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Presuming you're a tourist in Warsaw, the neighborhoods you're most likely to frequent are quite safe, even at night. Violent robberies which can be more common in parts of the U.S. aren't as big of a thing in Europe.
Most tourists would end up in the central districts of Śródmieście (the city center, which includes neighborhoods like Powiśle, the Old and New Towns and Muranów), Wola, Ochota and Mokotów, all of which are safe even at night. I've lived in Ochota and it's very quiet in the evening.
Some will say to avoid the eastern side of Warsaw, specifically Praga, which is less safe. However, during the day Praga is actually quite safe and it's a pleasant place to visit. Check out the Saska Kępa neighborhood when you get the chance to, as it's home to some very stately mansions and is known for its street art.
So long as you exercise common sense, as you should in any big city, you'll be fine.