Beginning C For Engineers Fall 2005

Beginning C For EngineersFall 2005 Lecture 3: While loops, For loops, Nested loops, and Multiple Selection Section 2 – 9/14/05 Section 4 – 9/15/05 Bettina Schimanski

Homework • Read Chapter 4 • HW 3 • Don’t forget the classroom change next week: • Section 2 (Wednesday): Sage 5101 • Section 4 (Thursday): Walker 5113

More C Operators int age; age = 8; age = age + 1; 8 age 9 age

PREFIX FORMIncrement Operator int age; age = 8; ++age; 8 age 9 age

POSTFIX FORM Increment Operator int age; age = 8; age++; 8 age 9 age

Decrement Operator int dogs; dogs = 100; dogs--; --dogs; 100 dogs 98 dogs

Which Form to Use • When the increment (or decrement) operator is used in a “stand alone” statementsolely to add one (or subtract one) from a variable’s value, it can be used in either prefix or postfix form • But… when the increment (or decrement) operator is used in a statement with other operators, the prefix and postfix forms can yield different results USE EITHER dogs--;--dogs ;

The += operator • If you wish to add a value to a variable and put the result back in the variable, use the += operator • ex: int stuff = 4; stuff += 3; /* same as stuff = stuff + 3; stuff is now 7 */ Adds the value on the right to the variable on the left.

Other similar operators • As you might expect, there are also operators for -=, *=, /=, and %=. x += y; /* same as x = x + y; */ z *= 3; /* same as z = z * 3; */ y /= 2; /* same as y = y / 2; */ i %= 5; /* same as i = i % 5; */

What is a loop? • A loop is a repetition control structure. • It causes a single statement or block to be executed repeatedly

Loop constructs in C • while • execute block of statements repeatedly as long as some condition is true • condition is tested before block of statements are executed • for • an abbreviation for a collection of statements that use a while loop for creating counting loops • ideal when the number of iterations of the loop is “known” beforehand • do – while • similar to while • condition is tested after block of statements are executed

While Statement SYNTAX while ( Expression) { . . // loop body . } NOTE: Loop body can be a single statement, a null statement, or a block.

Event-controlled Loops Keep processing data until a certain event happens, for example • a special value is entered (e.g. -1) to indicate that processing should stop • there is more data in the file • until the value of a flag changes 13

#include <stdio.h> int main( ) /* Example from Lecture 2 */ { int total = 0, num; printf( “Enter a number (-1 to stop ) ”); scanf(“%d”, &num); /* Read until the user enters -1 */ while (num != -1) { total = total + num; printf( “Enter a number (-1 to stop ) ”); scanf(“%d”, &num); } printf( “Total = %d”, total); return 0; }

#include <stdio.h> int main( ) { int total = 0, num; printf( “Enter a number (-1 to stop ) ”); scanf(“%d”, &num); /* Read until the user enters -1 */ while (num != -1) { total += num; printf( “Enter a number (-1 to stop ) ”); scanf(“%d”, &num); } printf( “Total = %d”, total); return 0; }

Flag-controlled Loops • Initialize a flag to 1(true) or 0 (false) • Use a meaningful name for the flag • A condition in the loop body changes the value of the flag • Test for the flag in the loop test expression

int numpos = 0; int isSafe = 1 /* initialize Boolean flag */ /*Loop while flag is true, i.e. while only positive numbers */ while (isSafe) { scanf(“%d”, &num); if ( num < 0 ) isSafe = 0;/* change flag value */ else numpos++; } printf(“%d positive numbers were entered\n”, numpos);

Count-controlled Loops contain: • An initialization of the loop control variable • An expression to test for continuing the loop • An update of the loop controlvariable to be executed with each iteration of the body • * Although count-controlled loops can be implemented using while loops, it is more common to use for loops for counting

Using For loops for Counting SYNTAX for ( initialization ; test expression; update ) { 0 or more statements to repeat } update initialization statement statement . . . test true false done

Example of Repetition int num; for ( num = 1; num <= 3; num++) { printf(“%d Potato\n”, num); } initialization test update

Example of Repetition ? num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } OUTPUT

