Loops. CSCI 112: Programming in C

Transcription

1 Loops CSCI 112: Programming in C

2 Loops allow repetition of code Remember the three types of control structure? Loops make up the repetition structure. Basic idea: repeat some block of code until some condition is met What kinds of things might loops help us achieve?

3 Loops allow repetition of code Remember the three types of control structure? Loops make up the repetition structure. Basic idea: repeat steps in our program over and over until some condition is met What kinds of things might loops help us achieve? Perform the same operation on a bunch of data, like processing payroll for a group of employees Gather user input without knowing how many data points they wish to enter Run some process indefinitely: Send readings from temperature sensor to some server

4 When to use a loop We know we want to do something more than once. Ask yourself: To solve this problem, do I need to repeat some steps more than once? If so, the next step is to figure out what type of loop fits the problem Can we figure out how many time the loop will repeat before runtime? If so, we need a counting loop Otherwise, the loop will be some form of a conditional loop.

5 Types of loops What types of loops are there?

6 Loop constructs What types of loops are there? In terms of language constructs, we have: For loop While loop Do-while loop

7 The general structure of a loop Three main components: Initialize a loop control variable Test the repetition condition (based on control variable) Run the loop body Change/update the control variable The order in which these actions take place depends on loop type

8 The for loop The while loop handles only repetition test in the construct As long as test evaluates to 1 (true), the loop continues for (<initialization expression>; <repetition test>; <update expression>) { // Loop body // This runs each time loop repeats }

9 The for loop The while loop handles only repetition test in the construct As long as test evaluates to 1 (true), the loop continues for (<initialization expression>; <repetition test>; <update expression>) { // Loop body // This runs each time loop repeats } Example for (int i = 0; i < 10; i++) { printf("%d \n", i); }

10 The for loop Let s write a loop that calculates 10! In other words,

11 The while loop The while loop handles only repetition test in the construct As long as test evaluates to 1 (true), the loop continues while (<repetition test>) { // Loop body } Example int i = 0; while (i < 10) { printf("%d", i); i++; }

12 The while loop Let s write a while loop that adds random values to a variable until the sum becomes greater than 1,000

13 The do-while loop Instead of testing the repetition condition before each iteration, dowhile checks at after. Guarantees loop will run at least once do { // Loop body, will run at least once } while (<repetition test>);

14 The do-while loop Example // Initialize loop control variable int input; do { printf("enter a value > "); scanf("%d", &input); printf("you entered %d \n", input); } while (input!= -99);

15 Types of loops At a high level, we have: Counting loops Sentinel controlled loops Input validation loop Endfilecontrolled loops General conditional loops

16 Counting loops We need to do something n times We can use a for loop or a while loop: Set *loop control variable* to an initial value of 0. While *loop control variable* < *final value*... Increase *loop control variable* by 1

17 Sentinel-controlled loop A sentinel is a value at the end of a sequence that says we re done Do-while or while are best suited Get a line of data While the sentinel value has not been encountered Process the data value Get another value

18 Input validation loop We want to ask a user for input, get (scanf) the input, and if it does not meet some condition, repeat Do-while or while are best-suited Get a data value If data value isn't in the acceptable range, repeat

19 Side note: functions as parameters We can pass function A into another function B as a parameter, and call function A from function B Return type and parameters must match up: int square(int x) { return x * x; } int another_function(int f(int), int value) { return f(value); } int main(void) { int result = another_function(square, 20); printf("result is %d \n", result); }