1. 16.216 ECE Application Programming
Instructor: Dr. Michael Geiger
Summer 2012
Lecture 4
Conditional statements: if and switch
Exam 1 Preview

2. ECE Application Programming: Lecture 4
Lecture outline
Announcements/reminders
Program 2 graded; regrades due Monday, 7/23
Program 3 due Friday, 7/20
Program 4 due Thursday, 7/26
Exam 1 next Tuesday, 7/24
Allowed one 8.5” x 11” sheet of notes
No electronic devices
Review
Operators
Output formatting
Today’s lecture
Conditional statements
Exam 1 Preview
Note: will not go through PE2 (conditionals/formatting), but will post last semester’s slides/code on web
5/6/2018
ECE Application Programming: Lecture 4

3. ECE Application Programming: Lecture 4
Review: C operators
Operator
Associativity
Innermost ( )
Left to right
Unary -, unary ~
Right to left
* / %
<< >>
NOTE: shift amt < 32
& ^ |
5/6/2018
ECE Application Programming: Lecture 4

4. ECE Application Programming: Lecture 4
Review: field width
Format specifier for printf() takes form:
%[flags][width][.precision]
Field width: minimum # chars to be printed
Never truncates—if more chars than width, prints all
Default behavior—right justify value, print spaces
printf(“%10d”, n); 
(8 spaces, then number 12)
Flags allow you to add sign (+), left justify (-), and add leading zeroes (0)
printf(“%+-10d”, n);  +12_______
(Number 12 with sign, then 7 spaces)
printf(“%010d”, n); 
To use a variable to specify field width, use * as the width
printf(“%*d”, n, n); 
(Field width = 12  10 spaces, then the number 12)
5/6/2018
ECE Application Programming: Lecture 4

5. ECE Application Programming: Lecture 4
Review: precision
Format specifier for printf() takes form:
%[flags][width][.precision]
Specifying precision:
# chars after decimal point for FP (%f, %lf)
Rounds last digit
printf(“%.1lf”, x);  3.8
printf(“%.0lf”, x);  4
Minimum # chars for integer (%d, %i, %u, %x)
Does not truncate; will pad with leading 0s
printf(“%.3d”, n);  012
Max # chars for string (%s)
printf(“%.5s”, “one two”);  one t
No effect for char (%c)
As with field width, can use * to specify that field width is a variable
5/6/2018
ECE Application Programming: Lecture 4

6. Formatted output: hexadecimal
To print a number in hex, use %x or %X
%x prints characters a-f in lowercase
%X prints characters A-F in uppercase
To show leading 0x, use the # flag
To show leading 0s, use precision with total # chars
Field width + 0 flag also works unless value = 0
Examples (assume var1 = 0x1A2B)
printf(“%x”, var1)  1a2b
printf(“%X”, var1)  1A2B
printf(“%#x”, var1)  0x1a2b
printf(“%.6x”, var1)  001a2b
printf(“%#.6x”, var1)  0x1a2b
5/6/2018
ECE Application Programming: Lecture 4

7. ECE Application Programming: Lecture 4
Decisions
Recall that flowcharts can include decisions
Conditionally execute some path
May want to:
Only perform operation if condition is true:
A = 0?
TRUE
FALSE
A = 0?
TRUE
X = x + 1
FALSE
5/6/2018
ECE Application Programming: Lecture 4

8. ECE Application Programming: Lecture 4
Decisions (cont.)
May want to:
Perform one operation if condition is true, another if false:
A = 0?
TRUE
X = x + 1
FALSE
X = x - 1
5/6/2018
ECE Application Programming: Lecture 4

9. ECE Application Programming: Lecture 4
Decisions (cont.)
May want to:
Check multiple conditions, in order
A = 0?
TRUE
X = x + 1
FALSE
B=1?
TRUE
X = x - 1
FALSE
X = 0
5/6/2018
ECE Application Programming: Lecture 4

