c - Why a 2D array is used for stack? Why cant a 1D array be used for a stack? -

here, s[20][20] declared in function pre_post. push operation involves pushing string stack s. how done? why cant execute program 1d array s s[20] stack??

#include<stdio.h> #include<string.h> #include<ctype.h> void push(char item[],int *top,char s[][20])    //pushes symbol item onto stack s {   *top=*top+1;                                  //top incremented   strcpy(s[*top],item);                         //item written onto stack }  char *pop(int *top,char s[][20])                //pops out topmost element of stack s {   char *item;   item=s[*top];   *top=*top-1;   return item; }  void pre_post(char prefix[],char postfix[])     //function convert prefix postfix {   char s[20][20];   int top,i;   char symbol,ch[2];   char *op1,*op2;    top=-1;   for(i=strlen(prefix)-1;i>=0;i--)              //scans expression in   reverse manner   {     symbol=prefix[i];                           //the last symbol scanned first     ch[0]=symbol;     ch[1]='\0';     switch (symbol)                             //checks whether symbol 1 among fiveoperators given.(next line)                                                 //if yes execute corresponding statements     {       case '+':       case '-':       case '*':       case '/':       case '^':         op1=pop(&top,s);                        //the topmost operand of stack s popped , stored op1         op2=pop(&top,s);                        //the next operand of stack s popped , stored op2          strcpy(postfix,op1);                    //copies op1 postfix         strcat(postfix,op2);                    //concatenates op2 postfix         strcat(postfix,ch);                     //concatenates ch postfix         push(postfix,&top,s);                   //pushes postfix onto stack s         break;       default:         if(isalnum(symbol))                     //it means switch case not satisfied i.e. symbol operand           push(ch,&top,s);                      //pushes operand onto stack s      }   }  }  int main() {   char prefix[20];   char postfix[20];   printf("\n\n enter prefix expression \n\n");   gets(prefix);   pre_post(prefix,postfix);   printf("\n\n postfix expression %s \n\n",postfix);   return 0; } 

you need 1-d array of characters represent string.

char str[20]; // array of characters can hold string               // of 19 characters. 

then, need 2-d array of characters represent array of strings.

char str[100][20]; // array 100 strings. each element of array                    // can hold string of 19 characters.