control flow of program
TRANSCRIPT
CS101 Introduction to computingCS101 Introduction to computing
Control Flow of ProgramControl Flow of Program
A. Sahu and S. V .RaoDept of Comp. Sc. & Engg.Dept of Comp. Sc. & Engg.
Indian Institute of Technology Guwahati
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Outline• Program flow control
–If‐else–Switch case–Switch case–Looping : while, for, do‐while
• Looping : while, for, do‐while
• Problem Solving• Problem Solving
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Structured Programming• All programs can be written in terms of only three control structures– Sequence, selection and repetition
• The sequence structure – Unless otherwise directed, the statements are executed in the order in which they are written.
Th l i• The selection structure – Used to choose among alternative courses of actionaction.
• The repetition structure– Allows an action to be repeated while someAllows an action to be repeated while some condition remains true.
Sequential ExecutionSequential Execution
Statement 1
Statement 2
...
Statement n
Compute the resonant frequency of an RLC circuitfrequency of an RLC circuit
#include <stdio.h>#include <math h>#include <math.h>int main() {
double l, c, omega, f; printf("Enter inductance in mH: "); //S1scanf("%lf", &l); //S2printf("Enter capacitance in microF: "); //S4p ( p ); //scanf("%lf", &c); //S5omega = 1.0/sqrt((l.0/1000)*(c/1000000)); //S6f = omega / (2 * M PI); //S7f = omega / (2 * M_PI); //S7printf("Resonant freq: %.2f\n", f); //S8return 0; //S9
}
Selective Execution : Flow chart( l f)(only if)
BooleanBoolean true
ExpressionExpression
Statement 1
Statement S
Selective Execution : Flow chartSelective Execution : Flow chart
BooleanBooleanExpr
true false
Expr
statement 1 statement 2
Selection: the if‐else statementSelection: the if else statementif ( condition ){
statement(s)/*if clause */}}else {
t t t( ) /* l l */statement(s) /*else clause */}
Nesting of if‐else Statementsgif ( condition1 ){
statement(s)}else if ( condition2 )else if ( condition2 ) {
statement(s)}}. . . /* more else clauses may be here */else{{
statement(s) /* the default case */}
Bad Example : 2 if 1 elseif ( n > 0 )
if ( a > b )az=a;
else z=b;
if ( n > 0 ){if (a> b)
if ( n > 0 )if ( a > b )
; if (a> b)z=a;
}elseif ( a > b )
z=a;else
else z=b;
z=b;
Indentation will not ensure result : elsematch with closest if Code of Red box behaves like Code of Green box
In Assembly language: No if‐else• Assembly language
– No support for [if else, No for loop, No while loop]– All higher construct get implemented
using if and goto statement goto statement uses Label
• If else get converted to if gotog g
if(a> b )if(a>b) goto L1:z=b;
z=a;else z=b;NextStmt;
z b;goto L2:
L1: z=a;
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NextStmt; L1: z a;L2: Next stmt
In Assembly language: No if‐elseif(a>b) goto L1:if(a>b) goto L1:
bbz=b;
2
z=b;
2goto L2:L1:
goto L2:L1:
z=a;L2:
z=a;L2:
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Next stmtNext stmt
Multi‐way if else : switch caseMulti way if else : switch case• If‐else : two way, if part and else part • To make it multi‐way: nested if‐else
– Confusing, lengthy
• C language provide – Switch caseSwitch case– Multi way selection– Range multi‐way selectionRange multi way selection
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The switch Multiple‐Selection Structure• switch
– Useful when expression is tested for multiple values– Consists of a series of case labels and an optional default case
– break is (almost always) necessaryswitch (<expression>){case <Value1> :case <Value1> :
<Action/Stmts for Value1>; break; case <Value2> :
<Action/Stmts for Value2>; break; . . . default: <Action/Stmts for DefaultValue>; / ;
break; }
Flowchart of Switch Statement
truecase acase a
action(s) break
case bcase b
action(s)break
false true
f l
.
false
.