10. ECE Application Programming: Lecture 4
if statements
Frequently want to conditionally execute code
Range checking
Error checking
Different decisions based on input, or result of operation
Basic conditional execution: if statement
Form:
if (<expression>)
<statement>
[ else brackets show
<statement> ] else is optional
5/6/2018
ECE Application Programming: Lecture 4

11. ECE Application Programming: Lecture 4
if statements (cont.)
<expression> can be any valid expression
Considered “false” if 0, “true” if nonzero
Can use comparisons:
Greater than/less than: > <
e.g. if (a < b)
Greater than or equal/less than or equal: >= <=
e.g. if (x <= 20)
Equal/not equal: == !=
e.g. if (var == 10)
5/6/2018
ECE Application Programming: Lecture 4

12. ECE Application Programming: Lecture 4
if statements (cont.)
<expression> can be any valid expression
Can combine multiple conditions using
Logical AND: &&
Logical OR: ||
e.g. if ((x < 3) && (y > 5))
Always put each condition in parentheses
Can test inverse of condition using logical NOT: !
e.g. if (!(x < 3))  equivalent to if (x >= 3)
These operators: not bitwise operators!
A & B is a bitwise operation
A && B has only 2 possible results: 0 or “non-zero”
5/6/2018
ECE Application Programming: Lecture 4

13. ECE Application Programming: Lecture 4
if statements (cont.)
<statement> can be one or more lines
If just one line, no additional formatting needed
if (x < 3)
printf(“x = %d\n”, x);
If multiple lines, statement is block enclosed by { } {
x = x + 3; }
else part is optional—covers cases if condition is not true
5/6/2018
ECE Application Programming: Lecture 4

14. ECE Application Programming: Lecture 4if
if (a > b) big = a; else big = b;
if (a+6*3-43) printf("wow is this not cool"); else printf("this is not cool");
5/6/2018
ECE Application Programming: Lecture 4

15. ECE Application Programming: Lecture 4
if (common pitfalls)
a single equals means ASSIGN.
a double equal must be used to check for equality.
x=12345;
if (x=3) { printf("x is 3\n"); } else { printf("x is not 3\n"); }
This code will ALWAYS print: x is 3
5/6/2018
ECE Application Programming: Lecture 4

16. ECE Application Programming: Lecture 4
if (example)
void main(void) { float a,b,c,disc; : scanf("%f %f %f",&a,&b,&c); if (a==0) { // statements } else { disc = b*b-4*a*c; if ( disc < 0 ) { // statements } else { // statements } } }
5/6/2018
ECE Application Programming: Lecture 4

17. Example: if statements
What does the following code print?
int main() {
int x = 3;
int y = 7;
if (x > 2)
x = x - 2;
else
x = x + 2;
if ((y % 2) == 1) {
y = -x;
if ((x != 0) && (y != -1))
y = 0; }
printf("x = %d, y = %d\n", x, y);
return 0;
5/6/2018
ECE Application Programming: Lecture 4

18. ECE Application Programming: Lecture 4
Example solution
int main() {
int x = 3;
int y = 7;
if (x > 2)  Condition is true, since 3 > 2
x = x - 2;  x set to 1
else
x = x + 2;
if ((y % 2) == 1)  Tests if y is an odd number--true condition {
y = -x;  y set to -1
if ((x != 0) && (y != -1))  First part of condition is true,
second part is false--overall false
y = 0; }
printf("x = %d, y = %d\n", x, y);  Prints: x = 1, y = -1
return 0;
5/6/2018
ECE Application Programming: Lecture 4

19. if (range checking - take 1)
int n;
printf("Enter a number 1 to 10: "); scanf("%d",&n); if (n > 10) { printf(“That’s not in range!"); } else if (n < 1) { printf(“That’s not in range!"); } else { printf("Good job!"); }
5/6/2018
ECE Application Programming: Lecture 4

