C Program To Find First And Follow In Compiler Design
Let’s try to find first and follow for given CFG with the help of a C program.
E=XY
X=ilove
Y=t4tutorials
C Program To Find First And Follow In Compiler Design
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// C program to calculate the First and // CFG_Follow sets of a given grammar #include<stdio.h> #include<ctype.h> #include<string.h> // Functions to calculate CFG_Follow void T4Tutorials(char, int, int); void CFG_Follow(char c); // Function to calculate First void Searching_First(char, int, int); int count, n = 0; // Stores the final result of the First Sets char FirstCalculation[10][100]; // Stores the final result of the CFG_Follow Sets char calc_CFG_Follow[10][100]; int m = 0; // Stores the production rules char production[10][10]; char f[10], first[10]; int k; char ck; int e; int main(int argc, char **argv) { int jm = 0; int km = 0; int i, choice; char c, ch; count = 4; // The Input grammar strcpy(production[0], "E=XY"); strcpy(production[1], "X=ilove"); strcpy(production[3], "Y=t4tutorials"); int kay; char done[count]; int ptr = -1; // Initializing the FirstCalculation array for(k = 0; k < count; k++) { for(kay = 0; kay < 100; kay++) { FirstCalculation[k][kay] = '!'; } } int point1 = 0, point2, xxx; for(k = 0; k < count; k++) { c = production[k][0]; point2 = 0; xxx = 0; // Checking if First of c has already been calculated for(kay = 0; kay <= ptr; kay++) if(c == done[kay]) xxx = 1; if (xxx == 1) continue; // Function call Searching_First(c, 0, 0); ptr += 1; // Adding c to the calculated list done[ptr] = c; printf("\n First(%c) = { ", c); FirstCalculation[point1][point2++] = c; // Printing the First Sets of the grammar for(i = 0 + jm; i < n; i++) { int lark = 0, chk = 0; for(lark = 0; lark < point2; lark++) { if (first[i] == FirstCalculation[point1][lark]) { chk = 1; break; } } if(chk == 0) { printf("%c, ", first[i]); FirstCalculation[point1][point2++] = first[i]; } } printf("}\n"); jm = n; point1++; } printf("\n"); printf("****************************************\n\n\n"); char donee[count]; ptr = -1; // Initializing the calc_CFG_Follow array for(k = 0; k < count; k++) { for(kay = 0; kay < 100; kay++) { calc_CFG_Follow[k][kay] = '!'; } } point1 = 0; int land = 0; for(e = 0; e < count; e++) { ck = production[e][0]; point2 = 0; xxx = 0; // Checking if CFG_Follow of ck // has alredy been calculated for(kay = 0; kay <= ptr; kay++) if(ck == donee[kay]) xxx = 1; if (xxx == 1) continue; land += 1; // Function call CFG_Follow(ck); ptr += 1; // Adding ck to the calculated list donee[ptr] = ck; printf(" CFG_Follow(%c) = { ", ck); calc_CFG_Follow[point1][point2++] = ck; // Printing the CFG_Follow Sets of the grammar for(i = 0 + km; i < m; i++) { int lark = 0, chk = 0; for(lark = 0; lark < point2; lark++) { if (f[i] == calc_CFG_Follow[point1][lark]) { chk = 1; break; } } if(chk == 0) { printf("%c, ", f[i]); calc_CFG_Follow[point1][point2++] = f[i]; } } printf(" }\n\n"); km = m; point1++; } } void CFG_Follow(char c) { int i, j; // Adding "$" to the CFG_Follow, set of the start symbol if(production[0][0] == c) { f[m++] = '$'; } for(i = 0; i < 10; i++) { for(j = 2;j < 10; j++) { if(production[i][j] == c) { if(production[i][j+1] != '\0') { // Calculate the first of the next // Variable (Non-Terminal ) in the production T4Tutorials(production[i][j+1], i, (j+2)); } if(production[i][j+1]=='\0' && c!=production[i][0]) { // Calculate the CFG_Follow of the Variable (Non-Terminal ) // in the L.H.S. of the production CFG_Follow(production[i][0]); } } } } } void Searching_First(char c, int q1, int q2) { int j; // The case where we // encounter a Terminal if(!(isupper(c))) { first[n++] = c; } for(j = 0; j < count; j++) { if(production[j][0] == c) { if(production[j][2] == '#') { if(production[q1][q2] == '\0') first[n++] = '#'; else if(production[q1][q2] != '\0' && (q1 != 0 || q2 != 0)) { // Recursion to calculate First of New // Variable (Non-Terminal ) we encounter after epsilon Searching_First(production[q1][q2], q1, (q2+1)); } else first[n++] = '#'; } else if(!isupper(production[j][2])) { first[n++] = production[j][2]; } else { // Recursion to calculate First of // New Variable (Non-Terminal ) we encounter // at the beginning Searching_First(production[j][2], j, 3); } } } } void T4Tutorials(char c, int c1, int c2) { int k; // The case where we encounter // a Terminal if(!(isupper(c))) f[m++] = c; else { int i = 0, j = 1; for(i = 0; i < count; i++) { if(FirstCalculation[i][0] == c) break; } //Including the First set of the // Variable (Non-Terminal ) in the CFG_Follow of // the original query while(FirstCalculation[i][j] != '!') { if(FirstCalculation[i][j] != '#') { f[m++] = FirstCalculation[i][j]; } else { if(production[c1][c2] == '\0') { // Case where we reach the // end of a production CFG_Follow(production[c1][0]); } else { // Recursion to the next symbol in case we encounter a "#" T4Tutorials(production[c1][c2], c1, c2+1); } } j++; } } } |
Output
C Code of First and Follow in Parsing[Download]
Rules of First and Follow
A tutorial with easy examples of Rules of First and Follow can be read here.