.case z case z
action(s)break
true
action(s)
default
false
action(s)
Multiway Switch Selection example int main(){//simple calculator int a=50,b=10, R;char choice;
int main(){//simple calculator int a=50,b=10, R;char choice;char choice;printf(“Enter choice”);scanf(“%c”,&choice);
char choice;printf(“Enter choice”);scanf(“%c”,&choice);
switch (choice){case ‘a’ : R=a+b; printf(“R=%d”,R); break;
switch (choice){case ‘a’ : R=a+b; printf(“R=%d”,R); break; case ‘s’ : R=a-b; printf(“R=%d”,R); break; case ‘m’ : R=a*b; printf(“R=%d”,R); break;case ‘d’ : R=a/b; printf(“R=%d”,R); break;
case ‘s’ : R=a-b; printf(“R=%d”,R); break; case ‘m’ : R=a*b; printf(“R=%d”,R); break;case ‘d’ : R=a/b; printf(“R=%d”,R); break;case d : R a/b; printf( R %d ,R); break;default : printf(“Wrong choice”) ; break;}t 0
case d : R a/b; printf( R %d ,R); break;default : printf(“Wrong choice”) ; break;}t 0return 0;
}return 0;}
Multiway Switch Selection example int main(){//simple calculator int a=50,b=10, R;char choice;
int main(){//simple calculator int a=50,b=10, R;char choice;char choice;printf(“Enter choice”);scanf(“%c”,&choice);
char choice;printf(“Enter choice”);scanf(“%c”,&choice);
switch (choice){case ‘a’ : R=a+b; printf(“R=%d”,R); break;
switch (choice){case ‘a’ : R=a+b; printf(“R=%d”,R); break; case ‘s’ : R=a-b; printf(“R=%d”,R); break; case ‘m’ : R=a*b; printf(“R=%d”,R); break;case ‘d’ : R=a/b; printf(“R=%d”,R); break;
case ‘s’ : R=a-b; printf(“R=%d”,R); break; case ‘m’ : R=a*b; printf(“R=%d”,R); break;case ‘d’ : R=a/b; printf(“R=%d”,R); break;case d : R a/b; printf( R %d ,R); break;default : printf(“Wrong choice”) ; break;}t 0
case d : R a/b; printf( R %d ,R); break;default : printf(“Wrong choice”) ; break;}t 0return 0;
}return 0;}
Multiway Switch Selection example
switch (choice){switch (choice){case ‘A’ : // no break, work for both A & a
// next statement automatically // get executed
case ‘A’ : // no break, work for both A & a// next statement automatically // get executed // g
case ‘a’ : R=a+b; printf(“R=%d”,R); break; case ‘S’ :case ‘s’ : R=a-b; printf(“R=%d” R); break;
// gcase ‘a’ : R=a+b; printf(“R=%d”,R); break; case ‘S’ :case ‘s’ : R=a-b; printf(“R=%d” R); break;case s : R=a-b; printf( R=%d ,R); break; case ‘M’ :case ‘m’ : R=a*b; printf(“R=%d”,R); break;
case s : R=a-b; printf( R=%d ,R); break; case ‘M’ :case ‘m’ : R=a*b; printf(“R=%d”,R); break;case ‘D’ :case ‘d’ : R=a/b; printf(“R=%d”,R); break;default : printf(“Wrong choice”) ; break;
case ‘D’ :case ‘d’ : R=a/b; printf(“R=%d”,R); break;default : printf(“Wrong choice”) ; break;p g}
p g}
Range Multiway Switch Selection example i ti tint x;scanf(“%d”,&x);switch (x){
int x;scanf(“%d”,&x);switch (x){case 1 ... 20:// 1 space three dots space 20
printf(“You entered >=1 and <=20”); break;
case 1 ... 20:// 1 space three dots space 20 printf(“You entered >=1 and <=20”); break;break;
case 21 ... 30 :printf(“You entered >=21 and <=30”); b k
break; case 21 ... 30 :
printf(“You entered >=21 and <=30”); b kbreak;
default : printf(“You entered < 1 and >31”) ;
break; default :
printf(“You entered < 1 and >31”) ; break;
}break;
}
Syntax = case <low_range> ... <high_range> :
Loops and RepetitionLoops and Repetition
• Loops in programs allow us to repeat blocks• Loops in programs allow us to repeat blocks of code U f l f• Useful for:– Counting
i i i i i– Repetitive activities– Programs that never end
• Because of looping feature of computer – We also name “Computer” as “Machine” (which can do repetitive mechanical work for us)do repetitive mechanical work for us)
Three Types of Loops/Repetition in CThree Types of Loops/Repetition in C
• while–top‐tested loop (pretest)f• for–counting loopcounting loop–forever‐sentinel
• do–bottom tested loop (posttest)–bottom‐tested loop (posttest)
The while loopTop‐tested loop (pre‐test)
while (condition)
Note that as in IF selection only one
( )statement;
Note that, as in IF selection, only one statement is executed. You need a block to repeat more than one statement (using { })repeat more than one statement (using { })
while (condition){while (condition){statements;
}}
while(condition)statement;
condition
truef l
statements
false
statements
Similar to the if statement
• Check the Boolean condition• If true, execute the statement/block
Repeat the above until the Boolean is false
In Assembly language: No while loop• Assembly language
– No support for [while loop]– All higher construct get implemented
using if and goto statement goto statement uses Label
• while get converted to if gotog g
while(Cond){ L1:if(!Cond)goto L2;STMTS;
}Next STMT;
L1:if(!Cond)goto L2; STMTS;goto L1;
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Next STMT; g ;L2:Next STMT
While statement using goto
L1:if(!C)goto L2L1:if(!C)goto L2
condition
L1:if(!C)goto L2
Statements
L1:if(!C)goto L2
Statementstrue false
Statements
goto L1
Statements
goto L1statements goto L1
L2
goto L1
L2L2:L2:
While loop
ditiditifalse
while(condition)statement;
conditioncondition
truewhile(condition){statement1;
statementsstatementsstatement1;statement2;
}}
int i = 10;hile(i > 0) {while(i > 0) {
printf("i=%d\n", i);i = i ‐ 1;i = i 1;
}
Forever loops and never loopsB th diti l b•Because the conditional can be
–“always true” : you can get a loop that runs forever– or “always false”, you can get a loop never runs at all.
