Iteration in C H&K Chapter 5

Iteration in CH&K Chapter 5 Instructor – GokcenCilingir Cpt S 121 (July 1, 2011) Washington State University

Control Structures • Remember, in the very first lecture we talked about how algorithms are put together, and discussed sequential, conditional (selection) and iterative instructions. • Sequential instructions • do them in the order given • Conditional instructions • do them if a condition is true • Iterative instructions • do them while a condition is true • So far, we’ve examined ways to translate sequential and conditional instructions we have in our algorithms into C programs. Today we’re going to see what C provides us to be able to carry out iterative instructions.

Motivation example • In Lab 2 Task 1, we wrote a program that does the following: • Carries out following task 10 times: • Reads a character from the input file • Prints corresponding ASCII value to the output file • Update statistics on numLines, numAlpha, and numDigits. • Prints out the final statistics to an output file ch = read_character(infile); ascii = determine_ascii_value(ch); print_int(outfile_ascii, ascii); numLines= number_lines(ch, numLines); numDigits = number_digits(ch, numDigits); numAlpha = number_alphas(ch, numAlpha);

Motivation example (cont’d) • In Lab 2 Task 1, we wrote a program that does the following: • Carries out following task 10 times: • Reads a character from the input file • Prints corresponding ASCII value to the output file • Update statistics on numLines, numAlpha, and numDigits. • Prints out the final statistics to an output file int counter = 0; while (counter < 10) { • ch = read_character(infile); • ascii = determine_ascii_value(ch); • print_int(outfile_ascii, ascii); • numLines= number_lines(ch, numLines); • numDigits = number_digits(ch, numDigits); • numAlpha = number_alphas(ch, numAlpha); • counter += 1; }

How to decide when a loop is needed • Sometimes, problem statement explicitly tells you to carry out a procedure a number of times, like in the case of the motivation example: read and process 10 characters. My rule of thumb is: if you need to do something more than 3 times, consider using a loop statement. • Sometimes, problem statement doesn’t imply the need for a loop, but the algorithm you design to solve the problem, may require a loop. Think about the Greatest Common Divisor (GCD) problem (introduced in Lecture 1). • Are any steps repeated in your algorithm? • No  No loop required • Yes  Do you know in advance how many steps are repeated? • No  Use a conditional loop • Yes  Use a counting loop

Loop patterns • Counting loop(for or while): executes a fixed number of times • Sentinel-controlled or Endfile-Controlled loop(for or while): process data until a special value is encountered, e.g., end-of-file • Input validation loop(do-while): repeatedly accept interactive input until a value within a specified range is entered • General conditional loop(while, for): repeatedly process data until a desired condition is met

Counter Loops (1) • Pseudocode for a counter loop: Set loop counter to 0 while (loop counter < final count) … (Do data processing) add 1 to loop counter endwhile

Counter Loops (2) • Implementing Counter Loops: the while loop while (<repetition-condition>) { <body> }

Counter Loops (3) • Notes on while loops: • <repetition-condition> is evaluated at beginning of loop. If it evaluates to true, the loop body is executed. If it evaluates to false, control shifts to first statement after loop body • <body> contains one or more C statements • After last statement in <body> is executed, control is shifted back to beginning of loop, and <repetition-condition> is re-evaluated. • “Progress” must be made within the loop. That is, something must be done so that <repetition-condition> eventually evaluates to false. Otherwise we have an “infinite loop”

Counter Loops (4) initializing the counter int counter = 0; while (counter < 10) { • ch = read_character(infile); • ascii = determine_ascii_value(ch); • print_int(outfile_ascii, ascii); • numLines= number_lines(ch, numLines); • numDigits = number_digits(ch, numDigits); • numAlpha = number_alphas(ch, numAlpha); • counter = counter + 1; } repetition condition advancing the counter

Counter Loops (5) • Another alternative for implementing Counter Loops: the for loop for (<initialization>; <repetition-condition>; <update-expression>) { <body> }

