Beginning C For Engineers Fall 2005

Beginning C For EngineersFall 2005

Lecture 3:While loops, For loops, Nested loops, and Multiple Selection

Section 2 – 9/14/05Section 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

8

int age;

age = 8;

age = age + 1;

age

9

age

PREFIX FORMIncrement Operator

8

int age;

age = 8;

++age;

age

9

age

POSTFIX FORM Increment Operator

8

int age;

age = 8;

age++;

age

9

age

Decrement Operator

100

int dogs;

dogs = 100;

dogs--; --dogs;

dogs

98

dogs

When the increment (or decrement) operator is used in a “stand alone” statement solely 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

Which Form to Use

dogs-- ; --dogs ;

USE EITHER

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; */

A loop is a repetition control structure.

It causes a single statement or block to be executed repeatedly

What is a loop?

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 */

elsenumpos++;

}

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 control variable 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

SYNTAXfor ( initialization ; test expression ; update ) {

0 or more statements to repeat

}

initialization update

teststatementstatement. . .

done

true

false

Example of Repetition

int num;

for ( num = 1 ; num <= 3 ; num++ ) {

printf(“%d Potato\n”, num);}

initialization test update

21

Example of Repetition num

int num;

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

OUTPUT

?


OUTPUT

1

int num;



OUTPUT

1

int num;


true


OUTPUT

1

int num;


1 Potato


OUTPUT

2

int num;


1 Potato


OUTPUT

2

int num;


1 Potato

true


OUTPUT

2

int num;


1 Potato

2 Potato


OUTPUT

3

int num;


1 Potato

2 Potato


OUTPUT

3

int num;


true

1 Potato

2 Potato


OUTPUT

3

int num;


1 Potato

2 Potato

3 Potato


OUTPUT

4

int num;


1 Potato

2 Potato

3 Potato


OUTPUT

4

int num;


1 Potato

2 Potato

3 Potato

false

Example of Repetition num 4

int num;


false

When the loop control condition is evaluated and has value false, control passes to the statementfollowing the for statement.

int count ;

for ( count = 0 ; count < 4 ; count++ ){ printf(“%d \n”, count);}

printf( “Done\n”);

Count-controlled Loop

OUTPUT: 0123Done

int count ;

for ( count = 0 ; count <= 4 ; count++ ){ printf(“%d \n”, count);}


Count-controlled Loop

OUTPUT: 01234Done

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*/}


Summing Values in Loops

int num, count;int total = 0;

for ( count = 0; count < 10; count++ ){

printf(“nter 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?

What Is Displayed By Each Program Segment?

for (i = 0; i < 10; i +=2){

printf(“%d\n”, i );}

for (j = 20; j >= 0; j -= 3){

printf(“%d\n”, j );}

02468

20171411852

Nested For Loops

for (j = 0; j <= 3; j ++) {

for (k = j; k < 5; k ++) {

printf(“%d %d\n”, j , k);}printf (“\n”);

}

What is printed to the screen?

0 00 10 20 30 4

1 11 21 31 4

2 22 3 2 4

3 33 4

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

case a

case b

case z

default

true

true

true

false

false

false

case a action(s)

case b action(s)

case z action(s)

break;

break;

break;

/* count the letter grades */

switch(grade) {case ‘A’:

aCount++; break;

case ‘B’:bCount++;break;

case ‘C’:cCount++;break;

case ‘D’:dCount++;break;

default:fCount++;

}

Place within a while loop to capture many grades:

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?

Error: What is printed out for bCount and cCount?


switch(grade) {case ‘A’:

aCount++; break;

case ‘B’:bCount++;/* No break! */

case ‘C’:cCount++;break;

case ‘D’:dCount++;break;

default:fCount++;

}

More than one case can have the same action(s).


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++;

}

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

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Beginning C For Engineers Fall 2005