Example of Repetition 1 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } OUTPUT

Example of Repetition 1 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } true OUTPUT

1 Potato Example of Repetition 1 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } OUTPUT

1 Potato Example of Repetition 2 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } OUTPUT

1 Potato Example of Repetition 2 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } true OUTPUT

1 Potato 2 Potato Example of Repetition 2 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } OUTPUT

1 Potato 2 Potato Example of Repetition 3 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } OUTPUT

1 Potato 2 Potato Example of Repetition 3 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } true OUTPUT

1 Potato 2 Potato 3 Potato Example of Repetition 3 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } OUTPUT

1 Potato 2 Potato 3 Potato Example of Repetition 4 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } OUTPUT

1 Potato 2 Potato 3 Potato Example of Repetition 4 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } false OUTPUT

Example of Repetition 4 num int num; for ( num = 1 ; num <= 3; num++) { printf(“%d Potato\n”, num); } false When the loop control condition is evaluated and has value false, control passes to the statement following the for statement.

Count-controlled Loop int count ; for ( count = 0 ; count < 4 ; count++ ) { printf(“%d \n”, count); } printf( “Done\n”); OUTPUT: 0 1 2 3 Done

Count-controlled Loop int count ; for ( count = 0 ; count <= 4 ; count++ ) { printf(“%d \n”, count); } printf( “Done\n”); OUTPUT: 0 1 2 3 4 Done

Equivalence with while loop • The following while loop is equivalent to the previous for loop (on the previous slide) int count ; count = 0; /* initialization */ while (count <= 4) /* test expression */ { printf(“%d \n”, count); count++; /* update loop variable*/ } printf( “Done\n”);

Summing Values in Loops int num, count; int total = 0; for ( count = 0; count < 10; count++) { printf(“Enter a numer:”); scanf (“%d”, &num); total += num; }

For Loop Variations • The counter variable can be initialized to anything. • The test expression can be any logical expression involving the counter variable. • The increment can be any value, positive or negative. • Examples: for (i = 0; i < 10; i += 2) for (k = 20; k > 0; k -= 1) for (j = 1; j != 6; j += 2) Warning: What happens in the last example?

for (i = 0; i < 10; i +=2) { printf(“%d\n”, i ); } for (j = 20; j >= 0; j -= 3) { printf(“%d\n”, j ); } What Is Displayed By Each Program Segment? 20 17 14 11 8 5 2 0 2 4 6 8

for (j = 0; j <= 3; j ++) { for (k = j; k < 5; k ++) { printf(“%d %d\n”, j , k); } printf (“\n”); } Nested For Loops 0 0 0 1 0 2 0 3 0 4 1 1 1 2 1 3 1 4 2 2 2 3 2 4 3 3 3 4 What is printed to the screen?

Multiple Selection • An algorithm may contain a series of decisions in which a variable or expression is tested • Many if…else if… else statements • Switch statement

Switch Statement true case a action(s) break; case a false true case b action(s) break; case b false true case z action(s) case z break; false default

Place within a while loop to capture many grades: /* count the letter grades */ switch(grade) { case ‘A’: aCount++; break; case ‘B’: bCount++; break; case ‘C’: cCount++; break; case ‘D’: dCount++; break; default: fCount++; }

Switch Statement • Each case can have one or more actions • Each break statement causes control to immediately exit the switch statement • What happens if you leave out a break?

/* count the letter grades */ switch(grade) { case ‘A’: aCount++; break; case ‘B’: bCount++; /* No break! */ case ‘C’: cCount++; break; case ‘D’: dCount++; break; default: fCount++; } Error: What is printed out for bCount and cCount?

/* count the letter grades */ switch(grade) { case ‘A’: case ‘a’: aCount++; break; case ‘B’: case ‘b’: bCount++; break; case ‘C’: case ‘c’: cCount++; break; case ‘D’: case ‘d’: dCount++; break; default: fCount++; } More than one case can have the same action(s).

The Lab • Tracing code by hand • Summing, Average • Max/Min • In <limits.h> there is INT_MAX and INT_MIN • These defined constants are useful for initializing your min and max variables

Beginning C For Engineers Fall 2005