Repetition and Iteration ANSIC Repetition We need a

Repetition and Iteration ANSI-C

Repetition • We need a control instruction to allows us to execute an statement or a set of statements as many times as desired. • All the programs we have written look like the one below:

#include <stdio. h> /* * pre-condition: int year, represents a year. * post-conditions: returns 1 if leap, 0 if not leap. */ int leap(int year){ return (year %100) && (year%400 == 0) || !(year % 100 == 0) && (year % 4 == 0); } int main(void){ int value; printf("Enter an integer representing a year>>"); scanf("%d", &value); if (leap(value)) printf(" %d is a leap year", value); else printf(" return (0); } %d is not a leap year", value);

Lousy main structure • This program calls on the function leap once, and then it terminates. • We will need to execute the program every time we want to try it with a different set of data.

While control structure while (condition) statement; • Or while (condition){ statement 1; statement 2; …. } Loop condition: a boolean expression Semantics: a. evaluates condition b. if true executes all statement Continues doing a. and b. until condition becomes false. When condition evaluates to false, while is then terminated.

Condition in the while • What kind of condition to write? • It depends on the number of times we want the while to execute. • But MORE importantly it depends on the computation the while is designed to performed. • In others words, we often do not know the number of times we want the while to execute • must establish: – The condition that will control the execution of loop – How the body of loop relates to the condition so that the condition becomes false only when the desired computation is finished and conversely.

Writing whiles to Repeat execution • If we just want to write a while to repeat the execution of a set of statements a certain number of times: 10 • must keep track of the number of times the loop executes via a count • loop condition must be true as long as count has not reached correct number • body of the loop must increment count every time it executes. int count = 0; while(count ? ? 10){/* statement 1; statement 2; …. count = count + 1; } = != < <= */

Writing whiles to Repeat execution • Or if we want to execute a set of statements up until we externally stop the program: while (1){ statement 1; statement 2; …. }

#include <stdio. h> /* * pre-condition: int year, represents a year. * post-conditions: returns 1 if leap, 0 if not leap. */ int leap(int year){ return (year%400 == 0) || !(year % 100 == 0) && (year % 4 == 0); } int main(void){ int value; while (1) { printf("Enter an integer representing a year, or ^C return to stop program>>"); scanf("%d", &value); if (leap(value)) printf(" %d is a leap year", value); else printf(" } return (0); } %d is not a leap year", value);

#include <stdio. h> /* * pre-condition: int year, represents a year. * post-conditions: returns 1 if leap, 0 if not leap. */ int leap(int year){ return (year%400 == 0) || !(year % 100 == 0) && (year % 4 == 0); } int main(void){ int value; int count = 0; while (count < 10) { printf("Enter an integer representing a year, or ^C return to stop program>>"); scanf("%d", &value); if (leap(value)) printf(" %d is a leap year", value); else printf(" %d is not a leap year", value); count = count + 1; } return (0); }

#include <stdio. h> /* * pre-condition: int year, represents a year. * post-conditions: returns 1 if leap, 0 if not leap. */ int leap(int year){ return (year%400 == 0) || !(year % 100 == 0) && (year % 4 == 0); } int main(void){ int value; int count = 1; while (count <= 10) { printf("Enter an integer representing a year, or ^C return to stop program>>"); scanf("%d", &value); if (leap(value)) printf(" %d is a leap year", value); else printf(" %d is not a leap year", value); count = count + 1; } return (0); }

scanf • This is a function included from the <stdio. h> library • As a function it is specified to return the number of tokens it succesfully read. scanf(“%d”, &value 1); scanf(“%d%d”, &value 1, &value 2); • In particular it will return 0 when it fails reading at all. • use this to structure loop: – place as condition of loop to read an integer for year, – to stop loop, do not enter an integer, enter a letter.

#include <stdio. h> /* * pre-condition: int year, represents a year. * post-conditions: returns 1 if leap, 0 if not leap. */ int leap(int year){ return (year%400 == 0) || !(year % 100 == 0) && (year % 4 == 0); } int main(void){ int value; printf("Enter an integer representing a year, or a non-blank character to indicate no more data>>"); while (scanf("%d", &value)!= 0){ if (leap(value)) printf(" %d is a leap yearn", value); else printf(" %d is not a leap yearn", value); printf("Enter an integer representing a year, or a non-blank character to indicate no more data>>"); } printf("Program stop by user. "); return (0); }

Indicating no more data • Terminating program by entering a letter is fine as we expect “legal data” to be integers. • So, entering something different from a digit, or blanks scanf fails to read it and returns 0 as the number of successful reads and our loop terminates. • But what if we are dealing with a program where numbers as well as characters are legal data? How to tell the loop that there is no more legal data to be entered? • User indicates no more data by ^Z return. • scanf returns – 1 when there is NO MORE data. • <stdio. f> represents – 1 using the constant EOF.

#include <stdio. h> /* * pre-condition: int year, represents a year. * post-conditions: returns 1 if leap, 0 if not leap. */ int leap(int year){ return (year%400 == 0) || !(year % 100 == 0) && (year % 4 == 0); } int main(void){ int value; printf("Enter an integer representing a year, or ^z return to indicate no more data>>"); while (scanf("%d", &value)!= EOF){ if (leap(value)) printf(" %d is a leap yearn", value); else printf(" %d is not a leap yearn", value); printf("Enter an integer representing a year, or ^z return to indicate no more data>>"); } printf("Program stop by user. "); return (0); }

Performing computations with whiles • Now for the most typical use of while loops. • Example: write a function that computes n! int fact(int number){ int result = 1; int value = 2; while ( value <= number){ result = result * value; value = value + 1; } return result; }

Performing computations with whiles • Write a function that computes 1+22 +…+n 2 int square. Addition(int number){ int result = 1; int value = 2; while ( value <= number){ result = result + value*value; value = value + 1; } return result; }

Performing computations with whiles • Write a function that computes the integer logarithm in base two of an integer number: • Log 2 (n) = y 2 y <= n, 2 y+1 > n. Looking for y. int natural. Log(int number){ int result = 0; int value = 1; while ( value*2 <= number){ value = value*2; result = result + 1; } return result; } int natural. Log(int number){ int result = 0; int value = 1; while ( (value = value*2) <= number) result = result + 1; return result; } Typical C-style of writing

While loop code structure • The while loop uses variables • These variables have to be appropriately initialized • The body of the loop has to make sure loop condition will eventually become false, otherwise you have an infinite loop. • And it should terminate exactly when desired computation is finished! … … Variable initialization … While (condition){ … Update appropriate variable(s) to eventually exit loop when computation is … achieved }

• • • Writing code is easy Writing correct, well-written code is not easy Writing loops is easy Writing correct loops is not. Therefore be: DISCIPLINED!!!!!