Assignment Announcements
3 out today
Assignment 3 and 4 logisitics
Exam 1 Thursday
55 minute exam (9:30 10:25)
Program Development Measurement g p
SLOC, person hours, logic defects
Topics for today Topics for today
Scope of variables Exam Review Exam Review
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Global (External) Variables
Variables declared outside functions are global External variables can be shared by several (all) functions:
#include <stdio.h> int i; /* i is a global variable */ void print_count(void) { printf("T minus %d and counting\n", i); } int main(void) { for (i = 10; i > 0; i) { print_count(); } return 0; }
E External variables retain their values throughout the execution of the program l i bl i h i l h h h i f h External variables should be used sparingly, if at all
If an external variable is assigned an incorrect value, it is difficult to identify the guilty function Functions that use external variables are hard to reuse
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Local Variables
Variables declared inside a function are local to that function Variables declared inside a function are local to that function Example:
float max(float a, float b) { float big; / big is a local variable / float big; /* big is a local variable */ if (a > b) { big = a; } else { big = b; } return big; return big; }
Local variable exists only when the enclosing function is executing L l Local variable is visible only to statements in the enclosing function i bl i i ibl l i h l i f i Names of local variables can be reused for other purposes in the same program same program
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Parameters
A function header can contain parameter declarations f h d d l
void swap( int i, int j) { int temp; int temp; temp = i; i = j; j = temp; j = temp; }
A variable declared as a parameter in a function header a ab e dec a ed as a pa a ete a u ct o eade exists only as long as the function is executing It is a local variable It derives its value from the arguments passed at function call time
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Inside of Blocks
A block of statements can contain declarations
if (i < j) {
int temp; int temp; temp = i; i = j; j = temp;
}
A variable declared in a block exists only as long as statements in the block are executing y A variable declared in a block is visible only to statements in the block A block can appear anywhere a statement is A block can appear anywhere a statement is allowed
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Scope Rules
When a declaration inside a block (or function) uses an identifier When a declaration inside a block (or function) uses an identifier that is already visible, the new declaration temporarily "hides" the old one At th At the end of the block, the identifier regains its old meaning d f th bl k th id tifi i it ld i
int i; /* Declaration 1 global */ void f (int i) { /* Declaration 2 a parameter */ i = 1; i = 1; } void g (void) { int i; ; / /* Declaration 3 a local */ / if (i > 0) { int i; /* Declaration 4 local to block */ i = 2; } Resolution rules: i = 3; } 1. Look for most local void h (void) { void h (void) { 2. Look for a parameter 2 Look for a parameter i = 4; 3. Look for a global }
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Exam Review
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Exam Topics
I Introduction to C (e.g., strengths, weaknesses) d i C( h k )
Chapter 1 and Lecture 1
Variables and Assignment Variables and Assignment
Chapters 2, 4, 7 and Lecture 2
Formatted I/O
Chapter 3 and Lecture 3
Control Statements
Chapters 5 6 and Lectures 4 5 Chapters 5, 6 and Lectures 4, 5
Arrays
Chapter 8 and Lectures 6, 7 p
Functions
Chapter 9 and Lecture 8
Scope of Variables
Chapter 10 and Lecture 9 (today)
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Requested Review Topics
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Loops -- two basic patterns
Countcontrolled loop: Number of iterations is determined before the loop starts Counts each iteration using a counter variable Counts each iteration using a counter variable Stop when the desired number of iterations has been performed:
for (i = 1; i < 100; i++ ) { printf ("the value of i is: %d ",i); }
Eventcontrolled loop: Before each iteration, check whether some event has occurred Continue until that event occurs
Number of iterations not known beforehand Number of iterations not known beforehand
The event signal is in the condition; may change during an iteration. Boolean variable often used to flag the signal:
while (condition) { / loop while the condition is true / while (condition) { /* loop while the condition is true */ /* do other stuff */ }
while Statement
while statement has the form
while ( condition ) statement
The body is executed repeatedly as long as the condition is true (has a The body is executed repeatedly as long as the condition is true (has a nonzero value)
condition is tested before each execution of the body
Example:
i = 10; /* count down from */ 10 while (i > 0) { while (i > 0) { printf ("T minus %d and counting\n", i); i; }
condition
F
T
Using a nonzero constant as the controlling condition creates an infinite loop:
while (1) { hil (1) { ... }
For loop
The for statement has the form
for ( expression1 ; expression2 ; expression3 ) statement
The following code has the equivalent semantics of the above for The following code has the equivalent semantics of the above for statement:
expression1; while (expression2) { hil ( i 2) { statement expression3; }
For example
for (i = 10; i > 0; i ) { printf("T minus %d and counting\n", i); i tf("T i %d d ti \ " i) }
Any or all of the three expressions may be omitted
Omitting the middle expression creates an infinite loop.
