#include <stdio.h>

#include "nekiheader.h"

int main(){
char ulaz[MAXLINE+1];
char i,j,k,c;
char operatori[MAXLINE / 2];
float operandi[MAXLINE / 2];
int brChar, brOperatora;
int operandPos, operatorPos, rb;
char errorF;

        while(1){
                operandPos=0;
            operatorPos=0;
                errorF=0;

                brChar = newreadline(ulaz,MAXLINE);
                printf("%s",ulaz);


                if(brChar){ // broj karaktera mora biti > 0
                        if(ulaz[0] <'0' || ulaz[0] > '9'){

                                continue;
                        }
                        for(i=0; i<brChar; i++){
                                if((ulaz[i] < '0' || ulaz[i] >  '9')
                                   && ulaz[i] != '+' && ulaz[i] != '-' && ulaz[i] != '*' && ulaz[i] != '/'){

                                                continue;
                                }
                        }
                        brOperatora=0;
                        for(i=0; i<brChar; i++){
                                if(ulaz[i] == '+' || ulaz[i] == '-' || ulaz[i] == '*' || ulaz[i] == '/')  brOperatora++;
                        }
                        if(brOperatora==0){

                                continue;
                        }

                        if(ulaz[brChar-1] =='+' || ulaz[brChar-1] == '-' || ulaz[brChar-1] == '*' || ulaz[brChar-1] == '/'){

                                continue;
                        }

                        for(i=0; i<brChar-1; i++){
                                if((ulaz[i] == '+' || ulaz[i] == '-' || ulaz[i] == '*' || ulaz[i] == '/') &&
                                        (ulaz[i+1] == '+' || ulaz[i+1] == '-' || ulaz[i+1] == '*' || ulaz[i+1] == '/')){

                                        break;
                                }
                        }
                        if(i<brChar-1) continue;

                }
                else continue;

                i=0;
                rb=brOperatora;
                while(rb){
                        sscanf(ulaz, "%f", &operandi[operandPos]);
                        operandPos++;

                        while(i<brChar){
                                if(ulaz[i] == '+' || ulaz[i] == '-' || ulaz[i] == '*' || ulaz[i] == '/' ){
                                        operatori[operatorPos] = ulaz[i];
                                        operatorPos++;

                                        k=0;
                                        for(j=i+1; j<brChar; j++,k++){
                                                ulaz[k] = ulaz[j];
                                        }
                                        ulaz[k] = 0;
                                        brChar -= i+1;
                                        i=0;
                                        break;
                                }
                                i++;
                        }
                        rb--;
                }
                sscanf(ulaz, "%f", &operandi[operandPos]);


                while(brOperatora){
                        for(i=0; i< brOperatora; i++){
                                if(operatori[i] == '*' || operatori[i] == '/') break;
                        }
                        if(i<brOperatora){
                                if(operatori[i] == '*')
                                        operandi[i] *= operandi[i+1];
                                else
                                        if(operandi[i+1]==0){
                                                 printf("Error - dijeljenje s 0!\n");
                                                 errorF = 1;
                                                 break;
                                        }
                                        else operandi[i] /= operandi[i+1];

                                for(j=i+1; j<brOperatora+1; j++){
                                        operandi[j] = operandi[j+1];
                                }

                                for(j=i; j<brOperatora-1; j++){
                                        operatori[j] = operatori[j+1];
                                }
                                brOperatora--;
                        }
                        else{
                                for(i=0; i< brOperatora; i++){
                                        if(operatori[i] == '+' || operatori[i] == '-') break;
                                }
                                if(i<brOperatora){
                                        if(operatori[i] == '+')
                                                operandi[i] += operandi[i+1];
                                        else
                                                operandi[i] -= operandi[i+1];

                                        for(j=i+1; j<brOperatora+1; j++){
                                                operandi[j] = operandi[j+1];
                                        }

                                        for(j=i; j<brOperatora-1; j++){
                                                operatori[j] = operatori[j+1];
                                        }
                                        brOperatora--;
                                }
                        }
                }
                if(!errorF)
                        printf("=%f\n",operandi[0]);
        }
        return 0;
}


char newreadline(char *buf, int max){
int c;
char *start, *end;

        start = buf;
        end = buf + max;
        while((c = getchar()) != '\n')
                if(buf < end) *buf++ = c;

        *buf = 0;
        return buf - start;
}






nekiheder.h

#ifndef KALK_H
#define KALK_H

#define MAXLINE 200

char newreadline(char *buf, int max);

#endif