A practical introduction to Pascal programming language - GIARA

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Iterative constructionsAlthough the exercise appears to be complete, it has a failing scenario when no ’a’ or ’b’ isincluded in the sequence. In such a case, we have that expectingB is set to true, and hence thesequence is classified as illegal. To solve it, we use a boolean variable to know whether an ’a’was found in the sequence. Note that, otherwise, any of such sequences is going to be valid.⊡Program 64: Exercise 3.24 (Version B)program c03e24;varc:char;someAappear:boolean;expectingB:boolean;result:boolean;beginwriteln(’Please input the sequence of characters ended by dot (.): ’);expectingB:=true;(* otherwise it is expecting an A *)result:=true;someAappear:=false;repeat(* 1- Reading the next value *)read(c);(* 2- Updating the other variables *)c:=lowercase(c);if (c=’a’) thenbeginsomeAappear:=true;if (expectingB) thenresult:=false(* I needed a B but an a was found *)elseexpectingB:=true(* I found an ’a’, now I need a ’b’ coming *)endelse if (c=’b’) thenexpectingB:=false; (* Error condition fulfilled *)until(c=’.’);if (((expectingB) or not(result)) and someAappear) thenwriteln(’The condition is not satisfied’)elsewriteln(’The condition is satisfied’)end.Exercise 3.25. Write a program that reads a sequence of integers ended by a zero (0) and displaysthe number of pairs of consecutive even numbers.□In this problem we need to remember whether the previous number was even or not. We usea boolean variable (prevIsEven) to perform such task. Note that we initialize it with the falsevalue, so that the first integer in the sequence will never be accounted for. Otherwise, having aeven number as first element of the sequence, we would count it as a pair.Program 65: Exercise 3.25program c03e25;varn:integer;prevIntIsEven:boolean;counter:integer;beginwriteln(’Please input the sequence of integers ended by zero (0): ’);57
A Practical Introduction to Pascal Programming Language⊡counter:=0;prevIntIsEven:=false;repeat(* 1- Reading the next value *)read(n);(* 2- Updating the other variables *)if (prevIntIsEven and (n mod 2=1)) thencounter:=counter+1;prevIntIsEven:=(n mod 2=1)until(n=0);writeln(’The situation happens ’,counter,’ times.’)end.Exercise 3.26. Write a program that reads a integers until a zero (0) is found and checks whetherthere exist some subsequence of 3 positions so that the sum of their elements is equal to 0.□The only complexity in this problem comes from remembering, at each step of the sequence,the value of the previous three values. Apart from that, we only need to sum the up each timeto see if the result is equal to 0.⊡Program 66: Exercise 3.26program c03e26;constMIN_INTEGER=-32500;varn:integer;prevInt,prevPrevInt:integer;result:boolean;beginwriteln(’Please input the sequence of integers ended by zero (0): ’);prevInt:=MIN_INTEGER;prevPrevInt:=MIN_INTEGER;result:=false;repeat(* 1- Reading the next value *)read(n);(* 2- Updating the other variables *)if (prevInt+prevPrevInt+n=0) thenresult:=true;prevPrevInt:=prevInt;prevInt:=nuntil(n=0);if (result) thenwriteln(’The situation happens at least once.’)elsewriteln(’The situation does not happen in the sequence.’)end.Exercise 3.27. Write a program that reads a sequence of integers and computes the number ofsubsequences of 3 positions so that their values are sorted in increasing order.□This problem is very similar to the one in the Exercise 3.26, exception made that we are nowchecking the ordering of the values, instead of their sum.58
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