INTRO_C_LECTURE 4.pdf for computer programming

Introduction to computer programming – C Lecture 4

Today’s Class • Flow Control • Control Structures • Conditional Statements

Intro-Decisions in Your Code • Programs execute top-down • Decisions break the top-down flow • The if statement tests a condition • When the condition is true, program flow is altered.

Flow Control • In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an imperative programming language from a declarative programming language. Wikipedia

Flow Control executed • In a program statement may be sequentially, selectively or iteratively. • Every program language provides constructs to support sequence, selection or iteration.

Types of Control Structures Flow of control through any given function is implemented with three basic types of control structures: • Sequential: default mode. ... Following a recipe • Selection: used for decisions, branching -- choosing between 2 or more alternative paths. ... If, if/else, switch • Repetition: used for looping, i.e. repeating a piece of code multiple times in a row….while, do/while, for

Useful tools Some useful tools for building programs or program segments • pseudocode - helps "think" out a problem or algorithm before trying to code it. • flowcharting - graphical way to formulate an algorithm or a program's flow. • stepwise refinement (top-down design) of algorithms

THE SEQUENCE CONSTRUCT Statement 1 Statement 2 Statement 3

Example of Sequential Structure #include <stdio.h> int main() { puts("I'm a computer!"); puts("Thrilled to meet you!"); return(0); } View code

SELECTION • The Selection construct means the execution of statement(s) depending upon a condition-test. If a condition evaluates to true, a course-of-action (a set of statements) is followed otherwise another course-of- action (a different set of statements). • This construct(selection construct) is also called decision construct because it helps in making decision about which set-of- statements is to be executed.

THE SELECTION CONSTRUCT. Condition ? Statement 1 Statement 2 Statement 1 Statement 2

ITERATION • Iteration construct means repetition of set of statements depending upon a condition test. Till the time of condition is true. ( or false depending upon the loop). A set of statements are repeated again and again. As soon as the condition become false (or true), the repetition stops. The iteration condition is also called ”Looping Construct”.

THE ITERATION CONSTRUCT Condition ? Statement 1 Statement 2 The Loop Body True False

THE SELECTION STATEMENT – if Statement • An if statement tests a particular condition, if the condition evaluated to true, a course of action is followed, i.e., a statement or a set of statement is executed. Otherwise if the condition evaluated to false then the course of action is ignored.

SYNTAX OF IF STATEMENT • if (condition) statement 1; The statement may consist of single or compound. If the condition evaluates non zero value that is true then the statement 1 is executed otherwise if the condition evaluates zero i.e., false then the statement 1 is ignored.

Example of if statement Example 1: if (age>18) printf(“The person is eligible for vote”) Example 2: #include <stdio.h> int main() { int a; printf("Type an integer: "); scanf("%d",&a); if( a >10 ) printf("%d is greater than 10.n",a); return(0); } Notice the semi colon

Example of if statement Theneed to evaluate the false condition would then give usthe following code. #include <stdio.h> int main() { int a; printf("Type an integer: "); scanf("%d",&a); if( a >10) { printf("Youtyped %d.n",a); printf("%dis greater than 10.n",a); } if(a < =10) { printf("Youtyped %d.n",a); printf("%dis less than or equal to 10.n",a); } return(0);

THE ELSE CLUASE • An if statement can optionally include an else clause. The else clause is included as follows: if (expression) statement1; else statement2; • If expression evaluates to true, statement1 is executed. If expression evaluates to false, statement2 is executed. • Both statement1 and statement2 can be compound statements or blocks. 2/17/2020

Flow chart of if statement if Condition ? Statement 1 Statement 2 Statement 1 Statement 2 els e tru e

IF - ELSE FORMAT if (condition) { Statement 1 Statement 2 } else { Statement 1 Statement 2 }

Example of if-else #include <stdio.h> int main() { int a; printf("Type an integer: "); scanf("%d",&a); if(a >10) { printf("You typed %d.n",a); printf("%dis greater than 10.n",a); } else(a < =10) { printf("You typed %d.n",a); printf("%dis less than or equal to 10.n",a); } return(0); Runcode

NESTED IFs • A nested if is an if that has another if in its body or in its else body. The nested if can have one of the following three forms Form1: if (expression 1) { if (expression 2) statement 1 else statement 2 } else body of else

NESTED IF contd.. • Form 2: if (expression 1) { if (expression 2) statement 1 else statement 2 if (expression 2) statement 1 else statement 2 ………. } else { }

THE if-else-if LADDER • A common programming construct in C is the if-else-if ladder, which is often also called as the if-else-if ladder because of its appearance. It takes the following general form. if (expression1) statement 1; else if (expression2) statement 2 else if (expression 3) statement 3 ………. else Statement 4;

Example #include <stdio.h> int main() { int a; printf("Type an integer: "); scanf("%d",&a); if( a >10 ) { printf("You typed %d.n",a); printf("%d is greater than 10.n",a); } else if( a <10) { printf("You typed %d.n",a); printf("%d is less than 10.n",a); } else { printf("You typed %d.n",a); printf("%d is 10.n",a); } return(0); } Run code

THE ? : ALTERNATIVE TO if • C has an operator that can be alternative to if statement. The conditional operator ? : • This operator can be used to replace the if statement of C.

CONDITIONAL OPERATOR ? : if (expression 2) statement 1 else statement 2 • The above form of if else statement can be replaced as, expression1?expression2:expression3;

CONDITIONAL OPERATOR ? : • For example char result; int marks; if (marks >50) { result =‘p’; } Else { result =„f‟; } • This can be alternatively written as, char result; int marks; Result =(marks >50) ? „p‟ :„f‟;

COMPARISON OF if AND ? : 1. compared to if –else sequence, ?: offers more concise, clean and compact code, but it is less obvious as compared to if. 2. Another difference is that the conditional operator ?: produces an expression, and hence a single value can be assigned or incorporated into a larger expression, where as if is more flexible. if can have multiple statements multiple assignments and expressions (in the form of compound statement) in its body. 3. When ?: operator is used in its nested form it becomes complex and difficult to understand. Generally ?: is used to conceal (hide) the purpose of the code.

Challenge- Group work • What is the output of the following C program fragment? #include <stdio.h> Int var =75 Int var2 =56 Int num; num =sizeof(var)? (var2>23 ?)((var ==75)? „A‟ :0) :0) :0; Printf(“%d”, num); Return o; }

Multiple Decisions • The C language lets you handle complex decisions by stacking a bunch of if else conditions. • Sometimes that structure gets a bit weird. • As an alternative you can employ the switch-case structure which is another decision making tool in the C-language.

Example #include <stdio.h> int main() { char a; printf("Your choice (A,B,C): "); scanf("%c",&a); switch(a) { case 'A': puts("Excellent choice!"); break; case 'B': puts("This is the most common choice."); break; case 'C': puts("I question your decision."); break; default: puts("That's not a valid choice."); } return(0); } Build and run the code lets see what it does

Challenge-Group Exercise • Code a decision making structure • Prompt for integer input • For values1,2,or3, display the words “red”, “green” or “blue.” • Flag invalid input for any other values the user inputs.

Solution • Lets see • 2 • 3 • 4 • Solutions • Stick to one solution, groups of 5 (tengwau@must.ac.ug)

INTRO_C_LECTURE 4.pdf for computer programming

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INTRO_C_LECTURE 4.pdf for computer programming