Counter Loops (6) • Notes on for loops: • <initialization> statement initializes the loop control variables before loop is executed the first time • <repetition-condition> is tested at beginning of loop. If it is true, loop <body> is executed. • <body> contains one or more C statements • After last statement in <body> is executed, control is shifted back to beginning of loop. Then, <update-expression> is executed. Finally, <repetition-condition> is re-evaluated. • As with while loops, the <update-expression> must define “progress.” That is, something must be done so that <repetition-condition> eventually evaluates to false. Otherwise we have an “infinite loop”

Counter Loops (7) repetition condition update expression initialization int counter; for(counter = 0; counter< 10; counter = counter +1) { • ch = read_character(infile); • ascii = determine_ascii_value(ch); • print_int(outfile_ascii, ascii); • numLines= number_lines(ch, numLines); • numDigits = number_digits(ch, numDigits); • numAlpha = number_alphas(ch, numAlpha); }

Aside: Compound Assignment Operators • Notice that the <update-expression>s in loops are often of the form:count = count + 1 • C defines special assignment operators to define statements of this form more compactly:

Aside: Increment and Decrement Operators (1) • The ++ and -- operators take a single variable as their operands. The side effect of the operator is to increment or decrement its operand by one: • count++ has the effect of count = count + 1 • count-- has the effect of count = count – 1 • Note: ++ and -- can be placed either before or after their variable operator: • Pre-increment or pre-decrement (e.g., ++count, --count ):value of expression is value of variable after the increment or decrement is applied • Post-increment of post-decrement (e.g., count++,count--):value of expression is value of variable before the increment or decrement is applied

Aside: Increment and Decrement Operators (2) • You try it: What are the values of i, j, and k after each of the following statements is executed? inti, j, k; i = 2; j = 3 + i++; k = 3 + ++i; i *= ++k + j--; i /= k-- + ++j;

Counter Loops (8) • Notice that we can rewrite our previous while loop with these new operators: int counter = 0; while (counter < 10) { • ch = read_character(infile); • ascii = determine_ascii_value(ch); • print_int(outfile_ascii, ascii); • numLines= number_lines(ch, numLines); • numDigits = number_digits(ch, numDigits); • numAlpha = number_alphas(ch, numAlpha); • counter++; }

Counter Loops (9) • Notice that we can also rewrite our previous for loop with these new operators: int counter; for(counter = 0; counter< 10; counter++) { • ch = read_character(infile); • ascii = determine_ascii_value(ch); • print_int(outfile_ascii, ascii); • numLines= number_lines(ch, numLines); • numDigits = number_digits(ch, numDigits); • numAlpha = number_alphas(ch, numAlpha); }

Example problem – solution with while Problem statement:Find average of 10 floating point numbers taken from the user int counter = 0; double sum = 0; double number = 0.0; printf ("Enter 10 floating point numbers for me:"); while (counter <10) { scanf("%lf", &number); sum += number; counter++; } printf("Average of them is: %lf\n", sum/counter); Accumulating a value

Example problem- solution with for Problem statement:Find average of10 floating point numbers taken from the user Notice the use of comma operator int counter = 0; double sum = 0; double number = 0.0; printf ("Enter 10 floating point numbers for me:"); sum = 0; for (counter = 0; counter <10; counter++) { scanf("%lf", &number); sum += number; } printf("Average of them is: %lf\n", sum/counter);

It’s OK to have loop increments other than 1!

Next Lecture… • We'll discuss some additional loop patterns: • Conditional loops • Sentinel-controlled loops & end-file-controlled loops • Flag-controlled loops

References • J.R. Hanly & E.B. Koffman, Problem Solving and Program Design in C (6th Ed.), Addison-Wesley, 2010 • P.J. Deitel & H.M. Deitel, C How to Program (5th Ed.), Pearson Education , Inc., 2007.

Iteration in C H&K Chapter 5