CS 101 Introductory Programming - Lecture 7: Loops In C & Good Coding Practices Presenter: Ankur Chattopadhyay
Feb 24, 2016
CS 101 Introductory Programming - Lecture 7: Loops In C & Good Coding
PracticesPresenter: Ankur Chattopadhyay
2
Command
Command
Command
Command
Command
Sequential Structure(straight-line structure)
Command
Test
Commandsto execute
if False
Command
Commandsto execute
if True
TrueFalse
Example Selection Structure(decision or branching structure)
Flowcharts for sequential, selection and iterative control structures
2Iterative Structure(looping structure)
Command
Commands
Command
Setup Done
Repetition Statements• Repetition statements allow us to execute
a statement multiple times• Often they are referred to as loops• Like conditional statements, they are
controlled by Boolean expressions• C has three kinds of repetition statements:– the while loop– the do loop– the for loop
The for Statement• A for statement has the following syntax:
for ( initialization ; condition ; increment ){ statement;}
The initializationis executed once
before the loop begins
The statement isexecuted until the
condition becomes false
The increment portion is executed at the end of each
iteration
Logic of a for loop
statement
true
conditionevaluated
false
increment
initialization
//Multiplication Algorithmint a, b, product, count;printf("Enter two non-negative numbers: ");scanf("%i %i", &a, &b);for (product = 0, count = 0; count < b; count = count + 1) { product = product + a;}printf("The product of %i and %i is %i\n", a, b, product);
//Sum of first n natural numbersint sum = 0; // the accumulatorfor (int i = 1; i <= n; i++) { sum += i; }
Examples of for loop: Lecture 6
Dissecting The for Statement• Each expression in the header of a for loop is
optional• If the initialization is left out, no initialization is
performed• If the condition is left out, it is always
considered to be true, and therefore creates an infinite loop– We usually call this a “forever loop”
• If the increment is left out, no increment operation is performed
The while Statement• A while statement has the following syntax:
while ( condition ){ statement;}
• If the condition is true, the statement is executed
• Then the condition is evaluated again, and if it is still true, the statement is executed again
• The statement is executed repeatedly until the condition becomes false
Logic of a while Loop
statement
true false
conditionevaluated
The while Statement• An example of a while statement:
int count = 1;while (count <= 5){printf(“%i”, count); count++; }
• If the condition of a while loop is false initially, the statement is never executed
• Therefore, the body of a while loop will execute zero or more times
Analyzing The while Statement• A while loop is functionally
equivalent to the following structure:
initialization;while ( condition ){ statement; increment;}
// for loop example: Lecture 6for (product = 0, count = 0; count < b; count = count + 1) { product = product + a;}Counter-Controlled Loop - Definite Repetition// equivalent while loop (note: a for loop is more appropriate in this case)product = 0;count = 0;while (count < b) { product = product + a; count = count + 1;}
Examples of while loop
Sentinel-Controlled Loop: Indefinite Repetition
Nesting an if statement inside a loopint keep_going = 1;
while (keep_going == 1) { // computation would go here ...
int answer; printf("Continue? (1 for yes, 0 for no) "); scanf("%i", &answer); if (answer == 0) { keep_going = 0; }}
Another Example of while loop
The do-while Statement• A do-while statement (also called a do
loop) has the following syntax:do{ statement;} while ( condition )
• The statement is executed once initially, and then the condition is evaluated
• The statement is executed repeatedly until the condition becomes false
Logic of a do-while Loop
true
conditionevaluated
statement
false
Example of a do Statement
• An example of a do loop:
• The body of a do loop executes at least once
int count = 0;do{ count++; printf(“%i”, count);} while (count < 5);
Comparing while and do
statement
true false
conditionevaluated
The while Loop
true
conditionevaluated
statement
false
The do Loop
Infinite Loops• The body of a while loop eventually must
make the condition false• If not, it is called an infinite loop, which
will execute until the user interrupts the program
• This is a common logical (semantic) error• You should always double check the logic
of a program to ensure that your loops will terminate normally
Example: Infinite Loop• An example of an infinite loop:
int count = 1;while (count <= 25){ printf(“%i”, count); count = count - 1;}
• This loop will continue executing forever
Good Coding Style Practice
• Always place braces around the body of a while loop.
• Advantages:– Easier to read– Will not forget to add the braces if you go back
and add a second statement to the loop body– Less likely to make a semantic error
• Indent the body of a loop - use spaces -- be consistent!
Some Case Studies/Sample Programs To Try Out
• Problem 1:- Write a C program that takes a series of integer
scores as inputs using a loop, exits if a score entered is negative and computes the average of all the scores entered.
• Problem 2:- Write a C program that takes an integer number
as input and determines whether the number is prime or not.
• Problem 3: Using loops in a C program print the following pattern - 1
1 2 1 2 3 1 2 3 4 1 2 3 4 5 (Hint: Use nested loops)