PSIgroup | CodingCpp / Procedural: Flow control structures

Trail Index | Procedural | Flow Control Structures

Flow control structures allow your program to deviate from the linear, one after the other sequence of instruction execution.

for - indexed loop
It has quite an odd syntax, is easy to get it wrong
if / then / else - conditional/alternative execution
while and do / while - condition loop
switch / case - multiple-choice execution
break - interfering with loops

/**
* Flow control structures
*/

#include <iostream>

int main() {
using namespace std;

// an indexed loop
int a=0;
for (int i=0;i<30;i++) {
a=a+.1; // would be nice to get a warning here...
cout<<i<<" step: "<<a<<endl;
}

// an if
if (a<1) {
cout<<"something wrong above...";
}

// an if-else structure
double b;
if (a>3) {
b=a/3;
} else {
b=a/2;
}

{ // a non-indexed while loop
double sum=0;
int count=0;
while (sum<10) {
sum+=.3;
count++;
}
cout<<"Sum="<<sum<<", count="<<count<<endl;
}

{
// a non-indexed while loop, with an "emergency exit"
double sum=0;
int count=0;
while (sum<10) {
++count;
if (count>10) break; // exit the loop NOW!
sum+=.3;
}
cout<<"Sum="<<sum<<", count="<<count<<endl;
// a break can be anywhere, inside any kind of loop
}

// like while() {}, but evaluate at end
// useful for looping at least once
do {
;//nothing
} while (false);

{ /*
* switch is like a whole bunch of if..else if.. else
* it's more readable and much more efficient
* (it does not need to evaluate a condition each time)
* but only works on an integer expression
*/
int idx=3;
switch (idx) {
case 1:
cout<<"one!\n";break;
case 2:
cout<<"two!\n";break;
default:
cout<<"not one and not two\n";
}
// note the use of break
}

return 0; // zero means all is well
}

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The peculiar syntax of "for" in C++ deserves to be discussed, considered that it's one of the most commonly used control structures. In BASIC, for example, the simplest indexed loop is

FOR i=0 TO 10
...
NEXT i

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and in Fortran

do i=1,10
...
enddo

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in C/C++, this is

int i;
for(i=0;i<=10;i++) {
...
}

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which makes explicit the initial assignment i=0, the repeat condition i<=10 and the increment operation i++, so that the for statement can be considered just a shorthand for

int i=0;
while (i<=10) {
...
i++;
}

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Please note also that in C++ you can use the more compact form below, where the index variable i is declared inside the for statement itself; this is not allowed in plain C. The two forms are nearly equivalent; the only difference is the scope of the i variable: in the C++ form it will only be defined inside the loop; with the C form it will exist in the whole outer block. In general the C++ form is to be preferred, because it makes impossible to use the index outside of the loop where it makes sense to use it.

for(int i=0;i<=10;i++) {
...
}

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In rare cases, for example where you have an if statement that uses break to exit the loop before the natural completion, you may want to check which index was processed last:

int i;
for(i=0;i<=10;i++) {
if ( a[i]==3 ) break;
}
if (i<10) cout<<" value 3 found in the array at index "<<i<<endl;

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(we will see the array data structures later)

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