20. if (range checking - take 2)
If there is only one statement needed for the true and/or false condition, the {} are not needed
int n;
printf("Enter a number 1 to 10: "); scanf("%d",&n); if (n > 10) printf(“That’s not in range!"); else if (n < 1) printf(“That’s not in range!"); else printf("Good job!");
5/6/2018
ECE Application Programming: Lecture 4

21. if (range checking - take 3)
Use the && or || as needed to check for multiple conditions
int n;
printf("Enter a number 1 to 10: "); scanf("%d",&n); Note these ( ) are needed
if ( (n > 10) || (n < 1) ) printf(“That’s not in range!"); else printf("Good job!");
5/6/2018
ECE Application Programming: Lecture 4

22. if (range checking - take 4)
Use the && or || as needed to check for multiple conditions
int n;
printf("Enter a number 1 to 10: "); scanf("%d",&n); Note these ( ) are needed
if ( (1 <= n) && (n <= 10) ) printf(“Good job!"); else printf(“That’s not in range!");
5/6/2018
ECE Application Programming: Lecture 4

23. if (range checking) (The WRONG WAY)
int n;
printf("Enter a number 1 to 10: "); scanf("%d",&n);
if (1 <= n <= 10 ) // THIS WILL NOT COMPILE printf("Good job!"); else printf(“That’s not in range!");
5/6/2018
ECE Application Programming: Lecture 4

24. Example: if statements
Write a short code sequence to do each of the following:
Given int x, check its value
If x is greater than 5 and less than or equal to 10, print x
Prompt for and read temperature as input (type double)
If temp is 90 or higher, print “It’s too hot!”
If temp is 32 or lower, print “It’s freezing!”
In all other cases, print “It’s okay”
Read 3 int values and print error if input problem
Values are separated by a comma
If fewer than 3 values read, print error message with number of values
Example: Error: only 2 inputs read correctly
5/6/2018
ECE Application Programming: Lecture 4

25. ECE Application Programming: Lecture 4
Example solution
Given int x, check its value
If x is greater than 5 and less than or equal to 10, print x
if ((x > 5) && (x <= 10))
printf(“%d\n”, x);
5/6/2018
ECE Application Programming: Lecture 4

26. Example solution (cont.)
Prompt for and read temperature as input (type double)
If temp is 90 or higher, print “It’s too hot!”
If temp is 32 or lower, print “It’s freezing!”
In all other cases, print “It’s okay”
int main() {
double temp;
printf(“Enter temperature: “);
scanf(“%lf”, &temp);
if (temp >= 90)
printf(“It’s too hot!\n”);
else if (temp <= 32)
printf(“It’s too cold!\n”);
else
printf(“It’s okay\n”);
return 0; }
5/6/2018
ECE Application Programming: Lecture 4

27. Example solution (cont.)
Read 3 int values and print error if input problem
Values are separated by a comma
If fewer than 3 values read, print error message with number of values
Example: Error: only 2 inputs read correctly
int main() {
int x, y, z; // Input values
int num; // # values read
num = scanf(“%d,%d,%d”, &x, &y, &z);
if (num < 3)
printf(“Error: only %d inputs read correctly”,
num);
return 0; }
5/6/2018
ECE Application Programming: Lecture 4

28. ECE Application Programming: Lecture 4
switch statements
Nesting several if/else if statements can get tedious
If each condition is simply checking equality of same variable or expression, can use switch
5/6/2018
ECE Application Programming: Lecture 4

29. switch/case statement - General form
switch ( <expression> ) { case <value1> : <statements> [ break; ]
case <value2> : <statements> [ break; ] .
. [ default: <statements> [ break; ] ] }
5/6/2018
ECE Application Programming: Lecture 4

30. switch/case statement
Check if <expression> matches any value in case statements
If <expression> == <value1>, execute <statements> in that case
If <expression> == <value2>, execute <statements> in that case
If <expression> does not equal any of the values, go to default case (if present)
5/6/2018
ECE Application Programming: Lecture 4

