iOS 科学计数法

科学计数法表示数字，并保留三位有效数字，记录一下

示例如下：

- (NSString *)getScientificStyleWithString:(NSString *)str{
    double num = [[NSString stringWithFormat:@"%@",str] floatValue];
    
    NSNumberFormatter * formatter = [[NSNumberFormatter alloc]init];
    formatter.numberStyle = NSNumberFormatterScientificStyle;
    
    NSString *resultStr;
    
    NSString * ttlString = [formatter stringFromNumber:[NSNumber numberWithDouble:num]];
    
    NSRange eRange = [ttlString rangeOfString:@"E"];
    
    if (eRange.location != NSNotFound) {
        NSString *dataStr = [ttlString substringToIndex:eRange.location];
        NSString *eStr = [ttlString substringFromIndex:eRange.location];
        
        resultStr = [NSString stringWithFormat:@"%.2f%@",dataStr.floatValue,eStr];
    }else{
        resultStr = ttlString;
    }

    return resultStr;
 }