CODEWITHSUNDEEP
for-loop
apss1943
apss1943

Reputation: 269

Is there alternative way to change index inside the for loop?

Assumed question I don’t wanna 7 in the random number so I do this. I know this is not a good example. Sorry for that first.

int temp[8];

for (int i = 0 ; i < 8 ; i ++ ){
    temp[i] = random(8);
    if (temp[i] == 7){
        temp[i] = random(8);
        i--;
    }
}

My ideal is don’t do i— where inside the for loop. Use extra space to record it then do the somehhing outside the first for loop. The code will become:

ArrayList i_index;
int count = 0 ;
int temp[8];

for (int i = 0 ; i < 8 ; i ++ ){
    temp[i] = random(8);
    if (temp[i] == 7){
        i_index.add(i) ;    
    }
}

for (int i = 0 ; i < 8 ; i ++ ){
    if ( i_index.get(count) == i ){
        temp[i] ++ ; //do something or whatever just not return 7 
        count ++ ;  
    }
}

Is that correct or is there any better ideal? Or hope someone can give a good example.

Upvotes: 2

Views: 107

Answers (4)

t3chb0t
t3chb0t

Reputation: 18675

I don't see anything wrong in manipulating the index like that. I do it my self very often.

For example lately I had to find items in an array that might come in pairs but not always so the loop looked similar to this (it's pseudocode):

for(int i = 0; i < itemCount; i++)
{
    item1 = items[i]
    if (i + 1 < itemCount)
    {
       item2 = items[i+1]
       if (item2 is a pair to item1)
       {
           ...do something
           i++ // item2 is already processed so skip it next time by increasing the i
       }
    }
}

If your algorithm requires it I would say it's totaly ok to manipulate the i. Sometimes there is no other way (like in my example).

EDIT: Now that you showed us the full case I still think it is ok in to change the i in your algorithm. However there are a few other solutions possible:

Example-1: you could use a while-loop inside the for loop

for (int i = 0 ; i < 8 ; i ++ )
{
    int num = random(8);
    while(num == 7)
    {
        num = random(8);
    }
    temp[i] = num;
}

Example-2: or the other way round by using a do-while inside the for loop

for (int i = 0 ; i < 8 ; i ++ )
{
    int num;
    do
    {
        num = random(8);
    } 
    while (num == 7);

    temp[i] = num;
}

Example-3: or just a single do-while loop until it finds 8 numbers different from 7:

int count = 0;
int num;

do
{
    num = random(8);
    if (num != 7)
    {
        temp[count] = num;
        count++;
    }
}
// Repeat as long as there are fewer then 8 items or the number is 7
while (count < 8 || num == 7);

Example-4: if you can use a collection other then an array, like a list or something where you can add items it can even be simpler:

int num;
bool isValidNumber;
do
{
    num = random(8);
    isValidNumber = (num != 7);
    if (isValidNumber)
    {
        numList.add(num);
    }
}
// Repeat as long as there are fewer then 8 items or the number is invalid
while (numList.count < 8 || !isValidNumber);

Upvotes: 1

Elliott Frisch
Elliott Frisch

Reputation: 201447

I believe you're trying to run the loop again with the same i (without decrementing it in the loop body). One option is to move the increment into the loop body and use a continue like

for (int i = 0 ; i < 8 ; ) { // <-- remove the i++ from here
    if (xxx){
        YYY; // do something
        // i--; // <-- don't need to subtract one from i
        continue; // <-- will rerun the loop with the same i
    }
    i++; // <-- add it here.
}

Another option would be an else like

for (int i = 0 ; i < 8 ; ) { // <-- remove the i++ from here
    if (xxx){
        YYY; // do something
        // i--; // <-- don't need to subtract one from i
    } else {
        i++; // <-- add it here.
    }
}

And then you don't need the continue;

Based on your newly updated post,

for (int i = 0 ; i < 8 ; i ++ ){
    temp[i] = random(7); // <-- 0,6 (will never be 7).
}

If you want a gap, I suggest you still reduce the range and test like,

for (int i = 0 ; i < 8 ; i ++ ){
    int r = random(8); // <-- assuming it's 8 exclusive,
    temp[i] =  (r != 7) ? r : 8; // <-- skip 7.
}

Upvotes: 0

Deeper
Deeper

Reputation: 129

i would like to try in this way since not sure what language you going to use and what mean by XXX and YYY action.

Example in PHP

$arr=array();

    for($x=0;$x<8;$x++)
    {
        if(xxx)
        {
            array_push($arr,$x);
        }
    }

Now we have a array which matched with XXX condition, so we just do YYY action based on what it stored in array.

foreach($arr as $y)
{
    YYY($y);
}

Upvotes: 0

Tyler Beason
Tyler Beason

Reputation: 193

Yea what I typically do is use a second index or counter variable like you've shown in your code.

count=0;
for(i=0,i<800,i++)
{
    if(x)
    {
       //do something using count
       //ex: array[count] = i+3*count;
       count++;
    }
}

I don't think your second loop is probably necessary, but it also isn't entirely clear what you are looking to do. Regardless, that's how you use a secondary counter inside a for loop.

Upvotes: 0

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