31. Switch statements and break
Each case is just a starting point—switch does not automatically skip other cases!
Example:
switch (x) {
case 0:
x = 3;
case 1:
x = x * 4;
default:
x = x – 1; }
If x == 0:
Start at case 0  x = 3;
Then, go to case 1  x = x * 4 = 3 * 4 = 12
Then, go to default:  x = x – 1 = 12 – 1 = 11
5/6/2018
ECE Application Programming: Lecture 4

32. Switch statements and break
Use break to exit at end of case
You may not always want to use break—will see examples later
Rewriting previous example:
switch (x) {
case 0:
x = 3;
break;
case 1:
x = x * 4;
default:
x = x – 1; }
5/6/2018
ECE Application Programming: Lecture 4

33. switch/case statement - example
#include <stdio.h>
int main() { char grd; printf("Enter Letter Grade: "); scanf("%c",&grd); printf(“You are ");
// continued next slide
5/6/2018
ECE Application Programming: Lecture 4

34. switch/case statement - example
switch (grd) { case 'A' :
printf("excellent"); break; case 'B' :
printf("good"); break; case 'C' :
printf("average"); break; case 'D' :
printf("poor"); break; case 'F' :
printf("failing"); break; default :
printf(“incapable of reading directions"); break; } return 0; }
5/6/2018
ECE Application Programming: Lecture 4

35. Example: switch statement
What does the program on the previous slides print if the user enters: A B+ c X
Recognize, of course, that it always prints:
Enter Letter Grade:
5/6/2018
ECE Application Programming: Lecture 4

36. ECE Application Programming: Lecture 4
Example solution
What does the program on the previous slides print if the user enters: A
You are excellent B+
Only first character is read—’B’
You are good c
This program is case-sensitive—’C’ and ‘c’ are two different characters!
Will go to default case
You are incapable of reading directions X
No case for ‘X’—goes to default case
5/6/2018
ECE Application Programming: Lecture 4

37. switch/case statement - Alt example
switch (grd) { case 'A' : case 'a': case 'B' : case 'b': printf("doing very well"); break; case 'C' : case 'c': case 'D' : case 'd': printf("not doing too well"); break; case 'F' : case ‘f': printf("failing"); break; default :
printf("incapable of reading directions"); break; } return 0; }
5/6/2018
ECE Application Programming: Lecture 4

38. ECE Application Programming: Lecture 4
Exam 1 notes
Allowed one 8.5” x 11” double-sided sheet of notes
No other notes
No electronic devices (calculator, phone, etc.)
Exam will last 50 minutes
Covers all lectures through today
You will not be tested on design process, IDEs
Material starts with basic C program structure
5/6/2018
ECE Application Programming: Lecture 4

39. Review: Basic C program structure
Preprocessor directives
#include: typically used to specify library files
#define: generally used to define macros
We’ll use to define constants
Main program
Starts with: int main()
Enclosed in block: specified by { }
Ends with return 0;
Indicates successful completion
Basic output
Call printf(<string>);
<string> can be replaced by characters enclosed in double quotes
May include escape sequence, e.g. \n (new line)
5/6/2018
ECE Application Programming: Lecture 4

40. Review: Data types, variables, constants
Four basic data types
int, float, double, char
Constants
Discussed viable ranges for all types
#define to give symbolic name to constant
Variables
Have name, type, value, memory location
Variable declarations: examples
int x;
float a, b;
double m = 2.35;
5/6/2018
ECE Application Programming: Lecture 4

41. ECE Application Programming: Lecture 4
Review: C operators
Operator
Associativity
Innermost ( )
Left to right
Unary -, unary ~
Right to left
* / %
<< >>
& ^ |
5/6/2018
ECE Application Programming: Lecture 4

42. Review: Operators and statements
Operators can be used either with constants or variables
More complex statements are allowed
e.g. x = ;
Parentheses help you prioritize parts of statement
Makes difference with order of operations
x = * 3
is different than
x = (1 + 2) * 3
5/6/2018
ECE Application Programming: Lecture 4

