How can I read and process part of file and write the rest to another file?

I need to read this csv file with Panda, perform some process on it and write the rest 10% of the data to another sheet.

Given this solution (https://stackoverflow.com/a/55763598/3373710), I want to do a process on the rest of store_data after taking out the 10% rows, however, elif condition prints the same rows of the original file, how can I fix my condition to skip the 10% rows?

store_data = pd.read_csv("heart_disease.csv")

with open("out1.csv","w") as outfile:
    outcsv = csv.writer(outfile)
    for i, row in store_data.iterrows():
        if not i % 10: #write 10% of the file to another file
            outcsv.writerow(row)
        elif i % 10:  #I need to do some process on the rest of the file
            store_data = store_data.applymap(str)