CSC 270 – Survey of Programming Languages

1. CSC 270 – Survey of Programming Languages Loops in C Modified from Dr. Robert Siegfried’s Presentation

2. Objectives • Repeating: While; Do While; Counting (for) • Counting - # of times • While – go on a condition • Do /While – do at least once, then continue on condition • Condition • Switch • Random number generation • Exit

3. Counting Loops (continued) for (count = start; count <= finish; count++) statement final value of the counter initial value of the counter variable used to count times through the loop Note: No shorthand declaration of counter so cannot do for (int count =

4. HelloAgain.c #include <stdio.h> /* * Hello again - this is a better way to write * "Hello, again" five times */ int main(void) { int i; for (i = 1; i <= 5; i++) { printf("Hello, again\n"); } return(0); } You try: ask how many times to repeat

5. The Revised HelloAgain.c #include <stdio.h> /* * Hello again - Write "Hello, again" as many times * as the user wants */ int main(void) { int i, total_times; unsigned int count; printf("How many times do you want to " "say \"hello\" ? "); scanf("%u", &total_times); for (count = 0; count < total_times; count++) printf("Hello, again\n"); return(0); }

6. Example: Averaging n Numbers Accumulating inside a loop • Let's start by outlining our algorithm: 1. Find out how many values there are. 2. Add up all the values. 3. Divide by the number of values 4. Print the result

7. averagen.c #include <stdio.h> /* * averagen - Find the average of n values where * the user enters n */ int main(void) { float sum, average, value; int num_values, current_value; //Find out how many values there are printf("How many values are you going to enter ? "); scanf("%d", &num_values);

8. averagegen.c continued /* Read in each value and add it to the sum */ sum = 0.0; for (current_value = 1; current_value <= num_values; current_value++) { printf("What is the next value ? "); scanf("%f", &value); sum = sum + value; } // Calculate and print out the average average = sum / num_values; printf("The average is %f\n", average); return(0); }

9. Counting loop vs Conditional Loop • When do we end? • While loop stops at any boolean test you choose • No automatic counter • Do /While – runs once first • While – tests the first time • Remember • – GO condition, not STOP condition

10. Syntax: While and Do/While Loops WHILE: while(condition) { statements } DO WHILE: do { statement(s) } (condition)

11. keepasking.c While Example #include <stdio.h> /* A simple example of how while works */ int main(void) { int number; /* Get your first number */ printf("Hi there. Pick a positive" " integer >>"); scanf("%d", &number);

12. /* Keep reading number as long as they are positive */ while (number > 0) { printf("Pick another positive" " integer>>"); scanf("%d", &number); } printf("%d is not a positive integer\n", number); return(0); }

13. Sentinel Value • Often conditional loops continue until some special value is encountered in the input which effectively tells the program to stop running the loop. This is called a sentinel value because it is the value for which we are watching. • ‘X’ is the sentinel value in the GPA algorithm’s main loop

14. gpa.c with sentinel value #include <stdio.h> /* * Calculates a grade point average assuming * that all courses have the same point value * and that A, B, C and D are passing grades and * that all other grades are failing. */ int main(void) { int num_courses = 0, total = 0; char grade; float gpa;

15. /* * Print the instructions and an * introductory message */ printf("This program calculates your grade" " point average\n"); printf("assuming that all courses have " "the same point\n"); printf("value. It also assumes that " "grades of A, B, C and D\n"); printf("are passing and that all other " "grades are failing.\n"); printf("To indicate that you are finished," " enter a grade of \'X\'\n\n");

16. /* Get the first course grade */ printf("What grade did you get in your " "first class?"); scanf("%c", &grade); /* Add up the numerical equivalents of the grades */ while (grade != 'X') { /* Convert an A to a 4, B to a 3, etc. and add it to the total */

17. if (grade == 'A') total = total + 4; else if (grade == 'B') total = total + 3; else if (grade == 'C') total = total + 2; else if (grade == 'D') total = total + 1; else if (grade != 'F') printf("A grade of %c is assumed to " "be an F\n", grade); num_courses++;