i t t 0int count=0;while(count !=0)
i tf(“Hi \ ”) // i tprintf(“Hi .. \n”);// never prints
hil ( t 1)//i idi !!!while (count=1)//insidious error!!!count = 0;
What is wrong with these statements?
How to count using while1. First, outside the loop, initialize the counter
variable2. Test for the counter’s value in the Boolean3 Do the body of the loop3. Do the body of the loop4. Last thing in the body should change the value
f th t !of the counter!1. i = 1;2 while(i <= 10) {2. while(i <= 10) {3. printf("i=%d\n", i);4. i = i + 1;;
}
The for loop• The while loop is pretty general.
–Anything that can be done using repetition can be done with a while loop
• Because counting is so common–There is a specialized construct–There is a specialized construct –Called : for loop.
• for loop–Makes it easy to set up a counting loop–Makes it easy to set up a counting loop
For loop: Three partsThree parts to a for loop (just like the while):while):– Set the initial value for the counterS t th diti f th t– Set the condition for the counter
– Set how the counter changes each time through the loop
f ( ){for ( count=1; count<=5; count++ ){statement;
}}
For Loop: Examplef ( )for(count=1; count<=5; count++)
printf(“count=%d\n”, count);
count = 1
falsecount <= 5count <= 5true
printf
count ++
For loop: Ascending for
for ( ctrl_var=init_val; ctrl ar < limit al
ctrl_var = init_val
ctrl_var <=limit_val; ctrl_var++) { statement;
cntrl varcntrl vartrue false
statement;}
<= limit_valcntrl_var<= limit_val
true false
statement
lctrl_var ++
For Loop : Descending for
for ( ctrl_var=init_val; ctrl ar > limit al
ctrl var = init val
ctrl_var >=limit_val; ctrl_var‐‐) { statement;ctrl_var = init_val statement;
}
cntrl_var>= limit_valcntrl_var>= limit_val
true false
statement
ctrl_var ‐‐
Precaution in CodingPrecaution in Coding
• Dangerous to alter within the body of g ythe loopcontrol variable ctrl var– control variable ctrl_var
– limit_var
• Components of the for statement can be a arbitrary statementsy– e.g. the loop condition may be a function callfunction call.
For loop : Examplesfor(i=1; i<=10; i++){
printf("%d\n" i);printf( %d\n , i);}
i = 1
i <= 10truefalse
printf
i ++
For loop Examples : Float ctrl variable for(t=1.7; t<3.5; t=t+0.1){
printf("%f\n", t);}
t = 1.7
t < 3.5truefalse
printf
t=t+0.1
For Loop: “one off” errorFor Loop: one off error• It is easy to get a for loop to be “one off” of the number you want.number you want. • Be careful of the combination of init_value and < vs. <=
–for(i=0; i<10; i++)–for(i=0; i<=10; i++)for(i 0; i< 10; i++)–for(i=1; i<10; i++)for(i=1; i<=10; i++)–for(i=1; i<=10; i++)
• Counting from 0, with <, is a good combination and good for invariants as welland good for invariants as well.
