Language: C++

#include<stdio.h>
#include<math.h>

int main(void){
	int N,i[10];
	char a[10]={'a','b','c','d','e','f','g','h','i','j'};
	scanf("%d",&N);
	for(i[0]=0;i<N;i[0]++){
		if(N>1){
			for(i[1]=0;i<N;i[1]++){
				if(N>2){
					for(i[2]=0;i<N;i[2]++){
						if(N>3){
							for(i[3]=0;i<N;i[3]++){
								if(N>4){
									for(i[4]=0;i<N;i[4]++){
										if(N>5){
											for(i[5]=0;i<N;i[5]++){
												if(N>6){
													for(i[6]=0;i<N;i[6]++){
														if(N>7){
															for(i[7]=0;i<N;i[7]++){
																if(N>8){
																	for(i[8]=0;i<N;i[8]++){
																		if(N>9){
																			for(i[9]=0;i<N;i[9]++)printf("%c%c%c%c%c%c%c%c%c%c\n",a[i[0]],a[i[1]],a[i[2]],a[i[3]],a[i[4]],a[i[5]],a[i[6]],a[i[7]],a[i[8]],a[i[9]]);
																		}else printf("%c%c%c%c%c%c%c%c%c\n",a[i[0]],a[i[1]],a[i[2]],a[i[3]],a[i[4]],a[i[5]],a[i[6]],a[i[7]],a[i[8]]);
																	}
																}else printf("%c%c%c%c%c%c%c%c\n",a[i[0]],a[i[1]],a[i[2]],a[i[3]],a[i[4]],a[i[5]],a[i[6]],a[i[7]]);
															}
														}else printf("%c%c%c%c%c%c%c\n",a[i[0]],a[i[1]],a[i[2]],a[i[3]],a[i[4]],a[i[5]],a[i[6]]);
													}
												}else printf("%c%c%c%c%c%c\n",a[i[0]],a[i[1]],a[i[2]],a[i[3]],a[i[4]],a[i[5]]);
											}
										}else printf("%c%c%c%c%c\n",a[i[0]],a[i[1]],a[i[2]],a[i[3]],a[i[4]]);
									}
								}else printf("%c%c%c%c\n",a[i[0]],a[i[1]],a[i[2]],a[i[3]]);
							}
						}else printf("%c%c%c\n",a[i[0]],a[i[1]],a[i[2]]);
					}
				}else printf("%c%c\n",a[i[0]],a[i[1]]);
			}
		}else printf("%c\n",a[i[0]]);
	}
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:8:21: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
    8 |         for(i[0]=0;i<N;i[0]++){
      |                    ~^~
a.cc:10:37: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   10 |                         for(i[1]=0;i<N;i[1]++){
      |                                    ~^~
a.cc:12:53: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   12 |                                         for(i[2]=0;i<N;i[2]++){
      |                                                    ~^~
a.cc:14:69: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   14 |                                                         for(i[3]=0;i<N;i[3]++){
      |                                                                    ~^~
a.cc:16:85: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   16 |                                                                         for(i[4]=0;i<N;i[4]++){
      |                                                                                    ~^~
a.cc:18:101: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   18 |                                                                                         for(i[5]=0;i<N;i[5]++){
      |                                                                                                    ~^~
a.cc:20:117: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   20 |                                                                                                         for(i[6]=0;i<N;i[6]++){
      |                                                                                                                    ~^~
a.cc:22:133: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   22 |                                                                                                                         for(i[7]=0;i<N;i[7]++){
      |                                                                                                                                    ~^~
a.cc:24:149: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   24 |                                                                                                                                         for(i[8]=0;i<N;i[8]++){
      |                                                                                                                                                    ~^~
a.cc:26:165: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   26 |                                                                                                                                                         for(i[9]=0;i<N;i[9]++)printf("%c%c%c%c%c%c%c%c%c%c\n",a[i[0]],a[i[1]],a[i[2]],a[i[3]],a[i[4]],a[i[5]],a[i[6]],a[i[7]],a[i[8]],a[i[9]]);
      |                                                                                                                                                                    ~^~