1. Conditionals, Loops, and Other Kinds of Statements
Assumptions:
Graduate level
Operating Systems
Making Choices about operation systems
Why a micro-century? …just about enough time for one concept
CIS 1057 Computer Programming in C
Fall 2013
(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements

2. Conditionals, Loops, and Other Statements
Review
Functions
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 2

3. Conditionals, Loops, and Other Statements
If-else Statements
if (expr)
statement1 /*do this if expr is non-zero*/
else
statement2 /*do this if expr is zero*/
The else clause is optional!
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 3

4. Conditionals, Loops, and Other Statements
If-else Examples
if (j > limit)
return j; /* note semicolon*/
else
j += stepValue; /* note semicolon*/
...
if (++k >= 4)
k = 0; /* increment k mod 4*/
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 4

5. If-else Examples (continued)
if (n < maxInput) {
scanf("string", &arg1, &arg2, …);
n++;
printf("string2", arg3, arg4, …);
} else { /* note NO semicolon*/
printf("Summary\n", arg5, …);
return n; }
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 5

6. Concatenated If-else Statements
if (expr1) statement /*do this if expr1 is non-zero*/
else if (expr2) statement /*i.e., expr1 == 0, expr2 != 0*/
else if (expr3) statement3 /expr1 and expr2 are zero, expr3 != 0*/
else if (expr4) statement4 /expr1, expr2 , expr3 all zero, expr4 != 0*/
else statement5 /*i.e., all expr are zero*/
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 6

7. Concatenated If-else Statements
Last else is always attached to last if
If it should be attached to any other if, use {} to control the flow of execution.
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 7

8. Conditionals, Loops, and Other Statements
Switch Statement
Somewhat like a concatenated if-else
Occasionally easier to read
Each arm is called a case
Evaluate switch expression, select the appropriate case (if any)
default case is optional
Difference from concatenated if-else
Need break statement to exit switch after a case
Otherwise, control falls through to next case
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 8

9. Switch Statement Example
int month, daysInMonth, leapYear; …
switch (month) { case 0: printf("January\n"); daysInMonth = 31; break; case 1: printf("February\n"); daysInMonth = leapYear ? 29 : 28; break; case 2: printf("March\n"); daysInMonth = 31; break; case 3: printf("April\n"); daysInMonth = 30; break; …
} // switch
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements

10. Switch Statement Example
case values must be constants
int month, daysInMonth, leapYear; …
switch (month) { case 0: printf("January\n"); daysInMonth = 31; break; case 1: printf("February\n"); daysInMonth = leapYear ? 29 : 28; break; case 2: printf("March\n"); daysInMonth = 31; break; case 3: printf("April\n"); daysInMonth = 30; break; …
} // switch
break statement needed to jump over subsequent cases
default case is optional (not shown)
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements

11. Conditionals, Loops, and Other Statements
Questions?
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 11

12. Conditionals, Loops, and Other Statements
Iterative Statement
while loop
for loop
do-while loop
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements

13. Conditionals, Loops, and Other Statements
while loops
while (expression) statement
Evaluate expression
If true, execute statement and then repeat
Repeat until expression becomes false
statement may be executed zero or more times!
Often a compound statement
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 13

14. Conditionals, Loops, and Other Statements
while loop example
Note initialization of sum and count
int sum = 0;
int count = 0;
int input;
while (scanf("%d", &input) != EOF){
sum += input; count++; printf("Input value of %f recorded.\n", input);
... }
if (count > 0)
printf("Average is %f\n", (double)sum/count);
else
printf("No inputs recorded\n");
What is this?
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 14

15. while loop examples (continued)
A program to count digits, white space characters, and other characters.
Includes a while loop with a switch statement inside
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 15

16. Conditionals, Loops, and Other Statements
do-while loop
Note: semicolon is required here
do statement while (expression);
Similar to while loop, but guaranteed to execute statement at least once
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 16

17. Conditionals, Loops, and Other Statements
Breaking out of a Loop
When it becomes necessary to terminate a loop prematurely
break; /*exits smallest containing switch or loop*/
When it becomes necessary to terminate the current iteration and start the next one
continue; /*terminates this iteration only*/
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 17

18. Conditionals, Loops, and Other Statements
Questions?
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 18

19. Conditionals, Loops, and Other Statements
for loop
Remember: zero is false, non-zero is true
A counting loop
for (expr1; expr2; expr3) statement
Evaluate expr1 to initialize
Evaluate expr2 to test
If true, execute statement
If not true, exit for loop
Evaluate expr3 to prepare for next iteration
Repeat expr2 to test
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 19

20. for loops and while loops
The for loop
for (expr1; expr2; expr3) statement
is exactly equivalent to the following
expr1; while (expr2) { statement expr3; }
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 20

21. The most common kind of for-loop
int i;
for (i = 0; i < limit; i++) {
...;
/* do something with ith entity */ }
Loop “body” is typically a compound statement
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 21

22. The most common kind of for-loop
int i;
for (i = 0; i < limit; i++) {
...;
/* do something with ith entity */ }
It is traditional in C that for-loops start counting at zero and test that the counter is less than the upper limit
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 22

23. The most common kind of for-loop
int i;
for (i = 0; i < limit; i++) {
...;
/* do something with ith entity */ }
This loop iterates limit times.
Iterations are numbered i = 0, 1, 2, …, limit-1
Reason:– arrays are indexed this way!
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 23

24. Conditionals, Loops, and Other Statements
Nested for-loops
int i, j;
for (i = 0; i < limit; i++) {
...; /*prepare subgroup i*/
for (j=0; j < limit2; j++) {
...;
/* do something with item j of
subgroup i*/ }
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 24

25. An Extension in Modern C Compilers
The following construct is legal in C99:–
for (int i = 0; i < limit; i++){
expression involving i; ...; printf(“Iteration %d completed.\n”, i);
...; }
The loop counter i is declared in for loop
Not visible outside of loop!
Common practice
Good programming style
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 25

26. Conditionals, Loops, and Other Statements
Notes on Loop Style
for (int i = 0; i < limit; i++) {
...; /*prepare for subgroup i*/
for (int j=0; j < limit2; j++) {
...;
/* do something with item j of
subgroup i*/
} /* end of loop j */
} /* end of loop i */
Declare loop variables in for-statements – C99
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 26

27. Conditionals, Loops, and Other Statements
Notes on Loop Style
for (int i = 0; i < limit; i++) {
...; /*prepare for subgroup i*/
for (int j=0; j < limit2; j++) {
...;
/* do something with item j of
subgroup i*/
} /* end of loop j */
} /* end of loop i */
Include a comment at end of each loop to show which loop it is
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 27

28. Conditionals, Loops, and Other Statements
Questions?
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 28

29. Conditionals, Loops, and Other Statements
An Example
int startingDay; /* init from user input*/
for (int month = 0; month < 12; month++) {
} // for month
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 29

30. An An Example (continued)
This variable is “global” to the loop. I.e., it is remembered from one iteration to the next.
int startingDay; /* init from user input*/
for (int month = 0; month < 12; month++) {
} // for month
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 30

31. An Example (continued)
int startingDay; /* init from user input*/
for (int month = 0; month < 12; month++) {
} // for month
At beginning of each iteration, startingDay indicates the day of the week on which that particular month starts.
It is the responsibility of the loop to update startingDay for the next month.
CIS 1057 Fall 2013
Conditionals, Loops, and Other Statements 31