For Loop: “one off” errorFor Loop: one off error• It is easy to get a for loop to be “one off” of the number you want. y• Be careful of the combination of init_value and < vs. <=
–for(i=0; i<10; i++)–for(i=0; i<=10; i++)
10 values: 0 to 911 values: 0 to 10
–for(i=1; i<10; i++)–for(i=1; i<=10; i++)C i f 0 i h i d bi i
9 values: 1 to 910 values: 1 to 10
• Counting from 0, with <, is a good combination and good for invariants as well
–As array indexing in C start with 0 : will be discussedAs array indexing in C start with 0 : will be discussed later
Nested For loop : Examplesfor(i=1; i<5; i++){for(i=1; i<5; i++){
for(j=1; j<4; j++){printf("%d * %d = %d\n",
i= 1
i, j, i*j);}
}
false
}
j= 1
i < 5true falsej= 1
j< 4tfalse
FOR LOOPj< 4
printf
true
i++J++
Equivalence of top‐tested while loop d f land for loop
• The following loop for(x=init; x<=limit; x++){
statement_list
• Is equivalent to
}
Is equivalent to
x=init;hil ( < li it) {while (x<=limit) {
statement_list;++x++;
}
For‐ever or infinite loopFor ever or infinite loop• Used for event driven case• Mostly event break the infinite loop using break statement : coming out of the loopbreak statement : coming out of the loop
while(1) { /* Loop until value is valid *//* Loop until value is valid */
}
for(;;) { /* Loop without testing *// Loop without testing /
}
For‐ever or infinite loop• Used for event driven case• Mostly event break the infinite loop using• Mostly event break the infinite loop using break statement while(1) {while(1) {
scanf(“%c”,&c);if(c==‘e’ || c==‘E’) { ( || ) {printf(“\nEntered the required
character e or E\n”);break; // coming out of the loop}
printf(“%c” c);printf(“%c”,c); }
For‐ever or infinite loop• Used for event driven case• Mostly event break the infinite loop usingMostly event break the infinite loop using break statement f ( ) {for(;;) {
scanf(“%c”,&c);if(c==‘e’ || c==‘E’) {if(c e || c E ) {
printf(“\nEntered the required character e or E\n”);
break; // coming out of the loop}
i f(“% ” )printf(“%c”,c);}
Finite for loop with : breakfor(i=1; i<=10; i++){
i = 1
for(i=1; i<=10; i++){printf("%d, ", i);if(i==5) break;
i < 10 false
( ) ;}
i <= 10
printf
true
1, 2, 3, 4, 5,printf
i 5true
1, 2, 3, 4, 5,
i == 5
false// coming out of the loop
i ++
Finite while loop with : breaki=0;
i = 0
i 0; while(i<=10){
printf("%d, ", i);if(i 5) b k
i < 10 false
if(i==5) break;i=i+1;
}i <= 10
printf
true
printf
i 5true
0, 1, 2, 3, 4, 5,
i == 5
false// coming out of the loop
i=i+1
Finite for loop with : continuefor(i=1; i<=10; i++){
i = 1
for(i=1; i<=10; i++){if(i==5) continue;printf("%d\n", i);
i < 10 false
p ( , );}
i <= 10true
true 0, 1, 2, 3, 4, 6, 7, 8, 9, 10, i == 5
false
true
// Ski if
, , , , , , , , , ,
printffalse // Skip a case if
condition satisfied
i ++
Finite while loop with : continue
i = 0i=0;While(i<=10){i++;
i 10i < 10true false
i++; if(i==5) continue;printf("%d, ", i);i 10i <= 10
i ++
printf( %d, , i); }
i ++
i 5true 1, 2, 3, 4, 6, 7, 8, 9, 10,
i == 5
false // Skip a case if printf condition satisfied
The do‐while loop• bottom‐tested loop (posttest)• One trip through loop is guaranteed i eOne trip through loop is guaranteed, i.e. statement is executed at least oncedo
statementwhile (loop condition);while (loop_condition);
do {do {statement1;statement2;
} hil (l diti )
Usually!} while (loop_condition);
do‐while loopdo {
statement;}while(condition);
statement;
true falseconditionconditiontrue false
do loop Examplesdo loop Examplesi = 0;d {do {
i++;i tf("%d\ " i)printf("%d\n", i);
} while(i < 10);
do {printf("Enter a value>0: ");printf( Enter a value>0: );scanf("%lf", &val);} while(val <= 0);} while(val < 0);
Bottom‐tested Equivalence• Bottom‐tested do loop (posttest)
do {statement;} while (condition);
• Similar to bottom‐tested forever loop
} while (condition);
p
for (;;) {statement_list;if (!condition) break;}}
Thanks
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