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Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Computer programming,Lecture 3,Lecture 3: Outline,Program Looping ,Kochan, chap.5,The,for,Statement,Relational Operators,Nested,for,Loops,Increment Operator,Program Input,for,Loop Variants,The,while,Statement,The,do,Statement,The,break,Statement,The,continue,Statement,Executing a program,Program = list of statements,Entrypoint,: the point where the execution starts,Control flow,: the order in which the individual statements are executed,Statement1,Statement2,Statement3,Statement4,Statement5,Statement6,Statement7,Statement8,Structure of a C program,#include ,int,main (void),int,value1, value2, sum;,value1 = 50;,value2 = 25;,sum = value1 + value2;,printf,(The sum of %i and %i is %in,value1, value2, sum);,return 0;,Entry point of a C program,Sequential,flow of control,Controlling the program flow,Forms of controlling the program flow:,Executing a sequence of statements,Repeating a sequence of statements (until some condition is met) (looping),Using a test to decide between alternative sequences (branching),Statement1,Statement2,Statement3,Statement4,Statement5,Statement6,Statement7,Statement8,Program Looping,Looping: doing one thing over and over,Program loop: a set of statements that is executed repetitively for a number of times,Simple example: displaying a message 100 times,:,printf(“hello,!n”);,printf(“hello,!n”);,printf(“hello,!n”);,printf(“hello,!n”);,printf(“hello,!n”);,Repeat 100 times,printf(“hello,!n”);,Program looping:,enables you to develop concise programs containing,repetitive processes that could otherwise require many lines of code !,The need for program looping,#include ,int,main (void) ,int,triangularNumber,;,triangularNumber,= 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8;,printf,(The eighth triangular number is %,in,triangularNumber,);,return 0;,What if we have to compute the 200-th (1000-th, etc) triangular number ?,Example problem: computing triangular numbers.,(The,n-th,triangular number is the sum of the integers from 1 through,n),In C: 3 different statements for looping: for, while, do,Example 200,th,triangular number,n=1,triangularNumber,=,triangularNumber,+ n,n=200,n=n+1,yes,no,triangularNumber,= 0,Print,triangularNumber,Statement,before,loop,init_expression,loop_condition,statement,loop_expression,Statement,after,loop,Example -,for,/* Program to calculate the 200th triangular number,Introduction of the for statement */,#include ,int,main (void),int,n,triangularNumber,;,triangularNumber,= 0;,for ( n = 1; n = 200; n = n + 1 ),triangularNumber,=,triangularNumber,+ n;,printf,(The 200th triangular number is %,in,triangularNumber,);,return 0;,The,for,statement,for (,init_expression,;,loop_condition,;,loop_expression,),program statement,init_expression,Program statement,loop_condition,Loop expression,yes,no,1,2,3,4,5,The,for,statement,for ( n = 1; n = 200; n = n + 1 ),triangularNumber,=,triangularNumber,+ n;,1,2,3,4,5,yes,no,How,for,works,The execution of a for statement proceeds as follows:,1. The initial expression is evaluated first. This expression usually sets a variable that will be used inside the loop, generally referred to as an,index,variable, to some initial value.,2. The looping condition is evaluated. If the condition is not satisfied (the expression is false has value 0), the loop is immediately terminated. Execution continues with the program statement that immediately follows the loop.,3. The program statement that constitutes the body of the loop is executed.,4. The looping expression is evaluated. This expression is generally used to change the value of the index variable,5. Return to step 2.,Infinite loops,Its the task of the programmer to design correctly the algorithms so that loops end at some moment !,/ Program to count 1+2+3+4+5,#include ,int,main (void),int,i, n = 5, sum =0;,for ( i = 1; i = n; n = n + 1 ),sum =,sum,+ i;,printf,(“%i %i %,in, i , sum, n);,return 0;,What is wrong here ?,Does the loop end?,Relational operators,Operator,Meaning,=,Is equal to,!