Homework / Exam
• Turn in HW3 today
• Exam 1 next class
– Open Book / Open Notes
– Recommended Use of Book / Notes in Exam:
•
•
•
•
•
Avoids reliance on “rote memorization” of details
Use for reference to tables, figures, etc.
Look up a specific syntax detail you’ve forgotten
Don’t expect to copy code for the answer to a problem!!
If you find yourself copying code – STOP! THINK!
• Start Chapter 4 for class after Exam
1
Statements / Blocks, Section 3.1
• An expression becomes a statement when it is
followed by a semicolon
x = 0;
• Braces are used to group declarations and statements
into a compound statement
{
x = 0;
y = 1;
}
/* Note: No semicolon after right brace */
2
if statements, Section 3.2
• Shortcut for “equal and not equal to 0” tests
• Can use:
if (expression)
if (!expression)
• In place of
if (expression != 0) if (expression == 0)
3
if-else Matching, Section 3.2
• Else is matched with the “closest previous
else-less if”
• Example (compiler won’t interpret as the code
has been indented on the left. See right side):
if (a)
if (a)
if (b)
if(b)
statement;
statement;
else
else
statement;
statement;
4
Switch, Section 3.4
• Consider cascading else-if sequence:
if (i == 1)
/* NOTE: Only one will execute */
statement-1;
else if (i == 2)
statement-2;
.
.
.
else if (i == 49)
statement-49;
else
statement-50;
/* Default or "catch-all"
*/
5
Switch
• Also have switch statement **LIKE JAVA**
switch (i) { /* in special limited situations */
case 1: statement-1;
break;
case 2: statement-2;
break;
. . .
case 49: statement-49;
break;
default: statement-50;
}
6
Switch
• The way the if-else cascade works is to test
– Test if i == 1
– If that fails, test if i == 2. If that fails, test if i == 3. . . .
– When we test i == 27, we have done 26 prior tests.
• With the switch statement, we achieve the same
effect, but probably faster:
–
–
–
–
–
Usually compiled into assembly language as a jump table
Array of "go to" instructions subscripted by the value of i
If i = 27 we "look up" the “go to” at address 27 in table
And execute only that one “go to”
Note the need for break statements. The default action is
to cascade down to the code for the next case!!
7
ASCII Hex to Integer
int axtoi (char s[ ])
{
int i, n, flag;
n = 0; flag = 1;
for ( i = 0; flag; i++) /* for (i = 0; ; i++) with ending 2 */
switch (s[i]) {
case '0': case '1': case '2': case '3': case '4': /* fall... */
case '5': case '6': case '7': case '8': case '9': /* through */
n = 16*n + (s[i] - '0');
break; /* need this so don't fall through
*/
8
ASCII Hex to Integer (Ending 1)
case 'a':case 'b':case 'c':case 'd':case 'e': case 'f':
n = 16*n + (s[i] - 'a' + 10);
break;
case ‘A’: case ‘B’: case ‘C’: case ‘D’: case ‘E’: case ‘F’:
n = 16*n + (s[i] - ‘A' + 10);
break;
default: /* stop “for” loop on non hex digit */
flag = 0;
break;
}
return n;
}
9
ASCII Hex to Integer (Ending 2)
case 'a': case 'b': case 'c': case 'd': case 'e': case 'f':
n = 16*n + (s[i] - 'a' + 10);
break;
case ‘A’: case ‘B’: case ‘C’: case ‘D’: case ‘E’: case ‘F’:
n = 16*n + (s[i] - ‘A' + 10);
break;
default: /* stop “for” loop on non hex digit */
goto ret;
}
ret: return n;
}
10
Loops –while and for, Section 3.5
• This “for” statement”
for (expr1; expr2; expr3)
statement;
• Is equivalent to this “while” statement:
expr1;
while (expr2) {
statement;
expr3;
}
11
For loop
• Note any part of for loop can be left out.
for (init; loop-test; increment)
• If init or increment expression is left out,
just not evaluated (program must initialize
and increment by other means)
• If loop-test is left out, assumes permanently
true condition and loops forever. (program
must break or goto to end to exit the loop)
12
Comma operator: ","
• Most often used in for loop statement
• Might have been useful for reverse () in hw1
for (i = 0, j = strlen(s) - 1; i < j; i++, j--)
• Pairs of expressions separated by "," are
evaluated left-to-right and type/value of
expression is type value of result.
• See K & R, pg 53, Precedence Table
13
Do … While, Section 3.6
• The do … while tests at the end of the loop
do {
statement(s);
} while (expression);
• Executes the statement(s) once even if the
“while” loop expression is false upon entry
• Used much less often than “for” and “while”
14
Break and Continue, Sections 3.7
• The break statement works for:
– for loop / while loop / do loop and switch.
– Brings you to end of loop or switch statement
ONE LEVEL ONLY.
• The continue statement works for:
– for loop / while loop / do loop, but not switch!
– It causes next iteration of enclosing loop to begin
15
GOTO and Labels, Section 3.8
• The goto statement breaks TWO OR MORE levels
• It “goes to” a statement with a label, e.g. ret:
• It may be OK to use goto if you always do it in a
FORWARD direction. (See axtoi, if use a goto ret;
then don't need flag or test in “for” loop.)
• K&R, pg66: “goto should be used rarely, if at all.”
• Note: Project coding standards may prohibit “goto”!
16
GOTO and Labels, Section 3.8
• Don’t implement if-else this way!
if (!condition) goto elselabel;
printf("if clause\n");
goto nextlabel;
elselabel:
printf("else clause\n");
nextlabel:
/* next statement in program */
17
Binary Search
/* binsearch:
find x in v[0] <= v[1] <= . . <= v[n-1]
returns subscript of x if found, -1 if not */
int binsearch ( int x, int v[ ], int n)
{
int low, high, mid;
low = 0;
high = n - 1;
18
Binary Search
while ( low <= high) {
mid = (low + high)/2;
if (x < v[mid])
high = mid - 1;
else if (x > v[mid])
low = mid + 1;
else
/* found match */
return mid;
}
return -1; /* no match */
}
19