CS101 Lecture 12

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Lecture 12
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What will I learn in this lecture?

• Know how if and switch C statements control the
  sequence of execution of statements.
• Be able to use relational and logical operators
  in the conditional part of an if or a switch
  statement.
• Related Chapters FER Chapter 3.6 6.3

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C Problem Example
1. Problem Definition Write a program that reads
a number and computes the square root if the
number is non-negative. 2. Refine, Generalize,
Decompose the problem definition (i.e., identify
subproblems, I/O, etc.) Input real
number Outputreal number 3. Develop
Algorithm (processing steps to solve problem)
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C Problem Example
Print enter value
Flowchart
Read value
False
True
value gt 0.0
Print sqrt(value)
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C Problem Example
/ C Program to compute the square root of a
positive number / include ltstdio.hgtinclude
ltmath.hgt void main(void) double value /
Declare variables. / / request user input /
printf("Please enter a non-negative number
") / read value / scanf("lf",
value) / Output the square root. / if
(value gt 0.0) printf("square_root(lf) lf
\n", value , sqrt(value))
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Selection Structures (Decision Statements)
if Selection Structure
if(expression) statement

• if expression evaluates to true, the statement
  is executed otherwise execution passes to the
  next statement in the program.

/ Error! Dont put semicolon here / / This is
an example of a logical error /
if(value gt 0.0) printf("square_root(lf) lf
\n", value,sqrt(value))
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C Problem Example (modified)
1. Problem Definition Modify the previous
program to notify the user when the input is
invalid.
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C Problem Example (modified)
Print enter value
Flowchart
Read value
False
True
value gt 0.0
Print sqrt(value)
Print invalid input
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C Problem Example (modified)
/ C Program to compute the square root of a
positive number / include ltstdio.hgtinclude
ltmath.hgt void main(void) double value /
Declare variables. / /request user input/
printf(Please enter a non-negative number )
scanf("lf", value) / read value / /
Output the square root. / if (value gt 0.0)
printf("square_root(lf) lf \n",
value,sqrt(value)) else printf("invalid
user input, please enter non-negative value\n")
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Math Library in C
- in header file math.h Arguments (parameters)
for each of the following functions are assumed
to be of type double. If not, a type double copy
is made for use in the function. To compile a
program that contains math functions you need to
use the -lm (Lm not 1m )option for gcc. gt gcc
file.c -lm
See next page
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fabs (x) - x (not the same as the abs(x)
function) sqrt (x) - square root of x
pow (x, a) - xa exp (x) - ex (e
2.718281828 ) log (x) - ln x loge x
log10 (x) - log10 x sin (x) - sine
function (x in radians) cos (x) - cosine
function (x in radians) tan (x) - tangent
function (x in radians) ceil (x) - smallest
integer gt x floor (x) - largest integer lt
x
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if - else Selection Structure
if (expression) statement1 else statement2

• if expression evaluates to true, statement1 is
  executed and execution skips statement2
• If expression evaluates to false, execution skips
  statement1 , statement2 is executed

