Repetition and Loops Explained

Repetition and Loops

• A loop is a control structure that repeats a group of steps (statements) in a program
• The loop body contains the statements that are repeated in the loop

Determining If a Loop is Needed

• To solve the problem, determine if any steps need to be repeated. If so, a loop is needed
• If the number of repetitions is known in advance, a counting loop is needed
• If you don't know when to stop the repetition, you will not be able to program the loop

Types of Loops

• Counting Loop: A loop with a fixed number of repetitions e.g., repeating 100 times
• Sentinel-Controlled Loop: Reads values from a file or keyboard and stops when a certain value (a sentinel) is read e.g., reading numbers until -99 is encountered
• End-of-File Controlled Loop: Reads values from a file and stops when there are no more values to read e.g., reading numbers until the end of the file is encountered
• Input Validation Loop: Keeps prompting for a value from the user until the user gets it right e.g., keep asking the user for a positive number until one is entered
• General Conditional Loop: Checks a condition and repeats statements in the loop body if true, stopping when the condition is false; this type encompasses all other loop kinds e.g., print numbers while they are below 100

Loops in C

• In C, all loops are implemented with general conditional loops
• A counting loop, such as repeating 100 times, starts a variable at 1. Then, it has a condition to stop the loop when the variable becomes larger than 100. Inside the loop, the program will increment the variable by 1 at each repetition

The while Statement

• The while statement repeats statements until a condition becomes false
• The syntax consists of while (condition) { statements executed/repeated if the condition is true }

Counting Loop Example Code

• int n; declares an integer variable n
• n = 1; initializes n to 1
• while (n <= 100) sets the condition: the loop continues as long as n is less than or equal to 100
• printf ("%d ", n); is the body, printing the value of n
• n = n + 1; updates n by incrementing it
• Within a while loop, initialization is performed once, then the condition is checked. If true, the body and update statements are executed, and the condition is checked again. If false, the loop ends

The for Statement

• The for statement repeats statements until a condition is false, functioning similarly to a while statement but using a slightly different syntax
• The Syntax is for (initialization; condition; update){ statements executed/repeated if condition is true }

Counting Loop With for Statement

• int n; declares an integer variable n
• for (n = 1; n <= 100; ++n) initializes n to 1, sets the condition for the loop to continue as long as n is less than or equal to 100, and increments n after each iteration
• printf ("%d ", n); is the body, printing the value of n

Execution of the for Statement

• In a for loop, initialization is performed once, then the condition is checked. If true, the body and update statements are executed, and the condition is checked again. If false, the loop ends

for vs. while

• A for loop is typically used when the number of iterations is known in advance (counting loops)
• A while loop is used when the number of iterations is not known in advance
• Both statements function similarly, with the variance being style.

Sentinel-Controlled Loop

• The following initializes number and sum: int number, sum;
• The following initializes the sum variable to 0: sum = 0;
• Prompts the user to enter a number to sum, the sentinel value to finish the loop is 0 through the use of the following code: printf ("Enter a number (Enter 0 to finish): ");
• The number is read by the user with scanf ("%d", &number);
• The main loop condition is checked in while (number != 0)
• Within the loop, the sum is computed. The user is also prompted to enter another number, and the loop will continue as long as the number is not zero
• The final output is done through the use of printf ("The sum is %d.\n", sum);
• In sentinel-controlled loops, reading is stopped when a certain value is read

EOF-Controlled Loop

• An End-of-File(EOF) controlled loop reads values until the end of the file is encountered
• This loop type uses the file fail status that occurs when trying to read data beyond the file's end

Status

• The reading status of a statement, like scanf, can be checked by assigning the statement to an integer variable
• If successful, the status will be the number of variables filled by the reading statement
• When reading from a file and no more numbers are available, the status becomes -1, also known as EOF

EOF-Controlled Loop I, II, III, IV, V

• Declares integer variables with int number, sum, status;
• Initializes the variable with sum = 0;
• Reads the values from the file with scanf
• Checks if it is the end of the file via status
• It continues looping, adding to the total, until it comes to the end of the file. Alternatives can be != -1, or == 1

The do-while Statement

• The do-while statement repeats the repetition in C slightly differently
• Unlike the while and for statements, the loop's body is executed at least once even if the condition is false, because the condition is checked at the end of the loop

The do-while Statement Layout

• The do structure houses any statements that are to be repeated by the program do { statements that will be repeated; } while (condition);

do-whileStatement Operations

• In a do-while loop, the initialization is performed once, then the body and update statements are executed, and finally the condition is checked
• If the condition is true, the body and update are executed once more. If the condition is false, the loop ends and the program continues to the next statement

Input Validation Loops

• The example input validation loop would initialize an int n;
• The do statement would prompt "Enter a number between 1 and 5" and allow the user to input that number in
• The while statement would continue as long as n < 1 or n > 5 with }while (n < 1 || n > 5);

Nested Loops

• A nested loop refers to a loop within a loop, with an inner loop inside an outer
• Inner and outer loops of a nested loop need not be of same control structure (e.g., one can be a while loop, the other a for loop)
• The inner loop must be entirely inside the outer loop (no overlaps)
• Each loop must be controlled by a different index (loop control variable e.g., i and j). The outer loop can be called i-loop and the inner called j-loop

A Nested Loop Example

• Suppose you want to display a 12x12 multiplication table; a natural approach is to use the outer loop for the multiplicand and the inner loop for the multiplicator
• The intent is to loop the outer loop 12 times and for each value, loop the inner loop 12 times as well