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CS 101 Introductory Programming - Lecture 7: Loops In C & Good Coding Practices Presenter: Ankur Chattopadhyay

2 Command Sequential Structure (straight-line structure) Command Test Commands to execute if False Command Commands to execute if True TrueFalse Example Selection Structure (decision or branching structure) Flowcharts for sequential, selection and iterative control structures 2 Iterative 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 initialization is executed once before the loop begins The statement is executed until the condition becomes false The increment portion is executed at the end of each iteration

Logic of a for loop statement true condition evaluated false increment initialization

//Multiplication Algorithm int 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 numbers int sum = 0; // the accumulator for (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 condition evaluated

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 6 for (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 loop int 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 condition evaluated 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 condition evaluated The while Loop true condition evaluated 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 )

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