18. // Get the next course grade printf("What grade did you get in the " "next class?"); /* * The \n is necessary so we can skip the * newline we entered when we pressed * the enter key. */ scanf("\n%c", &grade); }

19. /* * Divide the point total by the number of * classes to get the grade point average * and print it. */ gpa = (float) total / num_courses; printf("Your grade point average is" " %4.2f\n",gpa); return(0); }

20. GPA with Switch while (grade != 'X') { switch(grade) { case ‘A' : case ‘a' : total = total + 4; break; ….(handle b-d) case ‘F’ : break; default: printf(“%s is invalid”,grade); printf(“enter a new grade”); break; } }

21. Switch switch( <variable to examine> ) { case value to match : case optionally another value to match : statements to execute on either match break; case value to match : case optionally another value to match : statements to execute on either match break; default: statements to execute if no other match break; } }

22. Magic Number Problem - Random • The magic number game involves guessing a number and with each wrong guess, the player is told “too high” or “ too low”. The goal is to guess the number in the smallest number of tries. • We need a method for having the computer pick a number at random for the player to guess. • We will need to learn about how to use “library functions” to provide us with the magic number.

23. Designing the Magic Number Algorithm Input – The player’s guess(es) Output – A clue (“too high” or “too low”) and the number of guesses that it took. Initial Algorithm 1. Use the random number function to pick a number • Let the player make a guess • As long as the player hasn’t guessed the number, give the appropriate clue and let him/her guess again. • Print the number of tries

24. The Magic Number Program #include <stdio.h> #include <stdlib.h> /* * main() - The magic number game has the user * trying to guess which number between 1 * and 100 the computer has picked */ int main(void) { int magic, guess; int tries = 1; /* * Use the random number function to pick a * number */ magic = rand() % 100 + 1;

25. /* Let the user make a guess */ printf("Guess ?"); scanf("%d", &guess); while (guess != magic) { /* * Tell him whether it's too high * or too low */ if (guess > magic) printf(".. Wrong .. Too high\n\n"); else printf(".. Wrong .. Too low\n\n"); /* Let the user make another guess */ printf("Guess ?"); scanf("%d", &guess); tries++; }

26. /* Tell the user that (s)he won */ if (guess == magic) { printf("** Right!! ** "); printf("%d is the magic number\n", magic); } /* Tell the user how many guesses it took */ printf("You took %d guesses\n", tries); return(0); }1

27. Random Number not so Random? Magic number program: #include <stdio.h> #include <stdlib.h> int main(void) { int magic, guess; int tries = 1; /* * Use the random number function to pick a * number */ // srand( time( NULL)); magic = rand() % 100 + 1; printf("%i",magic); • Seed your random number generator using srand(seed) – time can be a good seed • For industrial strength, research your platform

28. Change Magic Number to Do While • The main loop in the magic number program becomes: do { /* Let the user make a guess */ printf("Guess: "); scanf("%d", &guess); /* If the user won, tell him/her */ if (guess == magic) { printf("** Right!! ** “ << endl); printf("%d is the magic number\n", magic); }

29. Revisiting the magic number program (continued) // Let the user make another guess if (guess > magic) printf(".. Wrong .. Too high\n\n"); else printf(".. Wrong .. Too low\N\n"); tries++; } while (guess != magic);

30. exit() • exit() allows the user to let a program terminate if the program detects an unrecoverable error. • The statement #include <stdlib.h> has to be included to use exit. • A non-zero status value should be returned when the program terminates abnormally.

31. Java Comparison Thus Far Credit: http://introcs.cs.princeton.edu/java/faq/c2java.html

32. More Java Comparison Credit: http://introcs.cs.princeton.edu/java/faq/c2java.html

33. Summary • Decisions • If / else if / else • switch • Loops • For counter must be created before loop starts • While OR Do while • Random # • rand gives # between 0 and high value • rand() % choices and maybe add 1 • Random – seed with srand • Exit • Requires stdlib.h; error condition should be negative • Exits entire program