=,Is not equal to,Is less than,Is greater than,=,Is greater or equal,The relational operators have lower precedence than all arithmetic operators:,a b + c,is evaluated as,a (b + c),ATTENTION ! Do not confuse:,the “is equal to” operator = and the “assignment” operator =,ATTENTION when comparing floating-point values !,Only comparisons make sense !,Example ,for,with a body of 2,/ Program to generate a table of triangular numbers,#include ,int,main (void),int,n,triangularNumber,;,printf,(TABLE OF TRIANGULAR NUMBERS,nn,);,printf,( n Sum from 1 to,nn,);,printf,(- -n);,triangularNumber,= 0;,for ( n = 1; n = 10; +n ) ,triangularNumber,+= n;,printf,( %i %,in, n,triangularNumber,);,return 0;,The,body,of the loop consists in a,block,of 2 statements,Increment operator,Because addition by 1 is a very common operation in programs, a special operator was created in C for this.,Increment operator,: the expression +n is equivalent to the expression n = n + 1.,Decrement operator,: the expression -n is equivalent to the expression n = n 1,Increment and decrement operators can be placed in front (,prefix,) or after (,postfix,) their operand.,The,difference between prefix and postfix,:,Example: if n=4:,a=n+ leads to a=4, n=5,a=+n leads to a=5, n=5,Program input,#include ,int,main (void),int,n, number,triangularNumber,;,printf,(What triangular number do you want? );,scanf,(%i, ,triangularNumber,= 0;,for ( n = 1; n in a later chapter !,Reads integer from keyboard,Its polite to display a message before,Nested loops,#include ,int,main (void),int,n, number,triangularNumber, counter;,for ( counter = 1; counter = 5; +counter ) ,printf,(What triangular number do you want? );,scanf,(%i, ,triangularNumber,= 0;,for ( n = 1; n = number; +n ),triangularNumber,+= n;,printf,(Triangular number %i is %,inn, number,triangularNumber,);,return 0;,Remember indentations!,for,loop variants,Multiple expressions,(comma between),for(i,=0 , j=10 ; ij ; i+ , j-),Omitting fields,(semicolon have to be still),i=0;,for( ; i10 ; i+ ),Declaring variables,for(int,i=0 ; i=10 ; i+ ),The,while,statement,while (,expression,),program statement,while ( number 0“);,printf,(“Give a new number: “);,scanf(“%i,“, ,The,while,statement,while (,expression,),program statement,statement,Loop_expression,yes,no,Loop with the test in the beginning !,Body might never be executed !,Example:,A program to find the greatest common divisor of two nonnegative integer values ,Example -,while,/* Program to find the greatest common divisor,of two nonnegative integer values */,#include ,int,main (void),int,u, v, temp;,printf,(Please type in two nonnegative integers.n);,scanf,(%,i%i, ,while ( v != 0 ) ,temp = u % v;,u = v;,v = temp;,printf,(Their greatest common divisor is %,in, u);,return 0;,Example:,A program to print out the digits of a number in reverse order ,Example -,while,/ Program to reverse the digits of a number,#include ,int,main (void),int,number,right_digit,;,printf,(Enter your number.n);,scanf,(%i, ,while ( number != 0 ) ,right_digit,= number % 10;,printf,(%i,right_digit,);,number =,number,/ 10;,printf,(n);,return 0;,Example ,while,not quite OK !,/ Program to reverse the digits of a number,#include ,int,main (void),int,number,right_digit,;,printf,(Enter your number.n);,scanf,(%i, ,while ( number != 0 ) ,right_digit,= number % 10;,printf,(%i,right_digit,);,number =,number,/ 10;,printf,(n);,return 0;,What happens if you enter number=0 ?,The,do,statement,do,program statement,while (,loop_expression,);,statement,loop_expression,yes,no,Loop with the test at the end !,Body is executed at least once !,Example ,do while,/ Program to reverse the digits of a number,#include ,int,main (),int,number,right_digit,;,printf,(Enter your number.n);,scanf,(%i, ,do ,right_digit,= number % 10;,printf,(%i,right_digit,);,number =,number,/ 10;,while ( number != 0 );,printf,(n);,return 0;,Which loop to choose ?,Criteria: Who determines looping,Entry-condition loop - for, while,Exit-condition loop - do,Criteria: Number of repetitions:,Indefinite loops -while,Counting loops - for,In C, you can actually rewrite any while as a for and,viceversa,!,Example:,while,vs,for,#include ,int,main (void),int,count = 1;,while ( count = 5 ) ,printf,(%,in, count);,+count;,return 0;,#include ,int,main (void),int,count;,for ( count=1; count=5;,count+ ) ,printf,(%,in, count);,return 0;,The,break,Statement,Can be used in order to immediately exiting from a loop,After a break, following statements in the loop body are skipped and execution continues with the first statement after the loop,If a break is executed from within nested loops, only the innermost loop is terminated,The,break,statement,Programming style: dont abuse break !,.,while ( number != 0 ) ,/ Statements to do something in loop,printf(Stop, answer 1: );,scanf,(%i, ,if(answer,= 1),break;,/ very bad idea to do this,Similar to the break statement, but it does not make the loop terminate, just skips to the next iteration,The,continue,statement,The,continue,statement,Continue also not so good style!,.,while ( number != 0 ) ,/ Statements to do something in loop,printf(“Skip,next statements answer 1: );,scanf,(%i, ,if(answer,= 1),continue;,/ not so good idea,/ Statements to do something in loop,/ If answer was 1 these statements are,/ not executed. They are skipped.,/ Go straight to the beginning of while,
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