Income tax on share trading income

Dear Sir,

If a 30-year-old does not have any source of income and only earns Rs. 4 lakhs per annum from short-term capital gains in the share market, with this being the only income per year, is the following understanding correct?

After applying the standard deduction and deductions under section 80C, if the remaining income falls below the taxable income slab, is income tax not applicable on the remaining balance, or is the 15% tax on short-term capital gains mandatory even if the total income falls under the non-taxable income slab?

Pleas clarify