Increment for loop by variable

Question

I am taking in sound as a float called scaledVol. I wish to change the spacing of the letters being drawn out by scaledVol.

This is the code snippet:

for (int i = 0; i < camWidth; i+=7){
    for (int j = 0; j < camHeight; j+=9){
        // get the pixel and its lightness (lightness is the average of its RGB values)
        float lightness = pixelsRef.getColor(i,j).getLightness();
        // calculate the index of the character from our asciiCharacters array
        int character = powf( ofMap(lightness, 0, 255, 0, 1), 2.5) * asciiCharacters.size();
        // draw the character at the correct location
        ofSetColor(0, 255, 0);
        font.drawString(ofToString(asciiCharacters[character]), f, f);
    }
}

where i sets the width between character spacing and j sets the height between character spacing.

Instead of incrementing by 7 or 9, I would like to increment by a float called scaledVol.

"Instead of incrementing by 7 or 9, I would like to increment by a float called scaledVol" have you tried replacing the 7 or 9 with a float called scaledVol? — Cogwheel
– Cogwheel, Commented Jan 7, 2014 at 17:19
I have tried to replace the 7 or 9 with scaledVol and whilst xcode compiles the project, it does not run — Ryanas
– Ryanas, Commented Jan 7, 2014 at 18:19

Basile Starynkevitch · Accepted Answer · 2014-01-07 20:13:21Z

3

Instead of incrementing by 7 or 9, I would like to increment by a float called scaledVol.

Then code:

 for (int i = 0; i < camWidth; i+=(int)scaledVol){

You may want to take the floor of, and ensure the conversion is done once, the increment; perhaps code instead

 int incr = (int) floor(scaledVol);
 assert (incr > 0);
 for (int i = 0; i < camWidth; i+=incr) {

Read more about floor(3), ceil(3), round(3), and IEEE floating point and rounding errors

Please use your debugger (e.g. gdb) to understand more.

You could use more C++ friendly casts e.g.

 int incr = int(floor(scaledVol));

or static_cast

 int incr = static_cast<int>(floor(scaledVol));

or perhaps even reinterpret_cast

 int incr = reinterpret_cast<int>(floor(scaledVol));

^{which might not work as you want, particularily if both numerical types have same size.}

edited Jan 7, 2014 at 20:13

answered Jan 7, 2014 at 17:19

Basile Starynkevitch

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5 Comments

Jarod42 Over a year ago

Hope that incr != 0

Ed Heal Over a year ago

You have the potential for rounding errors (hence looping too often)

Ryanas Over a year ago

I attempted this and it compiles except the debug application does not respond. Unfortunately, I'm not seeing any errors.

Gabriel Schreiber Over a year ago

c-style-casts are a bad idea. use int(scaledVol) instead of (int)scaledVol.

Ryanas Over a year ago

Thanks, managed to use this and get it working. Had to map my variable first but then your technique of the c++ friendly cast solved my issue. Thank you.

Ed Heal · Accepted Answer · 2014-01-07 17:20:48Z

1

Need something like

for (float i = 0.0f; i < camWidth; i+=scaleVol){

Assuming that camWidth is a float. If not cast it to a float.

This will also over come a problem with rounding errors when converting scaledVol to an int

answered Jan 7, 2014 at 17:20

Ed Heal

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Comments

lrineau · Accepted Answer · 2014-01-08 10:18:46Z

1

You can use float as the type of the two loop variables, and then cast them to int:

for (float x = 0; (int)x < camWidth; x+=scaledVol) {
  int i = (int)x;
  for (float y = 0; (int)y < camHeight; y+=scaledVol) {
    int j = (int)y;
    // the rest of the code using i and j
  }
}

Be careful that scaledVol has best be greater than 1, otherwise you will have consecutive values of i and j that are equal. Your treatment in `// the rest of the code`` may not like that.

edited Jan 8, 2014 at 10:18

answered Jan 7, 2014 at 17:23

lrineau

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