Used for counter controlled loops
Tracing Loop Execution
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Review Exercises
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Exercises: Fundamentals
Consider this program:
#include <stdio.h> main( ) { printf("Parkinson's Law:\n Work expands so as to "); printf("fill the time\n"); i tf("fill th ti \ ") printf("available for its completion.\n"); return 0; return 0; }
How many directives? What are they? How many directives? What are they? How many statements? What are they?
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Exercises: Fundamentals
Which of the following are keywords in C?
for If main printf while
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Exercises: Fundamentals
Write a program that computes the volume of a p g sphere with a 10 meter radius using the formula v = 4/3r3 Output the result Output the result Modify the program to take the radius as an input
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Exercises: Expressions
Write a program that asks the user to enter a g , p twodigit number, then prints the number with its digits reversed. Hint: If n is an integer, n % 10 is the last digit in n Hint: If n is an integer, n % 10 is the last digit in n and n/10 is n with the last digit removed.
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Exercises: Selection Statements
Write a program that determines the number of g p digits in a number input from the user. Assume the number has no more than 4 digits.
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Exercises: Selection Statements
Using a switch statement, write a program that asks the user for a twodigit number, then prints g , p the English word for that number. For example:
Enter a two-digit number: 45 two digit You entered the number forty-five.
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Exercises: Loops
Write a program that asks the user to enter two g , p y integers, then calculates and displays their greatest common divisor (GCD):
Enter two integers: 12 28 Greatest common divisor: 4
Hint: Use Euclid's algorithm to compute the GCD Hint: Use Euclid s algorithm to compute the GCD
Let m and n be variables containing the two numbers. Divide m by n. Save the divisor in m, and numbers Divide m by n Save the divisor in m and save the remainder in n. If n is 0, then stop: m contains the GCD. Otherwise repeat the process, p p starting with the division of m by n.
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Exercises: Functions
Rewrite the previous program to use one or more functions.
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Exercises: Loops and Functions
Write a program that asks the user to enter a , fraction, then converts the fraction to lowest terms (e.g., 6/12 would be converted to 1/2) Hint: to convert a fraction to lowest terms, first Hint: to convert a fraction to lowest terms, first compute the GCD of the numerator and denominator. Then divide both by the GCD. denominator. Then divide both by the GCD.
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Exercises: Arrays
Write a program that reads a 5 x 5 array of g p integers and then prints the row sums and the column sums
Enter row 1: 1 Enter row 2: 6 Enter row 3: 1 Enter row 4: 6 Enter row 5: 1 Row totals: 15 Column totals: 2 3 7 8 2 3 7 8 2 3 30 15 4 5 9 0 4 5 9 0 4 5 15 30 15 20 25 30 15
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Exercises: Functions
The following function, which computes the area g , of the triangle, contains two errors. What are they?
float triangle_area(float base, height) float product; { p product = base * height; g return (product / 2); }
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Exercises: Functions
Write functions that return the following values ( (assuming a and n are parameters where a is an g p array of int values and n is the (integer) length of the array) y)
The largest element in a The average of all elements in a g The number of positive elements in a
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Exercises: Test-Style Questions
Consider the following code segment. What is p the value of the variable x after the for loop?
int j, x = 2; for (j = 0; j <= 3; j++) { x = x + j * x; }
Hint: trace the execution of the loop
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Exercises: Test-Style Questions
Complete the function below named invertMatrix that takes a 2D array named v y and inverts it. The 2D array v has n x n elements.
void invertMatrix (int v[ ][ ], int n) {
}
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