For/if

[gloin]

Hello,

Tying to understand using a for without brackets...not sure why they are used. This example A executes differently than b. Thanks much

ex a

				
	for (j=0;j<3;j++) 
				if (j!=i) 
				A= A-B;

ex b

for (j=0;j<3;j++) 
	{			
            if (j!=i) 
	    A= A-B;
                
        }

[GCDEF]

Brackets are used to group statements.

There's no syntactic difference in your example, but consider

if(a == b)
    c = d;
    e = f;

or

if(a == b)
{
    c = d;
    e = f;
}

In the first example, e = f is executed unconditionally. In the second, only if the if statement is true.

[2kaud]

This example A executes differently than b

How? In what way? Have you traced through the code with the debugger? In both cases does i, A and B start with the same values? Post the actual code used where a executes differently to b.

Consider

#include <iostream>
using namespace std;

int main()
{
	int A;
	int j;

	const int B = 10;
	const int i = 2;

	A = 100;
	for (j = 0; j < 3; j++)
		if (j != i)
			A = A - B;

	cout << "a) " << A << endl;

	A = 100;
	for (j = 0; j < 3; j++)
	{
		if (j != i)
			A = A - B;

	}

	cout << "b) " << A << endl;
}

This displays

a) 80
b) 80

[gloin]

Sorry for the psudo code...

so when I run this I get the correct answers...if you remove the remarks it doesn't work correctly. Im not sure why when I add the brackets it wont work...

#include <iostream> 
#include <cmath> 
#include <iomanip> 
using namespace std; 

int main() 
{
	double z;
	double a[3][4] ={1,3,1,10,1,2,-1,6,2,1,2,10};
	double b[3]; 
	
	for (int i=0;i<3-1;i++) 
		for (int k=i+1;k<3;k++) 
		{   z=a[k][i] / a[i][i]; 
			for(int j=0;j<=3;j++) 
			a[k][j] = a[k][j] - z*a[i][j];
			
		}

	for (int i=3-1;i>=0;i--)
	
	{	b[i]=a[i][3]; 
		for (int j=0;j<3;j++) 
		//{
			if (j!=i)
			//{
				b[i]=b[i]-a[i][j] * b[j];
				b[i]=b[i]/a[i][i];
			//}
		//}
	}
	
 	int k = 1; 
 
 	for(int i=0;i<3;i++) 
	{cout<<"A"<<k<<" = "<<b[i]<<endl; 
		k++; 
	}
		
	system ("pause"); 
	return 0; 
	
}

[GCDEF]

I've already explained why. The second statement following your if will execute regardless whether the if statement evaluates true or false. Indentation is only for readability. It has no effect on the actual code.

In your first example, you only have one statement following each for or if. In the second, you have two statements after the if, but only one is controlled by the if.