NSString:

1.subStringToIndex:1  擷取字串,(擷取到1)

  NSString *str = @"我是一串字串";
    NSString *strJieQu = [str substringToIndex:1];
    NSLog(@"擷取str:%@", strJieQu);

2.subStringFromIndex:1 擷取字串（從1開始擷取）

NSString *str = @"我是一串字串";
    NSString *strJieQu = [str substringFromIndex:1];
    NSLog(@"擷取str:%@", strJieQu);
 

3. stringWithFormat:(NSString) 字串的萬能初始化方法

 NSString *str = @"我是一串字串";
    NSInteger num = 5;
    CGFloat floatNum = 3.14;
    NSString *strJieQu = [NSString stringWithFormat:@"%@, %ld, %f", str, num, floatNum];
    NSLog(@"擷取str:%@", strJieQu);

執行結果：我是一串字串, 5, 3.140000

4.stringByReplaceOccurrencesOfString:(NSString) withString:(NSString)字串的替換

    NSString *str = @"我是一串字串";
    NSString *strJieQu = [str stringByReplacingOccurrencesOfString:@"我" withString:@"你"];
    NSLog(@"擷取str:%@", strJieQu);

5.stringByAppendingFormat:(NSString) 字串的拼接

    NSString *str = @"我是一串字串";
    NSString *strJieQu = [str stringByAppendingFormat:@"%@, 我很happy", str];
    NSLog(@"擷取str:%@", strJieQu);
 

6. 判斷字串是否相等:
     isEqual:
     isEqualToString:(NSString);

   常用於if條件判斷

NSString *str = @"我是一串字串";
    NSString *strJieQu = [str stringByAppendingFormat:@"%@, 我很happy", str];
    if ([str isEqualToString:@"我是一串字串"]) {
        
        NSLog(@"一樣");
    }
    else
    {
        NSLog(@"擷取str:%@", strJieQu);
    }

7. compare : 字串比較

    NSString *str = @"我是一串字串";
    NSLog(@"擷取str:%ld", (long)[str compare:@"178"]);

8. hasPrefix: 判斷字串開頭是否相等

    NSString *str = @"我是一串字串";
    if ([str hasPrefix:@"我"]) {
        
        NSLog(@"YES");
    }
    else
    {
        NSLog(@"NO");
    }

9. hasSuffix: 判斷字串結尾是否相等

    NSString *str = @"我是一串字串";
    if ([str hasSuffix:@"我"]) {
        
        NSLog(@"YES");
    }
    else
    {
        NSLog(@"NO");
    }

12 floatValue:強轉CGFloat型