How can I skip some datas ?

	void FixedUpdate() {
		
		delta_time += Time.deltaTime; 

		if (frame < nv_data[0].positions.Length) { 

			for (int k = 0; k < body.Length; ++k) {
				
				if (body[k] != null) { 

					body[k].transform.localPosition = nv_data[k].positions[frame] / 1000;

				} 

				else
					continue;
			}
								
			if (frame < forces_dup.Length) { 

				if (frame >= 1) {

						float delta_x = (((nv_data[59].positions[frame] - nv_data[59].positions[frame-1]).z)*0.001f);
					vitesse = delta_x/Time.deltaTime;
					acceleration = (vitesse-ancienne_vitesse)/Time.deltaTime;
					ancienne_vitesse = vitesse;

						indice_3 ++;

						StreamWriter writer = new StreamWriter ("Masse-fois-Accelerations_FR_5Hz", true); 
						
						using (writer) {

							writer.WriteLine(Time.time + "\t" + 80*acceleration + "\t" + forces_dup[indice_3].x);
					}
				}	

			}

				frame++; 	
		}

}

void FixedUpdate() {

    delta_time += Time.deltaTime;

    if (frame < nv_data [0].positions.Length) {

                    if ((frame % 20) == 0) {

                            Debug.Log (frame);

                            for (int k = 0; k < body.Length; ++k) {

                                    if (body [k] != null) { 

                                            body [k].transform.localPosition = nv_data [k].positions [frame] / 1000;

                                    } else
                                            continue;
                            }

                            if (frame < forces_dup.Length) {

                                    if (frame >= 1) {

                                            float delta_x = (((nv_data [59].positions [frame] - nv_data [59].positions [frame - 1]).z) * 0.001f);
                                            vitesse = delta_x / Time.deltaTime;
                                            acceleration = (vitesse - ancienne_vitesse) / Time.deltaTime;
                                            ancienne_vitesse = vitesse;

                                            indice_3 ++;

                                            StreamWriter writer = new StreamWriter ("Masse-fois-Accelerations_FR_5Hz", true);

                                            using (writer) {

                                                    writer.WriteLine (Time.time + "\t" + 80 * acceleration + "\t" + forces_dup [indice_3].x);
                                            }
                                    }
                            }

                            frame++;
                    }
            }
}