1. By Hector M Lugo-Cordero September 3, 2008
Flow Control
By Hector M Lugo-Cordero
September 3, 2008
Department of Electrical and Computer Engineering

2. The Department of Electrical and Computer Engineering
Outline
Conditions
Selection ? if
if … else
If … else if … else
Switch
Cycles
While
Do while
For
Jump Statements
Break
Continue
Go to
Try and catch
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3. The Department of Electrical and Computer Engineering
Conditions
Conditions are expressions that can be either true or false
x > 0 is true if x is greater than zero
Operators
Equal ( == ): x == y  x equals y
Greater ( > ): x > y  x is greater than y
Less ( < ): x < y  x is smaller than y
Not equal ( != ): x != y  x is not equal to y
Greater or equal ( >= ): x >= y
Less or equal ( <= ): x <= y
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Conditions (cont.)
Complex conditions
Test more than one condition
And ( && ) is true if and only if both conditions are true
x >= 0 && x <= 9 is true if x is between 0 and 9 inclusive
Or ( || ) is true if one of the conditions is true
x < -3 || x > 10 is true if x is smaller than -3 or greater than 10
Not ( ! ) Negates the result of an expression
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Truth Tables A B
A && B
A || B !A
False
True
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Selections
Selections alter the flow of the program which is typically sequential (line by line)
Selections are made according to the value of a Boolean expression
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Selection (?)
Syntax
<var> = <condition> ? <true_value> : <false_value>
Execution
Evaluates <condition>
If true stores <true_value> into <var>
Else stores <false_value> into <var>
Example
int x = -9;
int y = x < 0 ? -1*x : x;
cout << “|” << x << “| = ” << y << endl;
Prints: |-9| = 9
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8. The Department of Electrical and Computer EngineeringIf
Syntax
if(<condition>){ … }
else if(<condition>){ … } //optional
else //optional
Execution
If the condition is true then the if block is executed
Optional
One can add else if to test other conditions in case the first is not met. The first that becomes truth will be executed and the others won’t be tested.
Else can be added if none of the conditions is met then execute the else code.
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9. The Department of Electrical and Computer Engineering
If (cont.)
Example
int x = 0;
cin >> x;
if(x == 0){
cout << “The number is 0” << endl; }
else if(x > 0){
cout << “The number is positive” << endl;
else{
cout << “The number is negative” << endl;
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10. The Department of Electrical and Computer Engineering
Switch
Syntax
switch(<var>) {
case <value1>: …
break;
case <value2>:
default: }
Execution
Evaluates the variable <var>
Executes the code in the block of the corresponding value
If no break is found it continues to evaluate each case
Default is executed if no case is true or no break is found
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11. The Department of Electrical and Computer Engineering
Switch (cont.)
Example
int x = 9;
x = x % 2;
switch(x) {
case 0:
cout < “x is even” << endl;
x = 1;
break;
case 1:
cout << “x is odd” << endl;
default:
cout << “This should never happen” << endl; }
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12. The Department of Electrical and Computer Engineering
Cycles
Repeats a single action or a block as long as the condition evaluated is true
It may vary the behavior depending on what cycle is used
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13. The Department of Electrical and Computer Engineering
While
Syntax
while(<condition>){ … }
Execution
Evaluates the condition and if true executes the while block
Examples
int i = 0;
while(i < 10){
cout << i << endl;
++i;
while(1){ … } or while(true) { … }
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14. The Department of Electrical and Computer Engineering
Do While
Syntax
do{ …
}while(<condition>);
Execution
Executes the do-while block and then checks the condition (guaranteed that will execute at least once)
Examples
int sum = 0;
int x = 0;
cout << “Enter the next non-zero number to add to the sum: ”;
cin >> x;
sum += x;
}while(x != 0);
cout << “The sum is “ << sum << endl;
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15. The Department of Electrical and Computer Engineering
For
Syntax
for(<data_type> <var> = <init_value>; <condition>; <increment>){ … }
Execution
Starts the variable <var> in <init_value> and checks the condition to see if it is true.
If so it executes the for block and then the increment.
Else exists the for loop.
Examples
for(int i = 0; i < 100; ++i){ cout << i << endl;}
for(double j = 0.0; j < 5; j += 0.5){ … }
for(long k = 1500; k >= 0; --k){ … }
int x = 3;
for(; x > 0; --x); //just runs 3 times and does nothing
for(; ;++x)
for( ; ; )
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16. Jump Statements (discouraged?)
Goto: jumps to a specific location in the code
Continue: Skips the rest of the current iteration in a cycle and passes to the check the condition
Break: exits the current control structure (cycle)
Syntax
goto <label>
continue;
break;
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17. Jump Statements (discouraged?)
Example
int x = 0
theloop:
cout << x << endl;
if(x == 5) //there is no { and } why ?
continue;
else if(x == 10)
break;
else{ //now there are the { and } 
++x;
goto theloop; }
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18. The Department of Electrical and Computer Engineering
Try and catch
Syntax:
try{
… throw <value> ... }
catch(<data_type> <var_name>){ …
Execution:
Tries to execute the block inside of the try
If an exception is thrown the catch block is executed. An exception is an error that shouldn’t occur and causes your program to crash
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Try and catch (cont.)
Example:
try{
if(x == 0)
throw “Division by zero exception”;
else
div = y/x; }
catch(char* exp){
cerr << exp << endl;
Note that we can throw strings, int, etc.
Later we will develop our own exceptions
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Try and catch (cont.)
// bad_alloc standard exception
#include <iostream>
#include <exception>
using namespace std;
int main () {
try {
int* myarray= new int[1000]; }
catch (exception& e) {
cerr << "Standard exception: " << e.what() << endl;
return 0;
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Try and catch (cont.)
Exception
Description
bad_alloc
thrown by new on allocation failure
bad_cast
thrown by dynamic_cast when fails with a referenced type
bad_exception
thrown when an exception type doesn't match any catch
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22. The Department of Electrical and Computer Engineering
Questions? ?
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