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ios 数组求和，求平均值的小技巧

NSArray *testArray = [NSArray arrayWithObjects:@"2.0", @"2.3", @"3.0", @"4.0", nil];

NSNumber *sum = [testArray valueForKeyPath:@"@sum.floatValue"];

NSNumber *avg = [testArray valueForKeyPath:@"@avg.floatValue"];