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if - else Selection Structure
We can also execute multiple statements when a
given expression is true
if (expression) statement1 statementn e
lse statement1 statementm
if (expression) statement1 statement2 stat
ementn
or
. ..
. ..
Example -
(what does this code do?)
if(b lt a) temp a a b b temp
. ..
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C Problem Example
1. Problem Definition Modify the previous program
to compute the followingYou must check that
the value is legal, i.e. value gt 1.0 or value
lt -1.0
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C Problem Example
Print enter value
Flowchart
Read value
value gt 1.0 or value lt -1.0
False
True
Print sqrt(valuevalue -1.0)
Print invalid input
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C Problem Example (modified)
/ Compute the square root of valuevalue-1.0
/ include ltstdio.hgtinclude ltmath.hgt void
main(void) double value / Declare
variables. / / request user input/
printf("Please enter value gt 1.0 or lt -1.0
") scanf("lf", value) / read value /
/ Output the square root. / if ((value gt
1.0) (value lt -1.0)) printf("square_root(
f) f \n", value,sqrt(valuevalue - 1.0))
else printf("invalid user
input\n") printf("input should be a value
gt 1.0 or lt -1.0 \n")
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Logical Expressions (Relational Operations)
In logical expressions (which evaluate to true or
false), we can use the following Relational
operators
Relational Operator
Type of Test
equal to (dont use ) !
not equal to gt
greater than gt greater than or equal
to lt less than lt
less than or equal to
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Logical Operators-Truth Tables
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Logical Expressions
In C the value for False is represented by the
value zero and True is represented by any nonzero
value. The value False can be any zero value such
as the number 0 or 0.0 or null character \0
or the NULL pointer.
Example 1
if ( 5 ) printf("True") / prints True /
Example 2
int x 0 / x declared as an integer variable
/ / and initialized to the
value 0 / if (x 0) / note the
error, should be used /
printf(" x is zero\n") /message not printed,
why?/
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Logical Expressions
Avoid using to test real numbers for
equality! Example double x .3333333333 /
ten digits of 3s /if (x 1.0/3.0)
printf("equal!\n")else printf(" not
equal!\n") / prints not equal! / if ( fabs(x
- 1.0/3.0) lt 1.0e-10 ) printf("equal!\n")
/ prints equal! / else printf(" not
equal!\n")
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Nested if - else Selection Structure
1. Problem Definition Write a program that
returns a letter grade based on a quiz score. The
input will be the integer score from a 10 point
quiz. The letter grades are assigned by9 - 10
A7 - 8 B5 - 6 C3 - 4
Dlt 3 F 2. Refine, Generalize,
Decompose the problem definition (i.e., identify
subproblems, I/O, etc.) Input integer
score Outputcharacter grade 3. Develop
Algorithm (processing steps to solve problem)
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C Problem Example
Print enter score
Flowchart
Read score
score 10 score 9
True
False
Print A
(continued on next slide)
(skip else part of statement)
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C Problem Example
False
score 8 score 7
False
True
Print B
(continued on next slide)
(skip else part of statement)
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C Problem Example
False
score 6 score 5
False
True
Print C
(continued on next slide)
(skip else part of statement)
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C Problem Example
False
score 4 score 3
False
True
Print D
Print F
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C Problem Example
/ C Program to compute the letter grade for a
quiz. / include ltstdio.hgt void main(void)
int score / Declare variables. / /
request user input / printf("Please enter a
score ") scanf("i", score) / read value
/ / Output the grade // continued on the
next slide /
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C Problem Example
if ((score 10) (score 9))
printf("A\n")else if ((score 8) (score
7)) printf("B\n") else if ((score 6)
(score 5)) printf("C\n") else if
((score 4) (score 3)) printf("D\n")
else printf("F\n") / end of program /
Unless are used the else matches the first
if in the code above.
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switch Selection Structure
1. Problem Definition Redo the previous problem
by using a switch statement rather than the
nested if-else statement.
Pseudo-code Algorithm print enter score read
score switch score score 9,10 print
A score 7,8 print B score 5,6
print C score 3,4 print D otherwise
print F
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C Problem Example
/ C Program to compute the letter grade for a
quiz. / include ltstdio.hgt void main(void)
int score / Declare variables. / /
request user input / printf("Please enter a
score ") scanf("i", score) / read value
/ / Output the grade / / continued on
the next slide /
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C Problem Example
switch (score) case 10 case 9
printf("A\n") break case 8 case
7 printf("B\n") break case 6
case 5 printf("C\n") break case
4 case 3 printf("D\n") break
default printf("F\n") / end of switch
/ / end of program /
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switch Selection Structure
The switch structure can be used when we have
multiple alternatives based on a value of type
int or type char (but not float or double). This
structure has the general form
switch (controlling expression) case
label1 label1_statement(s) case
label2 label2_statement(s) default defau
lt_statement(s)
(controlling expr. must evaluate to an integer or
a character value each case should end with a
break stmt.)
. ..