43. Review: bit manipulation
Bitwise operators: | & ^ ~
Used for desired logical operations
Used to set/clear bits
Bit shifts: << >>
Used to shift bits into position
Used for multiplication/division by powers of 2
Common operations
Setting/clearing/flipping individual bit
Setting/clearing/flipping multiple bits
5/6/2018
ECE Application Programming: Lecture 4

44. Review: printf() and scanf() basics
To print variables (or constants), insert %<type> in your printf() format string
%c: single character
%d or %i: signed decimal integer
%u: unsigned decimal integer
%x or %X: unsigned hexadecimal integer
%f: float
%lf: double
%s: string
Each %<type> must correspond to a variable or constant that follows
printf("a=%f, b=%f",a,b);
To read input, use same format specifiers in scanf() format string, followed by addresses of variables
scanf("%d %f",&hours,&rate);
5/6/2018
ECE Application Programming: Lecture 4

45. ECE Application Programming: Lecture 4
Review: field width
Format specifier for printf() takes form:
%[flags][width][.precision]
Field width: minimum # chars to be printed
Never truncates—if more chars than width, prints all
Default behavior—right justify value, print spaces
printf(“%10d”, n); 
(8 spaces, then number 12)
Flags allow you to add sign (+), left justify (-), and add leading zeroes (0)
printf(“%+-10d”, n);  +12_______
(Number 12 with sign, then 7 spaces)
printf(“%010d”, n); 
To use a variable to specify field width, use * as the width
printf(“%*d”, n, n); 
(Field width = 12  10 spaces, then the number 12)
5/6/2018
ECE Application Programming: Lecture 4

46. ECE Application Programming: Lecture 4
Review: precision
Format specifier for printf() takes form:
%[flags][width][.precision]
Specifying precision:
# chars after decimal point for FP (%f, %lf)
Rounds last digit
printf(“%.1lf”, x);  3.8
printf(“%.0lf”, x);  4
Minimum # chars for integer (%d, %i, %u, %x)
Does not truncate; will pad with leading 0s
printf(“%.3d”, n);  012
Max # chars for string (%s)
printf(“%.5s”, “one two”);  one t
No effect for char (%c)
As with field width, can use * to specify that field width is a variable
5/6/2018
ECE Application Programming: Lecture 4

47. Review: hexadecimal output
To print a number in hex, use %x or %X
%x prints characters a-f in lowercase
%X prints characters A-F in uppercase
To show leading 0x, use the # flag
To show leading 0s, use precision with total # chars
Field width + 0 flag also works unless value = 0
Examples (assume var1 = 0x1A2B)
printf(“%x”, var1)  1a2b
printf(“%X”, var1)  1A2B
printf(“%#x”, var1)  0x1a2b
printf(“%.6x”, var1)  001a2b
printf(“%#.6x”, var1)  0x001a2b
5/6/2018
ECE Application Programming: Lecture 4

48. ECE Application Programming: Lecture 4
Review: if statements
Conditional execution using if statements:
Form:
if (<expression>)
<statement>
[ else brackets show
<statement> ] else is optional
Expression frequently uses relational operators to test equality/inequality
< > <= >= == !=
e.g., if (x <= 5)
Can combine conditions using logical operators
AND : && OR: ||
e.g., if ((x <= 5) && (x > 0))
Can test if condition is false using logical NOT: !
e.g., if (!(x < 5))
5/6/2018
ECE Application Programming: Lecture 4

49. Review: switch statements
When checking multiple exact values for expression, more sense to use switch statement
switch (<expr>) {
case <val1> :
...
break;
case <val2> :
default: }
break allows you to exit switch statement after completing code
Otherwise, program will continue to run through cases until finding break
default covers any values without specific case
5/6/2018
ECE Application Programming: Lecture 4

50. ECE Application Programming: Lecture 4
Next time
Exam 1
Allowed only one 8.5” x 11” note sheet
Please arrive as close to on time as possible!
Reminders
Program 3 due Friday
Program 4 due Thursday, 7/26
5/6/2018
ECE Application Programming: Lecture 4