God, the All-knowing doesn't make the wrong decision, all his acts are the products of full wisdom. To repent means to have sorrow and grief in your heart for a wrong action or unfair action, God can't be like this Definition of 'Repent' Both of the above sections are technically correct, though both make different assumptions about the word "repent." "Repent" simply means to change one's mind or to change one's course of action. In the verses cited above, God chooses to change His course of action. In this case the word "repent" doesn't imply any sin. The more common usage of the word "repent" in Christian circles refers to repentance of sin. However the writer of Holy Book, who is not Moses, used expressions that do not fit God, God is high above any human imagination, and even the best of words can't tell about how far his greatness is. In the story of Noah, Noah spent 600 hundred years trying to rectify their concepts and morals but they rejected and challenged him brazenly. Noah kept preaching among them but in vain were his efforts, he warned them against that day on which God would punish them all but they mocked him and even asked him to bring on them God's wrath. It was clear that they were people with callous hearts, they instilled in their children's minds that Noah was a liar, a generation after a generation. It was a due retribution that they received for their long stubbornness. However , Noah and his family except his wife and a son who didn't believe in him, and those who followed him embarked on the Ark and they all survived. I don't know why the writer here made God like a man who repented for his retribution from a nation that didn't accept to believe though it prophet spent among them 950 years, please note the Holy book says that Noah was 600 years old when he embarked on the ship. What is really strange is that writer of the Holy Book always makes God repent in many other places in the Holy Book:

Genesis 6:6 And it repented the Lord that he had made man on the earth, and it grieved him at his heart. Exodus 32:14 And the Lord repented of the evil which he thought to do unto his people.

Deuteronomy 32:36 For the Lord shall judge his people, and repent himself for his servants. 1 Samuel 15:11 It repenteth me [God] that I have set up Saul to be king. 1 Samuel 15:35 The Lord repented that he had made Saul king over Israel. 2 Samuel 24:16 The Lord repented of the evil, and said to the angel that destroyed the people, it is enough: stay now thine hand. 1 Chronicles 21:15 The Lord beheld, and he repented him of the evil, and said to the angel that destroyed, It is enough, stay now thine hand. Isaiah 38:1-5 In those days was Hezekiah sick unto death. And Isaiah ... said unto him, Thus saith the LORD, Set thine house in order: for thou shalt die, and not live. ... Thus saith the LORD ... I have heard thy prayer, I have seen thy tears: behold, I will add unto thy days fifteen years. Jeremiah 15:6 I [God] am weary of repenting. Jeremaih 18:8 I [God] will repent of the evil that I thought to do unto them. Jeremaih 26:3 That I [God]may repent me of the evil, which I purpose to do unto them. Jeremiah 26:13 The Lord will repent him of the evil that he hath pronounced against you. Jeremiah 26:19 The Lord repented him of the evil which he had pronounced against them. Jeremaih 42:10 For I [God] repent me of the evil that I have done unto you. Amos 7:3, 6 The Lord repented for this. Jonah 3:10 God repented of the evil, that he had said that he would do unto them. However the abovementioned verses contradict what God really said about himself: Numbers 3:19 God is not a man that he should lie; neither the son of a man that he should repent. 1 Samuel 15:29 The Strength of Israel will not lie nor repent: for he is not a man, that he should repent. Ezekiel 24:14 I the LORD have spoken it: it shall come to pass, and I will do it; I will not go back, neither will I spare, neither will I repent.