I am a student of 21 years. i have cough without cold alternate days . and also i have 3-4 times stool per day. these problems are staying with me since last year. my weight 80kg, height 5.5 ft. and have no disease history. i walk 3.5 km everyday?

Your weight is higher than a desired . Try to reduce your intake of food by about one third, not easy, but necessary. Make a diary recording all you eat and drink, for two weeks and then reduce, as you can manage. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol or street drugs.

You are very young to be having a persistent cough without a diagnosis. Additionally, 4 stools per day, is on the upper side (are they loose?). I would like for you to see a Primary care Doctor because there are several causes to look into. You may have Gastrointestinal Reflux causing you to cough, maybe just wrong diet and posture, even cystic fibrosis, etc.