practice. When you are ready, we will go on to a program with loops and conditional statements combined and working together. 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 understandable to you at this point. It contains a loop (lines 7 & 17) with two if statements within it (lines 8 & 9 and lines 10 through 16), and another loop (lines 11 through 15) within one of the if statements. You should make careful note of the formatting used here. The "begin" is at the end of the line which starts the control and the "end" is lined up under the control word such that it is very clear which control word it is associated with. You will develop your own clear method of formatting your code in time but until then it is suggested that you follow this example. An easily made error should be pointed out at this time. If an extraneous semicolon were put at the end of the if statement in line 8, 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 at the end of line 8 to see the error. FINALLY, A MEANINGFUL PROGRAM Load TEMPCONV and study its structure. Notice the header block that defines the program and gives a very brief explanation of what the program does. This program should pose no problem to you in understanding what it does since it is so clearly documented. Run it and you will have a list of Centigrade to Fahrenheit temperature conversions with a few added notes. Page 22 CHAPTER 4 - The Pascal loops and control structures 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 compared to the last program, it is difficult to understand what it does by studying the listing. This program, like the last should be easily understood by you, so we will go on to our next Pascal control structure. THE REPEAT UNTIL LOOP The next two Pascal constructs 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" construct which is the loop that is evaluated at the bottom. Retrieve the file REPEATLP to see an example of 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. It is up to you, the programmer, to insure that the loop will eventually terminate. Compile and run REPEATLP to observe the output. THE WHILE LOOP The file WHILELP contains an example of another new construct, the "while" loop. This uses the "while" "do" reserved words and will execute one Pascal statement (or one compound statement bounded with begin and end) continuously until the boolean expression between the two words becomes FALSE. This loop is also indeterminate and could, like the repeat until loop, never terminate. You should therefore exercise care in using it. There are two basic differences in the last two loops. The repeat until loop is evaluated at the bottom of the loop Page 23 CHAPTER 4 - The Pascal loops and control structures and must therefore always go through the loop at least one time. The while loop is evaluated at the top and may not go through even once. This gives you flexibility when choosing the loop to do the job at hand. Compile, run, and examine the output from the example program WHILELP. THE CASE STATEMENT The final control structure introduces one more 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 in line 9 is the variable used to make the selection. Following that, the various selections are listed as a possible value or range, followed by a colon, a single Pascal statement, and a semicolon for each selector. 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. Be sure to compile and run this example program. Load and display the sample program BIGCASE for another example of a case statement with a few more added features. This program uses the identical structure as the previous program but in line 11 a range is used as the selector so that if the value of Count is 7, 8, or 9 this selection will be made. In line 12, three different listed values will cause selection of this part of the code. Of greater importance are the compound statements used in some of the selections. If the variable Count has the value of 2, 4, or 6, a compound statement will be executed and if the value is 3, a for loop is executed. If the value is 1, an if statement is executed which will cause a compound statement to be executed. In this case the if statement will always Page 24 CHAPTER 4 - The Pascal loops and control structures be executed because TRUE will always be true, but any Boolean expression could be used in the expression. Be sure to compile and run this program, then study the output until you understand the result thoroughly. This brings us to the end of chapter 4 and you now have enough information to write essentially any program desired in Pascal. You would find that you would have a few difficulties if you attempted to try to write a very big program without the topics coming up in the next few chapters. The additional topics will greatly add to the flexibility of Pascal and will greatly ease programming in it. 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 25